Chapter 3

Global Mapping of CDR Opportunities

Authors

Hélène Pilorgé, Ben Kolosz, Grace C. Wu, and Jeremy Freeman

3.1

3.1 —

Introduction

 

As discussed in Chapter 2, achieving net-negative GHG emissions globally will require large-scale development of a portfolio of CDR systems. Additional considerations alongside carbon accounting will be required for each strategy, and many of these are fundamentally spatial: What land area can support a CDR system without competing with human activities (e.g., food production, settlements) and without disturbing natural habitats? Are construction materials available, and what do they cost? What are the social and environmental risks associated with each CDR system related to their location? Can the components of the CDR system be recycled or reused across deployments?

This chapter uses a geospatial approach to highlight global opportunities for siting biological and technological CDR systems that leverage available opportunities, but avoid competing with human activities or habitat conservation. For technological CDR systems, mapping both low-carbon energy resources and storage potential helps identify opportunities for co-location and minimizes transportation distances. In areas where several CDR systems are viable, deployment will involve complex decision-making processes that must include regional stakeholders, policymakers, and local communities. We attempt to provide more details on activities that have the potential for more expansive development. This section should be considered a coarse review. Any actual project would require a more thorough study to assess feasibility and to develop deployment plans.

3.2

3.2 —

Enhancing biological systems for CDR

Each year, approximately 30 percent of human-caused carbon emissions are absorbed into terrestrial ecosystems (Friedlingstein et al., 2019), including forests, soils, and other vegetation. As reviewed in Chapter 2, several human interventions have the potential to “enhance” these biological CDR systems, by either increasing the rate of carbon dioxide removal, extending the permanence of storage, or both. Here, we examine approaches to quantifying and evaluating the technical potential of biological carbon dioxide removal strategies, and their risks and constraints, in particular as they relate to spatial aspects of land use, ecology, and climate.

 

3.2.1

Forests

Interventions related to enhancing carbon dioxide removal by forests can take several forms, including reforestation, afforestation, agroforestry, avoided conversion, and improved forest management (IFM) (See Chapter 2 for definitions.) (Griscom et al., 2017; Anderson et al., 2017). In terms of their effect on the global carbon cycle, reforestation, afforestation, and agroforestry are more clearly forms of carbon dioxide removal insofar as they primarily drive new biomass growth, whereas avoided conversion combines of carbon dioxide removal and avoided emissions. Avoided conversion and IFM prevent emissions associated with deforestation, which reduces tree and soil carbon stocks. They may additionally result in continued carbon dioxide removal, depending on the specifics of the forest’s ecological dynamics (e.g., species, age).

Estimating the potential of any of these approaches begins with considering the location: Where is the project taking place? What do we know about the local ecosystem? And what are the potential interactions with the climate system? A key parameter is above-ground forest carbon. Typically, above-ground forest carbon is estimated by first estimating above-ground biomass, and then multiplying biomass by a conversion factor (approximately 0.5) reflecting the typical carbon concentration in the woody tissues of trees and plants, which vary in their elemental composition depending on the underlying compounds (e.g., lignins [~60 – 72% C], cellulose [44% C], and hemicelluloses [28 – 30% C]) (Martin et al., 2018). Plants grow biomass over time with rates and growth forms that vary by species, climate, soil, and other location-specific parameters. There is additional carbon below ground in roots and in soil organic matter. Efforts to quantify and map these dimensions spatially have thus far relied on a combination of ground observations, remote sensing, and mechanistic modeling. Remote sensing, particularly satellite imagery, is critical for spatially comprehensive large-scale observations, especially in remote areas. It can be used to assess potential forest-based interventions relative to current land use, track changes in forests over time, and interpolate ground measurements where they are unavailable (Hansen et al., 2013; Dubayah et al., 2020; Schimel et al., 2019; Badgley et al., 2017; Homer et al., 2020; Blackard et al., 2008; Spawn et al., 2020). Remote sensing must be validated and complemented with ground information, such as the Forest Inventory Analysis provided by the United States Forest Service, the RAINFOR network in the Amazon, or the African Tropical Rainforest Observation Network in Africa (Malhi et al., 2002; Tinkham et al., 2018; Hubau et al., 2020). These kinds of data, however, are not necessarily available globally, or in interoperable formats. Mechanistic simulation models of biomass growth are useful for comparing and integrating empirical measurements and linking forest carbon dynamics to larger-scale climate models (Fisher et al., 2018). Especially important is incorporating land-atmosphere interactions, because increased vegetation in some areas changes an area’s surface roughness, albedo, and evapotranspiration in ways that contribute to warming, thus eliminating or potentially negating the climate benefit of carbon dioxide removal (Laguë et al., 2019; Bright et al., 2015; Jones et al., 2013a; Jones et al., 2013b). Integrated satellite, ground, and model-based analysis will be critical to identify and prioritize opportunities for forest-based activities that increase carbon dioxide removal.

A key limiting factor for CDR involving forests is the permanence of storage, especially in a changing climate. Carbon stored in forests is sensitive to disturbances such as wildfire, drought, and insects, all of which are sensitive to the climate, and thus depend on spatial location and are likely to change over time (Anderegg et al., 2020; Seidl et al., 2017; Pugh et al., 2019; Giglio et al., 2013; Hicke et al., 2013). Fire in forests results in approximately 6.6 GtCO2 of emissions per year (Chuvieco et al., 2016; van der Werf et al., 2017). The historical record of wildfire occurrences (which can be assessed through satellite imagery) has been modeled as a function of climate variables to describe spatial variability, and strongly suggests that risks will increase in the future, particularly in areas that become drier or have more extreme temperatures (Williams and Abatzoglou, 2016; Barbero et al., 2015; Moritz et al., 2012). Drought decreases productivity and carbon stocks through tree and plant mortality. As a particularly striking example, the California drought in 2011 − 2015 killed more than 140 million trees, resulting in a cumulative ecosystem carbon loss of ~600 Mt CO2, approximately 34 percent of the state's total greenhouse gas emissions over that period (Sleeter et al., 2019; Anderegg et al., 2020). Recent progress has been made characterizing the physiology of drought-induced mortality, but spatially explicit datasets and predictive models remain an area of active research. Finally, anthropogenic disturbance is also a major risk factor from a CDR perspective, whether due to changes in land ownership, conversion of land to agricultural practice, or other factors that result in stored carbon being released back into the atmosphere. However, forests intensively managed to supply long-lived timber products can potentially contribute to carbon storage.

Some studies have tried to identify the potential scale and, in some cases, the spatial distribution of reforestation and other opportunities related to forest carbon (Figure 3.1) (Griscom et al., 2017; Bastin et al., 2019). At least one study (Bastin et al., 2019; Cook-Patton et al., 2020) inflated the potential of reforestation by a factor of ~3 – 10 by failing to account for critical dimensions around climate feedbacks, ecology, land use, and human activity more generally (Veldman et al., 2019; Skidmore et al., 2019; Lewis et al., 2019; Friedlingstein et al., 2019). Significant work remains to rigorously characterize both the potential and the risks of these approaches across space and time.

Figure

3.1

Above-ground carbon sequestration rate in potential reforestation areas (data from Cook-Patton et al., 2020 and Griscom et al., 2017).

cdr primer map 3 1

Above-ground carbon sequestration rate in potential reforestation areas (data from Cook-Patton et al., 2020 and Griscom et al., 2017).

 

3.2.2

Agriculture & grasslands (soil organic carbon)

Land-based carbon dioxide removal strategies for agricultural lands and grasslands have the potential to store 2 − 6.6 GtCO2eq/yr globally in the form of soil organic carbon (SOC) (Bossio et al., 2020; Zomer et al., 2017). Interventions include changes from conventional tillage to no tillage, applying soil amendments like biochar, adding cover crops, grazing improvements (e.g., reducing intensity and using legumes), and restoration of degraded grasslands. Soil carbon dioxide capture is appealing because there are agricultural practices that can, in principle, both increase soil and crop health, and thus productivity, and increase soil organic carbon. Additionally, SOC enhancement in existing agricultural landscapes, in contrast to several other terrestrial CDR strategies like reforestation or grassland restoration, does not lead to land-use competition.

Non-intervention-specific mapping of potential opportunities for carbon sequestration in agricultural landscapes requires spatial data on the extent of croplands and pasturelands (Zomer et al., 2017) as well as the degree to which the soil organic carbon has been depleted due to historic and current agricultural land use (Sanderman et al., 2017). Combining results from a statistical model that predicts current SOC stocks using climate, topography, geology, and land information with results from a spatially-explicit database of historical land use, Sanderman et al.(2017) constructed a global map of SOC change since the dawn of agriculture. Their map shows that SOC losses are attributed to both cropping and grazing activity. The midwestern United States, large swaths of Europe, and eastern and central China have lost significant absolute amounts of SOC, primarily due to cropping (Sanderman et al., 2017). Rangelands in Argentina, southern Africa, and Australia have experienced the greatest losses in terms of percentage of historic SOC (Sanderman et al., 2017). These hotspots of loss offer opportunities for implementing SOC restoration strategies, keeping in mind, however, that only half to two-thirds of depleted SOC can be recovered (Lal, 2004) through human interventions with currently developed strategies. 

Another approach to mapping SOC sequestration potential is to model the potential increase in SOC due to interventions as a sigmoidal (“S”-shaped) function, with parameters derived from fitting models to observational data (Zomer et al., 2017). This approach assumes that if soil is properly managed to increase SOC (with the method depending on the location and soil type), SOC will increase rapidly initially, but then decrease as SOC saturates and sequestration reaches an equilibrium. Zomer et al. (2017) apply these sigmoidal SOC recovery functions with parameters representing medium and high sequestration rates (Sommer and Bossio, 2014) to a gridded global map of current SOC content within agricultural land (Figure 3.2). The resulting maps show several hotspots for SOC sequestration potential − in particular, the midwestern U.S., south-central Canada, eastern Mexico, most of Europe, Ethiopia, Sudan, most of India, and several countries in Southeast Asia. 

Yet, no such studies mapping the potential or suitability of specific cropland CDR strategies exist (e.g., cover cropping, no-till, soil amendments). Spatially, modeling the SOC sequestration potential of these strategies would require a detailed soil map, cropping system map (e.g., US Department of Agriculture’s Cropland Data Layer, or CDL), and a tool that predicts SOC potential per parcel of land given average farm performance parameters (e.g., USDA’s COMET tool) based on strategy-specific literature on sequestration efficacy (e.g., crop species performance in certain agro-ecological regions) (Poeplau and Don, 2015).

Figure

3.2

Figure 3.2. Medium scenario of the annual increase in soil organic carbon in the first 30 cm of soil (data from Zomer et al., 2017). Units are tC/ha/yr, range is 0 to 1.2.

cdr primer map 3 2

Figure 3.2. Medium scenario of the annual increase in soil organic carbon in the first 30 cm of soil (data from Zomer et al., 2017). Units are tC/ha/yr, range is 0 to 1.2.

There are no global mapping studies on grassland restoration opportunities. However, regional and country-level analyses provide several possible approaches for CDR mapping (Fargione et al., 2018; Winowiecki et al., 2018). Marginal farmland, abandoned farmland, and degraded land (due to overgrazing) are candidates for grassland restoration. For example, Fargione et al. (2018) mapped areas in the U.S. where cropland was abandoned to grassland between 2008 and 2012, estimated using remote sensing products such as the USDA CDL and the National Land Cover Database (Lark et al., 2015), and used those areas to estimate SOC restoration potential from a gridded soil map (Hengl et al., 2017). Other grassland SOC sequestration opportunities include strategic conversion of low-productivity (marginal) croplands into high-diversity, productive perennial grasses for bioenergy (Awasthi et al., 2017; Gelfand et al., 2013; Tilman et al., 2006). Combining the above approaches with maps of SOC debt produced by Sanderson et al. (2017) can help identify the most promising locations for grassland restoration or improved grazing for carbon sequestration.

Through managed grazing intensity, soil amendment (e.g., fertilizer or liming) is another strategy for storing or restoring SOC in grasslands (Eze et al., 2018; McSherry and Ritchie, 2013). While no maps of modeled SOC sequestration potential for pasture improvement strategies exist, a global meta-analysis of the response of SOC stock to various grassland management strategies shows that there are geographically-specific factors like climate that can help predict the response of grassland SOC to management interventions (Eze et al., 2018). Generally, more-intensive grazing leads to greater declines in SOC, but the negative effects of grazing on SOC are generally more significant in tropical and subtropical climates than in temperate climates. While heavy liming does partially offset grazing’s depletion of SOC, this strategy can succeed only with acidic soils (land leached by heavy and persistent rainfall) and is partially dependent on the location’s geology. However, the majority of the world’s grasslands are in climatic zones with lower mean average precipitation. Nitrogen fertilizer does increase SOC, particularly in climates with high mean average temperature and high mean average precipitation (Eze et al., 2018), but several studies have found that there are diminishing returns in carbon sequestration per unit of added nitrogen. Moreover, high-nitrogen fertilizer inputs could lead to nitrous oxide emissions, which have significantly higher greenhouse gas potency than carbon dioxide (See Supplement 4.1 in Chapter 4 and Chapter 1).

A few existing studies have estimated the economic potential or economic cost of the mapped suitable areas for agricultural and grassland carbon dioxide removal strategies, and they offer a more realistic estimate of realizable potential (Smith et al., 2008; Fargione et al., 2018; Griscom et al., 2017). However, social and economic conditions are likely to be the most significant determinants of realizable potential. 

 

3.2.3

Wetlands, peatlands, and seagrass

Recent studies show that global peatland distribution by ecosystem and area is 83.3 percent boreal, 4 percent temperate, and 12.7 percent tropical (Leifeld and Menichetti, 2018). Using maps of global peatland distribution and maps of land use and climate, Leifeld and Menichetti (2018) estimated the potential GHG emissions if peatlands were to be fully degraded. For estimating current degree of degradation of peatlands, the study overlaid cropland areas with peatland areas and assumed that all peatlands used for cultivation are fully degraded or drained, since peatlands need to be drained before they can be cultivated (Leifeld and Menichetti, 2018). These estimates were validated using the area of degraded peatlands reported by each country (Joosten, 2010). Leifeld and Menichetti (2018) estimate the global carbon sequestration potential from peatland restoration ranges from 0.08 to 0.92 GtCO2eq/yr (Leifeld and Menichetti, 2018), while Griscom et al. (2017) estimate a maximum global potential of 0.8 GtCO2eq/yr with uncertainty in the upper range of about 2.4 GtCO2eq/yr, based on expert elicitation. Opportunities for peatland restoration are globally distributed with hotspots in Southeast Asia (Indonesia), parts of Southern Africa, Northern Europe (United Kingdom, Sweden, Finland, Germany), South America (Brazilian Amazon, Venezuela, Bolivia), and the state of Alaska. However, several studies highlight the high uncertainty of peatland extent estimates globally and the need for improved mapping to guide restoration and mitigation efforts (Griscom et al., 2017; Leifeld and Menichetti, 2018; Page et al., 2011). 

Seagrass meadows are responsible for storing about 18 percent of the carbon taken up by ocean sediments globally, or about 176 − 410 MtCO2 per year (Kennedy et al., 2010). These marine habitats have been lost at an annual rate of about 110 km2 since 1980, and more than 29 percent of known historical seagrass beds have disappeared (Waycott et al., 2009). The median estimate of the carbon emitted by this habitat loss is about 150 MtCO2eq per year (with a range of 50 − 330 MtCO2eq) (Pendleton et al., 2012). Restoration of lost or degraded seagrass meadows is thus a significant carbon sequestration opportunity. While there is an understanding of broad distributional trends of historic and current seagrass extent, less than a quarter of seagrass beds have been mapped globally, and locations are typically point observations without meadow extent estimates (Short et al., 2016). These mapped locations along with inferred and modeled habitat studies suggest that seagrass is very widely distributed and found along coasts of every continent except Antarctica (Short et al., 2016). Seagrasses are particularly abundant off the coasts of Australia, North America (including the Caribbean Islands), and Southeast Asia. Restoration potential mapping, however, relies on a spatial understanding of where seagrass meadows have been lost. Waycott et al. (2009) performed a meta-analysis of seagrass studies to qualitatively identify potential hotspots for seagrass meadow loss. Known areas of loss include the southwestern coast of Australia, eastern Canada, the Gulf of Mexico, the Mediterranean Sea, and the Baltic Sea (Waycott et al., 2009). Seagrass extent in other regions of the world is not well mapped.

 

3.2.4

Approaches for prioritizing biological carbon dioxide removal strategies across the landscape

Given the vast area requirements for terrestrial CDR, many of the terrestrial strategies described in this chapter could exacerbate the growing degree of land use competition among food production, climate change mitigation strategies like solar and wind development, urbanization, and conservation. Given finite land resources, policymakers, land managers, and communities must weigh the benefits and challenges of land-intensive CDR strategies. For example, whether to reforest or manage existing forested lands for biomass energy (with or without carbon capture and storage) or for carbon sequestration remains controversial (Favero et al., 2020). Other studies have explored expanding or repurposing existing cropland for perennial herbaceous bioenergy crops like switchgrass and miscanthus. These decisions consider more than the technical suitability of a CDR strategy (e.g., technical assessments like Griscom et al., 2017) by accounting for the economics and policies to incentivize particular CDR strategies. Socioeconomic studies on the spatial planning or land use allocation of CDR strategies often use economic models driven by biophysical data that typically fall within the partial and general equilibrium modeling category of climate and land use modeling (Favero et al., 2020; Herrero et al., 2013; Michetti, 2012). 

These economic models are typically embedded within the structure of integrated assessment models (IAMs) for simulating land use and land use change due to changes in the agriculture and forestry sectors through the supply (constrained by biophysical potential) and demand for key commodities in each sector in response to commodity prices. These models attempt to simulate the land use and climate effects of certain policies, such as a bioenergy demand policy at the national level (Favero et al., 2020; Herrero et al., 2013; Schmitz et al., 2014). For example, Favero et al. (2020) used the global timber model (GTM) to characterize bioenergy policy impacts on carbon emissions, carbon sequestration, and natural forest ecosystem services. The authors found that some inefficient bioenergy policy designs, such as taxing carbon dioxide emissions from biofuels, can lead to the loss of natural forests and forest carbon stocks, whereas efficient policies can protect natural forests while increasing total forest carbon. 

3.3

3.3 —

Technological CDR systems

 

3.3.1

Defining the best locations for technological CDR systems

Maximizing the CDR potential of engineered technologies requires co-locating the steps of the process as much as possible and using low-carbon energy. DAC plants can, in theory, be located anywhere, provided they have a source of energy (heat and/or electricity) and sufficient land. The DAC plants should thus be installed next to the energy production site if the source of heat is fixed. On account of the lower heat quality requirements associated with solid sorbent-based DAC approaches today, these plants can in some cases be sited at some industrial sites with sufficiently high-temperature waste heat that provides the necessary thermal energy - although these opportunities are limited. At other sites, waste heat at lower temperature may be available, which could be upgraded to high-enough temperatures using industrial heat pumps powered by electricity. This would substantially increase the options for DAC locations, at the cost of increased capital expense for these heat pumps. Ideally, the CO2 sequestration site should also be co-located with the CO2 capture site to minimize transportation costs and potential leakage through pipelines, thus maximizing the CDR potential. However, sequestering CO2 near densely-populated areas might be risky and encounter strong public resistance (Wong-Parodi and Ray, 2009). Alternatively, captured CO2 can be transported from its source or point of capture to CO2 sequestration sites. For example, transportation is proposed for a maximum of 100 km for sequestration in sedimentary reservoirs. Offshore sequestration is also a possibility. 

Alternatively, in locations where large volumes of CO2 could be captured but appropriate sequestration sites are distant, efforts can be combined to create transportation networks (Fry et al., 2017; Morbee et al., 2011). Increasing the pipeline length would require the addition of booster pumps but would not significantly increase the cost of transportation. For instance, increasing the length of a pipeline from 100 km to 200 km would add $2/tCO2 to pipeline costs (Grant and Morgan., 2018). The pipeline transportation network might need to be more developed in some areas like Europe, where carbon hubs could be sited in various locations throughout the region and the COcould be transported for sequestration in sedimentary formations below the North Sea.

This section also aims to highlight locations that minimally compete with human activities and avoid natural habitat loss. Maps show potential locations for low-carbon sources of energy such as photovoltaic (PV), concentrated solar (CSP) (see Box 3.1), and wind energy, which require large land areas on “converted lands” (Baruch-Mordo et al., 2019). Converted lands are lands already disturbed by human activities (e.g., human settlements, agricultural land, roads, and dams).

Supplement

3.1

Concentrated solar power (CSP)

Concentrated solar power (CSP) technology harnesses the sun’s heat to generate steam, which is fed into a turbine, producing clean electricity. CSP is ready to be deployed internationally, and several large-scale CSP plants currently exist in California and, most recently, Africa. There are several CSP plant designs, including the power tower system, the parabolic trough system, the linear fresnel system, and the parabolic dish system (Zhang et al., 2013). The tower, parabolic trough, and linear fresnel systems possess different solar mirror shapes, but all three focus the sun's heat toward tubes where a working fluid (e.g., molten salts, oil, or steam) is circulated. The heat is transmitted via a heat exchanger to another working fluid (e.g., steam) for electricity generation (Zhang et al., 2013). In a parabolic dish system, the heat is commonly sent directly to a heat engine at the focal point of the mirror (SolarPACES, 2018).

In order to minimize energy loss inside the pipes, a new design that requires a single storage tank for the molten salt, as opposed to two tanks for the tower system, was recently successfully tested with a 25 kW demonstrative prototype in the UAE (Tetreault-Friend et al., 2020). Concentrated Solar Power on Demand (CSPonD) consists of a concentrated solar power receiver that also acts as a thermal energy storage tank. The hot salts are in the upper part of the tank and separated from the cold salts by an insulated divider plate. The divider plate moves down during the day and allows the cold salts to move into the upper part to be heated. To produce electricity, the hot molten salt is circulated through a heat exchanger that creates steam, and then is stored back in the cold section of the tank. During the night, the divider plate moves up to ensure a constant flow of hot molten salt and steady production of electricity (Gil et al., 2017).

For PV solar energy, the collection of energy by CSP is intermittent. However, the thermal energy produced during the day by current CSP technologies using molten salts can be stored for several hours, allowing for a shorter night gap compared to PV technologies, or even run continuously (Ortega et al., 2008). CSP heat can be adapted to play other roles, such as water desalination (Department of Energy, 2019). The costs of CSP with storage are greater than the costs of PV with storage, so these systems have not gained traction for renewable electricity production. However, since 80 percent of the total energy requirements of today’s leading DAC technologies are thermal (Beuttler et al., 2019), this technology could be a helpful source of energy for DAC.

Our analysis also aims to avoid inhibiting the transition from high-carbon emissions to low-carbon electricity generation technologies. For instance, if a proposed PV plant can replace a coal-fired power plant, this use of solar electricity should be the priority, as opposed to powering a DAC plant with solar electricity next to a coal-fired power plant. This is the most efficient use of low-carbon electricity generation for reducing carbon dioxide emissions. Coal-fired plants are shown in this study to raise awareness for local priorities in the energy transition to low-carbon sources of energy and CDR.

 

3.3.2

Low-carbon sources of energy

DAC systems based on solid sorbents require an energy distribution of roughly 80 percent thermal and 20 percent electric (National Academies of Sciences, Engineering, and Medicine, 2019). Geothermal, CSP, nuclear power (Figures 3.3 and 3.4), and biomass all can provide both forms of energy. Other systems, such as electro-swing adsorption systems (Voskian and Hatton, 2019), need only electricity to run, which can be provided by solar PV and wind (Figure 3.3). To increase public acceptance, these technologies should not compete for space with urban areas, high-value regions for food production, biofuel feedstocks for difficult-to-decarbonize end uses, or natural ecosystems. PV, CSP, and onshore wind on converted lands that would have a low impact on the existing biosphere are shown in Figure 3.3 (Baruch-Mordo et al., 2019). Locations were selected based on renewable resource quality and technical suitability (i.e., slope, aspect, soil type, land cover type). Offshore wind potential maps exclude marine protected areas (Badger et al., 2015; Protected Planet, 2020). 

The energy generated by some renewable technologies such as solar and wind is temporally variable. Best-in-class solar power plants have a capacity factor as high as 35.2 percent, whereas best-in-class wind power plants have a capacity factor as high as 52 percent (Vimmerstedt et al., 2019). These types of variable renewable energy systems require alterations to ensure adequate compatibility within a technological CDR system. One option is to provide enough capacity at the plant for excess energy to be generated during operating hours and stored to operate the capture system continuously. This would be necessary for capture systems where repeated startup and shutdown can cause problems (i.e., solvent-based DAC, where the temperature of the calciner must reach 900ºC and the energy requirements are 80 percent thermal energy [National Academies of Sciences, Engineering, and Medicine, 2019]). This is also important for increasing cost-efficiency, as it ensures maximum utilization of capital-intensive capture systems. A second option is to overbuild the DAC component so it can capture the same amount of CO2 but operate only during energy generating hours. This option is viable only for systems where repeated startup and shutdown is possible (i.e., some sorbent-based DAC and electrochemical DAC technologies) and where the DAC component features lower capital costs.

Figure

3.3

Figure 3.3. Heat and electricity production opportunities: concentrated solar power (CSP), photovoltaic (PV), and on offshore wind (Badger et al., 2015; Baruch-Mordo et al., 2019; Protected Planet, 2020). Note that all regions suitable for CSP are also appropriate for solar PV.

cdr primer map 3 3

Figure 3.3. Heat and electricity production opportunities: concentrated solar power (CSP), photovoltaic (PV), and on offshore wind (Badger et al., 2015; Baruch-Mordo et al., 2019; Protected Planet, 2020). Note that all regions suitable for CSP are also appropriate for solar PV.

Opportunities for onshore wind, PV, and CSP are shown in 109 countries on converted lands, (already-transformed lands where the installation of these technologies at a large scale would have a low impact on natural habitats and human settlements) (Baruch-Mordo et al., 2019). Offshore wind locations were retrieved from the Global Wind Atlas repository (Badger et al., 2015), and protected marine areas were removed from the dataset (Protected Planet, 2020). Dark brown, dark red, and purple regions show the best opportunities for CSP, PV, and wind, respectively. Note that all CSP locations are also suitable for PV.

Figure 3.4 represents geothermal opportunities, with the aggregate geothermal potential at the country level shown for key countries (Geothermal Energy Association, 2015), along with the geographically-specific geothermal heat flow on land (Davies, 2013). Geothermal energy requires less land than other renewable energy technologies and can operate at high capacity factors, which means that it can operate continuously without the intermittency of wind and solar energy. Typical capacity factors for geothermal power plants range between 70 and 80 percent. The surface heat flow of the Earth is much higher along mid-oceanic ridges but is far less accessible when areas are 2,500 – 3,000 meters below sea level, with the exception of Iceland. Hence, only onshore surface heat flow is represented.

Figure

3.4

Figure 3.4. Energy opportunities for powering DAC plants and electrochemical plants: geothermal, nuclear, and hydropower. The green, yellow, black, and blue dots are geothermal, operating nuclear, under-construction nuclear power plants, and dams under construction, respectively, with the size proportional to the plant capacity. Note that the scales are different for geothermal power plants. The dark red dots indicate the geothermal potential per country, for countries where data are available. The red scale represents the surface heat flow. Geothermal energy is likely to be more available when the surface heat flow is higher (Davies, 2013; Geothermal Energy Association, 2015; Global Energy Observatory, 2018; IAEA Power Reactor Information System, 2019; Vimmerstedt et al., 2019; Zarfl et al., 2015).

cdr primer map 3 4

Figure 3.4. Energy opportunities for powering DAC plants and electrochemical plants: geothermal, nuclear, and hydropower. The green, yellow, black, and blue dots are geothermal, operating nuclear, under-construction nuclear power plants, and dams under construction, respectively, with the size proportional to the plant capacity. Note that the scales are different for geothermal power plants. The dark red dots indicate the geothermal potential per country, for countries where data are available. The red scale represents the surface heat flow. Geothermal energy is likely to be more available when the surface heat flow is higher (Davies, 2013; Geothermal Energy Association, 2015; Global Energy Observatory, 2018; IAEA Power Reactor Information System, 2019; Vimmerstedt et al., 2019; Zarfl et al., 2015).

Using geothermal as a low-carbon energy source depends greatly on location and power plant technology, as some geothermal power plants can release amounts of CO2 comparable to natural gas or even coal-fired power plants (Aksoy, 2014). The world average emissions factor for geothermal is estimated to be around 122 gCO2/kWh, with the highest values in the Menderes and Gediz grabens (depressed areas of the Earth’s crust bordered by parallel faults) in Turkey reaching up to 1,300 gCO2/kWh (Aksoy, 2014; Fridriksson et al., 2016). Geothermal power plants also release other more potent GHGs: hydrogen sulfide (about 3.0 wt%), methane (about 0.15 wt%), and ammonia (about 0.29 wt%) (Fridriksson et al., 2016). The release of GHGs by geothermal power plants depends on the type of plant. In a binary plant configuration, the fluid is never in contact with the atmosphere. It is pumped from the subsurface, the heat is recovered with a heat exchanger, and the fluid is injected back into the geothermal reservoir. Binary plants represent 15 percent of worldwide geothermal power plants (Geothermal Energy Association, 2015) and do not emit GHGs from geothermal fluids. However, most geothermal power plants are condensate plants (dry steam or flash plants), representing 84 percent of the worldwide installed capacity (Geothermal Energy Association, 2015). These plants are open systems that release GHGs from the fluid or steam, which originate from the host rock of the geothermal reservoir or the mantle bodies that warm up the reservoir. GHG emissions vary depending upon the location and management of the reservoir, and in some cases, reinjection of fluids depleted in GHGs can decrease GHG emissions, with time, at condensate plants (Fridriksson et al., 2016).

Different configurations exist for geothermal plants with various requirements for the temperature of the geothermal brine. Flash and dry steam power plants use geothermal fluid with temperatures higher than 180oC, and binary power plants use temperatures in excess of 100oC (U.S. Department of Energy [DOE], 2019). Flash plants use cooling towers before sending geothermal fluid back into the ground. Existing plants are unlikely to be retrofitted, but new geothermal power plants could include solid sorbent-based DAC modules in place of the cooling towers (DiPippo, 2005, 2004; Snyder et al., 2017), taking advantage of the remaining heat for CO2 capture. The geothermal fluid is sent back underground to ensure the sustainability of the geothermal reservoir. Associating DAC with existing geothermal installations is the cheapest energy option, as a large share of the capital cost of geothermal energy is drilling the wells. Using DAC modules as a cooling step before using geothermal energy is another possibility for pairing DAC with existing geothermal installations, as electricity generation requires lower temperatures than that of the geothermal brine. Otherwise, geothermal wells could be drilled solely for the purpose of providing heat to a DAC plant, but the cost of capture would be significantly higher. The co-development of geothermal power plants and DAC for transitioning from fossil fuel-based energy production to renewable energy and capturing current and past CO2 emissions is an attractive proposition (Baker et al., 2019; McQueen et al., 2020).

Another low-carbon source of energy is nuclear power. Nuclear reactors represented in Figure 3.4 are operational reactors whose current licenses expire after 2025 (other reactors that are not on the present map could see their license renewed beyond 2025), and reactors under construction (Global Energy Observatory, 2018; IAEA Power Reactor Information System, 2019). Given the dangers and controversies surrounding nuclear energy production, the goal here is not necessarily to encourage nuclear energy development, but rather to take advantage of the energy produced by existing facilities while they are still in operation (McQueen et al., 2020). Hence, they would act as a transitional energy source.

Hydropower is also a renewable source of energy, and proposed or under-construction dams are represented in Figure 3.4 (Zarfl et al., 2015). However, hydropower development is highly subject to social and environmental siting controversies, resulting in high cost overruns and long lead times, due to their well-documented impacts on terrestrial and freshwater ecosystems as well as human livelihoods.

As shown on Figures 3.3 and 3.4, energy sources for CDR are heterogeneously distributed. Generally speaking, the quality of solar energy and, to a lesser extent, wind energy is latitude-dependent. The highest latitudes for solar energy generation are 45o for CSP and 60o for PV, and their resource quality is highest close to the equator and decreases toward highest-latitude limits. In order of energy opportunity, optimal locations are in Africa, the Middle East, Australia, and South America. 

Conversely, the quality of wind energy is highest offshore and at higher latitudes due to higher wind speeds. In order of energy opportunity, promising locations include southern South America, New Zealand, the South Coast of Australia, and Northern Europe. More limited opportunities are found onshore at lower latitudes, the most promising being the Great Plains in the U.S.

High-quality geothermal resources are related to plate tectonics. Oceanic ridges and subduction zones disturb the surface heat flow and can trigger higher surface heat flow and higher temperatures in the subsurface over hundreds of kilometers from the tectonic plate edges depending on the local geologic context. Most of the world's opportunities lie within the vicinity of the Ring of Fire: the western U.S., Kamchatka (Russia), Japan, Taiwan, the Philippines, Papua New Guinea, and New Zealand. In order of energy opportunity, optimal locations are in Iceland, the Rift Valley in East Africa, Indonesia, Turkey, Western Europe, and Northern Africa.

Both nuclear and hydropower energy depend on large water supplies. Nuclear power is mostly generated in densely populated areas of developed countries (Europe, North America, and the east coast of Asia) on the coast, or on major rivers. Hydropower is typically developed in mountainous areas because it requires water reservoirs or large elevation gradients between the source of water and the hydroelectric turbine. Most dams under construction are in the Himalayas and South America, regions with high terrestrial and aquatic biodiversity.

Very few countries could develop all six low-carbon energy generation options in this chapter (solar CSP, solar PV, wind, geothermal, nuclear, and hydropower), but most countries have the potential to develop at least one of them. Locations with the least potential are northern Canada and northern Asia, but they are also places with low population density.

 

3.3.3

Locations for the sequestration of CO2

The biosphere, soils, and the hydrosphere currently store large amounts of CO2. Section 3.2 detailed the potential opportunities and risks of storage in the biosphere. The lithosphere has the largest storage potential with the lowest chance of CO2 re-release to the atmosphere in sedimentary basins or mafic and ultramafic rocks. The assignment of a technological readiness level (TRL) to a given technology is a method of assigning its maturity. This is described in greater detail in Figure 4.4 of Chapter 4. The sequestration of CO2 in deep sedimentary formations containing saline aquifers or depleted oil and gas reservoirs possesses the highest TRL and has the capacity to sequester past, present, and future emissions permanently (Szulczewski et al., 2012). Even with a low TRL, mafic and ultramafic rocks are promising avenues for CO2 sequestration, as carbon mineralization occurs much more quickly in these rocks.

This subsection focuses on potential resources for CO2 sequestration. The geological formations proposed are thus potentially usable for the sequestration of CO2, without making any statement on their technical, economic, legal, or socioeconomic feasibility. For any of these resources, local studies will be required to more precisely assess their capacity for CO2 sequestration. This information is not exhaustive; valuable data is scattered across the literature and other opportunities may exist in addition to those presented here.

 

3.3.4

CO2 sequestration in sedimentary formations

No global database of sedimentary formations suitable for CO2 sequestration exists today. There are local studies with variable degrees of certainty and granularity. In Europe, the CO2StoP study was completed in 2014 for the European Commission (Poulsen et al., 2014). In the United States, the U.S. Geological Survey (USGS) issued a national assessment of geologic carbon dioxide storage in 2013 (USGS, 2013), and the Gulf Coast Carbon Center (GCCC) features a database that highlights sequestration of greenhouse gases in brine (Hovorka et al., 2012).

At the global level, Bradshaw and Dance (2005) issued a map of a high-level estimate of the prospects of sedimentary basins for CO2 sequestration, later used by the IPCC (Metz et al., 2005). The authors classified the sedimentary basins into three types: 1) world-class petroleum basins: basins flagged as “high priority” or “frontier” basins by the USGS World Petroleum Assessment, 2) prospective basins: small petroleum basins and non-highly-deformed sedimentary basins (basins that have endured tectonic activity and have broken-up or irregular rock layers), and 3) non-prospective basins: highly deformed sedimentary basins and other geological provinces, including fold belts, metamorphic, and igneous rocks.

Global sedimentary basin maps are available and provide other characteristics such as type of basin, how well they are explored, and the maximum thickness of basin sediments (CGG Robertson, 2019). More granular data about the thickness of the sediments were gathered in one global dataset by Laske et al. (2013) (Exxon Production Research Company, 1985; Laske et al., 2013). To maintain a supercritical state, which reduces the risks of leakage, CO2 needs to be sequestered at pressures greater than 73.8 bars, corresponding to geostatic pressures occurring deeper than 800 meters. In order to ensure safe injection and trapping of CO2, the threshold of 1,000 meters is preferred. Combining these two datasets might help select basins with sediment thickness greater than 1,000 meters and help identify areas within these basins that are deeper than 1,000 meters. This combined information provides a rough guide to areas that can be explored for future CO2 sequestration projects.

Additional information helps narrow the choices for suitable sedimentary formations. It is important to ensure that large amounts of CO2 can be injected in formations and stay trapped in the subsurface pore space. This requires pairing permeable geological formations with a regional seal, which is a caprock of the formation, or barrier, that is impermeable to CO2. One rock that has the appropriate seal properties is shale, which has low permeability to both CO2 and hydrocarbons. The presence of a seal is critical in identifying an appropriate geologic sequestration site. Additionally, the presence of major oil and gas fields is a good indicator of a large reservoir and a reliable caprock since the same mechanisms that have reliably stored oil and gas for millions of years will be storing CO2 in a dedicated sequestration project. In addition, regulatory frameworks and accessibility might define the feasibility of a CO2 sequestration project. The Global CCS Institute (Consoli, 2016; Consoli and Wildgust, 2017) used these parameters to investigate prospective and potential basins in major countries across the world. The Institute also studied the capacity of prospective basins, indicators for storage readiness, and the legal and regulatory framework of each country. This study does not consider some parameters, including injectivity, well placement, regulations, and commercial drivers for CO2 sequestration. Therefore, the authors define a prospective sedimentary basin as one suitable to sequester human-caused CObased on current knowledge and data. In most cases, a prospective basin has published CO2 sequestration assessments. In those nations that have not completed a CO2 sequestration assessment, a basin could also be defined as prospective if it hosts major gas fields or is known to have suitable geology based on existing data.

Regional studies can provide more detailed information in specific areas, and multi-country assessments are of great interest as they aim to provide highly detailed, homogeneous data across large geographic areas. Here we present one multi-country study in Europe (CO2StoP) and discuss two datasets for the U.S. (Hovorka et al., 2012; Poulsen et al., 2014; U.S. Geological Survey (USGS), 2013). Other local studies exist, including the GEODISC program in Australia (Bradshaw and Rigg, 2001).

CO2StoP is a regional study that was conducted mainly in the European Union and some other European countries as a collaboration of volunteer countries (Poulsen et al., 2014). In this study, a reservoir formation is defined as a “mappable body of rock that is continuous in the subsurface and which is both porous and permeable” and a storage unit as “a part of a reservoir formation that is at depths greater than 800 meters and which is covered by an effective cap rock.” Each storage unit may contain one or more daughter units, which are defined as “structural of stratigraphic traps which have the potential to immobilize CO2 within them” (e.g., domes in deep saline aquifers sealed by caprocks or proven oil and gas fields). For each of the assessment units, the study provides general information (unit type [e.g., whether the reservoir is a saline aquifer or a hydrocarbon field], lithology, geographic area, country, geological basin) and specific information for pore-volume estimation (area, average thickness, average area of net sand, average ratio of net-versus-gross in the vertical direction, and average porosity). This project in particular has developed its own tool to calculate the capacity of the storage and daughter units and uses a geographical information system (GIS) for their localization. The geology of Europe is diverse, and there are sedimentary basins of various sizes in every country. Like the Global CCS Institute study (Consoli, 2016; Consoli and Wildgust, 2017) at the world level, this study shows that countries investigating their territory for sedimentary basins suitable for CO2 sequestration find several locations worth investigating further for potential CO2 injections in subsurface pore space.

The 2013 National Assessment of Geologic Carbon Dioxide Storage Resources provides geologic storage information down to the storage assessment unit (SAU) level, which is defined as a “mappable volume of rock that consists of a porous reservoir.” For each SAU, the list of parameters includes: 

  • Area, thickness, depth from surface of the SAU, and density of CO2  at depth;

  • Thickness, porosity, and permeability of the net porous interval, which is the part of the SAU that contains an appropriate lithology with sufficient porosity to store CO2; and

  • Trapping pore volume and efficiencies of trapping mechanisms.

Each parameter has a minimum, maximum, and likely value. These parameters can be used to identify the best SAUs in a given area and calculate other indicators, like the injectivity of CO2 in a given SAU.

In the U.S., the Gulf Coast Carbon Center (GCCC) brine database focuses on deep saline aquifers (Hovorka et al., 2012). It contains most of the following parameters for each basin, in some cases associated with a map showing their geographical variability:

  • Depth of the formation

  • Permeability or hydraulic conductivity 

  • Thickness (as well as the net sand thickness and the thickness of the seal) 

  • The percentage of impermeable shale in the aquifer

  • Rock/water reaction

  • Hydrocarbon production

  • Low direction

  • CO2 solubility in its brine (temperature, pressure, and water salinity at depth)

  • Porosity 

  • Water chemistry

  • Rock mineralogy

Both U.S. datasets, from the USGS and the GCCC, offer high-level information on the potential physical reaction of the basin to CO2 injections (capacity, pressure build-up) and, to some extent, the efficiency of trapping mechanisms likely to occur in the reservoir. The GCCC also provides information (rock/water reaction, water chemistry, and rock mineralogy) that can help predict the response of the reservoir rock to CO2 input. CO2 can indeed trigger dissolution and precipitation in the reservoir and alter the porosity and the permeability of the formation.

The GCCC also created the MATLAB module Enhanced Analytical Simulation Tool for CO2 storage capacity estimation and uncertainty quantification (EASiTool), which estimates the total CO2 that can be injected into a sedimentary formation over a given period of time through a given number of wells, accounting for the formation’s parameters. This information is vital to evaluate a CO2 sequestration project’s potential (Ganjdanesh and Hosseini, 2018, 2017; Hosseini et al., 2006).

 

3.3.5

Global map of prospective sedimentary basins for CO2 sequestration

All of the studies presented above have been used to create a global dataset of prospective basins for CO2 sequestration (Figure 3.5). Mainly based on the study from Consoli (2016), this map shows potential opportunities in countries that are large emitters of GHGs and offers a high-level understanding of how much information we have from each dataset and the suitability of each formation.

The GCCC and USGS datasets provide great detail. This map uses basin shapes from the USGS dataset, and basins that have injectivities lower than 0.25 MtCO2/yr (Baik et al., 2018) were discarded because they are not suitable for CO2 injection and sequestration. The CO2StoP study in Europe has a high level of knowledge and provides a GIS file that was used for this map. Studies from Consoli (2016) and Consoli & Wildgust (2017) were used to select basins in the Robertson Basins & Plays GIS shapefile, and other sources were used when necessary (Geoscience Australia, 2017; Pitman et al., 2012; Tartarello et al., 2017; Campbell et al., 2015). Consoli (2016) ranked the countries, using four categories: full, moderate, limited, and very limited knowledge (Consoli, 2016). This information is reflected in different colors on the map.  

Offshore basins offer attractive opportunities for CO2 sequestration (Offshore Storage Technologies Task Force, 2015; Ringrose and Meckel, 2019) due to their depth and remoteness. Offshore basins identified as highly prospective by Bradshaw and Dance (2005) were selected in the Robertson Basins & Plays GIS shapefile (Bradshaw and Dance, 2005; CGG Robertson, 2019) and displayed in this map. All datasets (detailed above) were combined with the map of sediment thickness by Laske et al. (2013) and resampled into a 3 km x 3 km raster. This gives a high-level estimate of the areas that might be too shallow for CO2 injections (< 1,000 m). 

This map (Figure 3.5) shows that in most countries that are large GHG emitters, numerous opportunities exist for future onshore or offshore CO2 sequestration. Operating, future, and completed projects for CO2 sequestration are shown on the map, along with current CO2-EOR locations in the U.S (Global CCS Institute, 2019). Most current and future activity is in Australia, East Asia, Europe, and North America.

Figure

3.5

Figure 3.5. Prospective sedimentary basins for CO2 sequestration in countries that are large emitters of GHGs (not crosshatched). Potential sequestration basins (colors) correspond to basins with assessment studies. Colors correspond to various levels of knowledge, and shades of the same color correspond to various sediment thicknesses. Prospective areas (grey) correspond to basins without assessments but with a sedimentary thickness greater than 1,000 meters (Campbell et al, 2015; CGG Robertson, 2019; Consoli, 2016; Consoli and Wildgust, 2017; Geoscience Australia, 2017; Global CCS Institute, 2019; Hovorka et al., 2012; Laske et al., 2013; Pitman et al, 2012; Poulsen et al., 2014; Ringrose and Meckel, 2019; Tartarello et al, 2017; U.S. Geological Survey (USGS), 2013).

cdr primer map 3 5

Figure 3.5. Prospective sedimentary basins for CO2 sequestration in countries that are large emitters of GHGs (not crosshatched). Potential sequestration basins (colors) correspond to basins with assessment studies. Colors correspond to various levels of knowledge, and shades of the same color correspond to various sediment thicknesses. Prospective areas (grey) correspond to basins without assessments but with a sedimentary thickness greater than 1,000 meters (Campbell et al, 2015; CGG Robertson, 2019; Consoli, 2016; Consoli and Wildgust, 2017; Geoscience Australia, 2017; Global CCS Institute, 2019; Hovorka et al., 2012; Laske et al., 2013; Pitman et al, 2012; Poulsen et al., 2014; Ringrose and Meckel, 2019; Tartarello et al, 2017; U.S. Geological Survey (USGS), 2013).

 

3.3.6

Capacity of sedimentary basins for CO2 sequestration

Attempts have been made to estimate global and regional capacity potential for CO2 sequestration. Lower estimates show that the global resource available is roughly 7,000 GtCO2, over three times as much capacity as the total GHG emitted since the beginning of the Industrial Revolution (~ 2,035 +/- 205 GtCO2 [Le Quéré et al., 2015]). The amount of CO2 that has to be sequestered by 2050 to reach the 2C climate goal is estimated to be 10 GtCO2/yr (National Academies of Sciences, Engineering, and Medicine [NASEM], 2019; United Nations Environment Programme [UNEP], 2017), requiring between 10,000 and 14,000 injection wells (Ringrose and Meckel, 2019). Beyond the CO2 sequestration capacity of each country or region, Pozo et al. investigated the question of equity between countries for CO2 sequestration (Pozo et al., 2020). Here we present two approaches for capacity estimates: a top-down approach using global datasets to provide a uniform dataset and estimate local resources (Kearns et al., 2017) and a bottom-up approach aggregating numerous local studies to identify local resources and ultimately provide a global dataset (Consoli and Wildgust, 2017). The areas studied to create the capacity estimates are presented in Table 3.1.

Kearns et al. used a holistic approach to build a homogeneous worldwide dataset (Kearns et al., 2017). Their methodology consists of a modified version of a method developed by the International Energy Agency Greenhouse Gas R&D Programme (IEAGHG). The IEAGHG assumes that about half of global sedimentary basins’ areas are covered by an adequate seal. The low bound assumes a closed system, where pressure is unable to dissipate, whereas the high bound assumes an open system, where the pressure build-up is negligible. This leads to the sequestration of 0.037 GtCO2 per 1,000 km3 of sedimentary basin for the lower bound and 0.26 GtCO2 per 1,000 km3 of rock for the higher bound. Contrary to the IEAGHG method, which assumes a fixed thickness of sedimentary layers, Kearns et al. used the sediment thickness from Laske et al. (2013) to calculate the CO2 sequestration potential (Laske et al., 2013). The authors distinguished between onshore, offshore technical, and offshore practical storage as some offshore basins might be difficult to access. Offshore practical resources are located within 200 miles of land masses larger than 10,000 km2 and exclude waters deeper than 300 meters and latitudes higher than the polar circles. These criteria significantly reduce the offshore capacity of coasts with narrow and steep continental shelves (e.g., Africa and Japan), and the capacity of northern countries (e.g., Russia and Canada). Total capacity of onshore and practical offshore sedimentary formations ranges from 8,000 to 55,000 GtCO2. While this approach primarily estimates global and regional capacity ranges, it also identifies specific sedimentary basins for carbon dioxide storage.

Consoli & Wildgust took a different approach to identify prospective basins. They studied assessments from countries that are major GHG emitters (Consoli and Wildgust, 2017). As stated above, to be considered prospective, sedimentary basins should contain permeable saline formations that are paired with a regional seal or have existing oil and gas fields. They also need to be reachable by trucks and equipment (or in the case of marine basins, ships). And finally, they need to be available for use in accordance with the laws and regulations of the country in which they are located. Regional assessments tend to differ in methodology, quality, and physical and chemical parameters provided, which makes direct comparisons difficult. In that context, multi-national surveys are key to harmonizing the methodology and data available. The authors also indicated that their methodology is not appropriate to estimate total resources for CO2 sequestration, as a more detailed, site-scale study of prospective basins is needed to estimate actual resources before starting any CO2 sequestration project. 

Despite all these challenges and the variability of geology from country to country, substantial CO2 sequestration resources are available in most regions of the world. Most of the countries that published regional assessments have identified sufficient resources to support multiple sequestration projects. This approach helps identify prospective basins worth further investigation.

Table

3.1

Capacity for CO2 sequestration across the world. Geographic areas correspond to Kearns et al. emissions prediction and policy analysis (EPPA) regions. Within these regions, capacities from Consoli & Wildgust are restricted to countries without hashes on Figure 3.5 (Consoli and Wildgust, 2017; Kearns et al., 2017).

cdr primer table 3 1

Capacity for CO2 sequestration across the world. Geographic areas correspond to Kearns et al. emissions prediction and policy analysis (EPPA) regions. Within these regions, capacities from Consoli & Wildgust are restricted to countries without hashes on Figure 3.5 (Consoli and Wildgust, 2017; Kearns et al., 2017).

In summary, sedimentary formations potentially suitable for CO2 sequestration are distributed around the world. Detailed local field studies and careful subsurface modeling are necessary to select appropriate injection sites. Together with technical or economical feasibility, factors such as population density, public acceptance, and local regulations are essential to ensure the success of injection projects. Also, estimated capacities (Table 3.1) suggest that the CO2 sequestration resource should not be a limiting factor. In order of the most collective pollution since the start of the Industrial Revolution, the top emitters of CO2 are North America, Europe, China, and Australia. Figure 3.5 shows these regions all have potential CO2 sequestration opportunities that might require long-distance COtransportation, depending on the co-location of CO2 sources and potential sinks. This could be partly overcome by using other rock types for CO2 sinks.

 

3.3.7

CO2 sequestration in mafic and ultramafic rocks

Mafic (basalts) and ultramafic (peridotites and serpentinites) rocks also have the potential to sequester CO2, as discussed in Section 2.1. The technology has a lower technology readiness level (TRL) than CO2 injection in sedimentary formations, but these rocks are much more reactive with CO2 and would present interesting alternatives to injection in locations where sedimentary basins are nonexistent, too shallow, or unable to permanently store CO2 at depth. These rocks are found around the globe but are not homogeneously distributed. Basalts are associated with current or past volcanism, while ultramafic rocks are located in present or past mountain ranges. In addition, basalts, peridotites, and serpentinites are part of the oceanic floor, where they are widely abundant, but they are not considered accessible for CDR due to their significant depths. (The ocean floor is an average of 5,000 meters below sea level.) The map (Figure 3.6) presents ultramafic rocks on land, basalts on land, and basalts offshore, excluding marine protected areas and exclusive economic zones (200 nautical miles, or about 370 kilometers, from shore) (Flanders Marine Institute, 2019; Hartmann and Moosdorf, 2012; Johansson et al., 2018; Kelemen, 1998; Protected Planet, 2020; Whittaker et al., 2015).

Figure

3.6

Figure 3.6. Location of mafic (basalts) and ultramafic (peridotites and serpentinites) rocks for carbon mineralization with the CO2 sequestration potential in gigatonnes (Coleman and Irwin, 1974; Flanders Marine Institute, 2019; Hartmann and Moosdorf, 2012; Johansson et al., 2018; McGrail et al, 2017; Protected Planet, 2020; Sigfússon et al, 2018; Whittaker et al., 2015).

cdr primer map 3 6

Figure 3.6. Location of mafic (basalts) and ultramafic (peridotites and serpentinites) rocks for carbon mineralization with the CO2 sequestration potential in gigatonnes (Coleman and Irwin, 1974; Flanders Marine Institute, 2019; Hartmann and Moosdorf, 2012; Johansson et al., 2018; McGrail et al, 2017; Protected Planet, 2020; Sigfússon et al, 2018; Whittaker et al., 2015).

 

3.3.8

CO2 sequestration in alkaline wastes

A variety of alkaline waste materials from the mining and construction and demolition industries possess significant potential to sequester large volumes of CO(Renforth, 2019). Recent research has identified at least 12 types of waste material that can capture CO2 through mineralization or enhanced weathering (Renforth, 2019). These alkaline materials can be broken down into their constituent components based upon their chemical composition, as indicated in Table 3.2. The functional unit of each alkalinity source is indicated in the first column, followed by the current estimated lifecycle emissions in kgCO2. The method for calculating mineral carbonation and enhanced weathering potential is outlined in Renforth (2019).

Table

3.2

Waste alkalinity materials and their lifecycle emissions, carbon mineralization, and enhanced weathering potential (adapted from Renforth, 2019)

cdr primer table 3 2

Waste alkalinity materials and their lifecycle emissions, carbon mineralization, and enhanced weathering potential (adapted from Renforth, 2019)

It is possible to capture CO2 from ash residue as a byproduct of gasification within the BECCS negative-emission system, with a potential of 186.2 ± 126.1 kgCO2/tash (Vassilev et al., 2013). This enhances the potential impact of BECCS beyond standalone estimates of emissions reduction. The biomass composition from the four samples in Table 3.2 was based on a combination of agricultural residue; herbaceous, woody, and animal-based biomass; and algae (Renforth, 2019). The average ash composition of the biomass is approximately 6.9 ± 1.1 wt% (Vassilev et al., 2013; Mckendry et al., 2002; de jong et al., 2014; Demirbas, 2014; Zanzi, 2001). In terms of energy content, the ash has an average higher heating value (includes the latent heat of vaporization of water) of 19.1 ± 0.3 GJ/tash

Cement also possesses the latent ability to sequester CO2, with a high carbonation and enhanced weathering potential of 510 and 773 kgCO2/tcement, respectively (Vanoss and Padovani, 2003). There is potentially even higher activity during building demolition, particularly when cement has been mixed with concrete, as the particle size has been reduced and the material is exposed to the elements (Washbourne et al., 2015). This enhanced sequestration rate is equal to approximately 85 tCO2/ha/yr. Some cement formulations may include a mixture of clinker and gypsum at a ratio of 9:1. Clinker in turn is created by heating limestone in a kiln to ~1500 °C, which also produces cement kiln dust (CKD) as a byproduct. CKD has the ability to sequester CO2 up to 330 ± 11.6 kgCO2/tCKD. Within the U.S., a small number of cement plants produce CKD at a CKD-to-clinker ratio of 1:10, with an average CO2 sequestration potential of 115 ± 17 kgCO2/tclinker

More than 300 million tonnes of lime is produced globally every year (USGS, 2018), with an estimated carbon mineralization potential of 776.9 ± 12.9 kgCO2/tlime and enhanced weathering ratio of 1165 ± 19.4 kgCO2/tlime. Future annual carbon dioxide removal ratios for lime are predicted to be between 60 and 143 MtCO2/tlime by 2100 (Renforth, 2019). Lime is a versatile material and, between 1975 and 2003 within the U.S. and E.U., it was used in a variety of industries (Farell, 2009; EuLA, 2014). More specifically, of the 300 million tonnes of annual lime production, roughly 41 ± 1% was used in steel production, 27 ± 0.6%  was used in chemical production, and 22 ± 0.9% was used in environmental applications such as acid neutralization. As lime is highly reactive, it may recarbonate over time. Renforth (2019) suggested that 20 percent of lime use is intrinsically linked to reaction with CO2. For example, 8.5 percent of lime production reacts with the CO2 from biomass in the recovery of sodium hydroxide from the Kraft process in the paper manufacturing industry (Renforth, 2019). Lime is also used to dilute extremely low-pH acids, such as sulfuric acid in the drainage of mine wastes. 

Waste from metal mining offers great potential to sequester CO2, mainly due to the large quantities of ultrabasic rocks within the host material. Significant quantities of such waste are produced in the metal mining process (Renforth, 2019). Most carbon mineralization research has targeted the host rocks of rare materials and metals, such as ultramafic rocks hosting platinum group elements (PGEs), , before kimberlite pipes hosting diamonds (Mervine et al., 2018), and more common materials such as nickel. Both PGE and diamonds offer potential carbon dioxide removal of 100-200 MtCO2/yr. Nickel is split into two categories of tailings, Ni-laterite and Ni-sulphide. Laterite has a carbon sequestration potential of 251 ± 26.7 and 377.2 ± 40.1 kgCO2/tlaterite for mineral carbonation and enhanced weathering, respectively, while sulphide offers potential savings of 367.5 ± 7.6 and 555.3 ± 11.7 kgCO2/tsulphide. Carbonating mine tailings can have other benefits, such as providing physical stability to tailings (Vanderzee et al., 2018), dust reduction, immobilization of toxic metals (Hamilton et al., 2016), and neutralizing hazardous materials like asbestos (Assima et al., 2013). Asbestos and brucite have high carbonation rates, as carbonation has been detected in their drainage waters. However, it is unclear if the entire formation of rock can actually weather at a meaningful reaction rate (Harrison et al, 2013).

The aluminum industry produces on average approximately 3.45 ± 0.04 tonnes of red mud (also known as bauxite residues) for every tonne of aluminum produced (Bertram et al., 2017). According to Power et al. (2011), around 120 Mt of red mud is produced globally, with a current stock of 3 Gt. The substance has a high carbon dioxide sequestration capacity of 44  – 66 kgCO2/tred mud. The carbonation and enhanced weathering potential were based on chemical composition alone and equate to 46.8 ± 8.1 kgCO2/tred mud and 128.3 ± 18.1 kgCO2/tred mud, respectively.

Roughly 500 Mt of slag is manufactured every year (USGS, 2016). For every tonne of steel produced, an additional 185 ± 5 and 117 ± 6 kg/tsteel of blast furnace and steel slag are formed as a byproduct of the crude steel production process, respectively (USGS, 2018). Pig iron production in blast furnaces may be reduced due to movement toward a circular or regenerative economy over the next decades (Neelis and Patel, 2006), although the production of steel from scrap has declined slightly since the 1970s (from 45 to 35 percent). It is difficult to predict how current stocks (25 Gt [Krausmann et al., 2017]) will be used over the next century. It is estimated that the total carbonation potential of slag will equate to 320 – 870 MtCO2/yr, with an enhanced weathering potential of 480 – 1,300 MtCO2/yr (Renforth, 2019). 

Figure 3.7 illustrates the location of the most notable waste alkalinity feedstocks. In particular, the U.S. has high quantities of fly ash, cement kiln dust, and steel slag as a legacy of its high industrial output.

Figure

3.7

Selected locations of waste alkalinity sources for operational periods between 2003 and 2007. Only datasets with declared capacity are shown. Location data were taken from the USGS mineral resources data system. Capacity was not included due to uncertainty in the quality of the dataset and age range. The datasets will need to be updated to ensure accurate mineral carbonation and enhanced weathering potentials (Kirchofer et al., 2013; USGS., 2003; 2010).

cdr primer map 3 7

Selected locations of waste alkalinity sources for operational periods between 2003 and 2007. Only datasets with declared capacity are shown. Location data were taken from the USGS mineral resources data system. Capacity was not included due to uncertainty in the quality of the dataset and age range. The datasets will need to be updated to ensure accurate mineral carbonation and enhanced weathering potentials (Kirchofer et al., 2013; USGS., 2003; 2010).

3.4

3.4 —

Siting technological CDR systems

Combining low-carbon energy resources (described in Section 3.3.2) with COcapture (described in Section 2) and sequestration sites (described in Section 3.3.3) assists in identifying potentially feasible locations for technological CDR systems. These proposed systems are defined as “potential” (see Section 1.3 for definitions of the terminology), as the characteristics of each location are unique, and ultimately a life-cycle analysis (LCA) would be necessary to assess the CDR potential of a specific system in a given location (Section 4). Before launching any project, it is essential to assess the feasibility of building a low-carbon energy plant (or pairing the DAC plant with an existing source of low-carbon energy), and the potential for geologic sequestration in that location. Below, we propose some examples of technological CDR systems and the potentially relevant locations for each. We focus on geologic sequestration in sedimentary basins as this technology has a higher TRL. This exercise can be considered a first step toward more thorough local analysis.

Technological CDR approaches available today operate with heat and electricity. All of the low-carbon sources of thermal energy identified above (geothermal, CSP, nuclear, and biomass) currently convert most of their thermal energy into electricity. For energy plants dedicated to supplying a DAC plant, the share of thermal energy converted into electricity could be reduced to match the needs of this DAC plant, i.e., 80 percent thermal, 20 percent electric. (Actual proportions will depend on the conversion efficiency from heat to electricity). Since most of the heat provided is too low for solvent-based DAC but matches sorbent-based DAC requirements (~100oC), the section below focuses on this latter technology. If only electricity is available, the heat for sorbent-based DAC could be provided by industrial heat pumps. In the future, DAC plants that need only electric energy, such as the electro-swing adsorption systems developed by Voskian and Hatton (2019), might enter the market. Opportunities for electric-powered DAC are also shown below.

 

3.4.1

Geothermal / nuclear / hydropower energy – sorbent-based DAC – sedimentary reservoirs

As highlighted in Section 3.3.2, potential geothermal energy, existing nuclear power plants, and potential hydropower facilities are unevenly distributed around the globe, according to the specificities of each of these resources. Figure 3.8 identifies opportunities for pairing geothermal, nuclear, and hydropower facilities with DAC within 100 kilometers of sedimentary basins. Pairing sorbent-based DAC with geothermal, nuclear, or hydropower would require using the geothermal heat or the waste heat from the nuclear plant and part of the electricity generated at the facility, particularly in the case of hydropower, where the dam would have to provide the electricity necessary for heat generation via industrial heat pumps. The plant’s carbon dioxide-capturing potential will depend on the pairing configuration: the type of plant for geothermal facilities, and the share of energy allocated to DAC for all above-cited sources of energy. A case study in the U.S. details calculations of carbon dioxide capture potential when a sorbent-based DAC plant is paired with geothermal or nuclear energy (McQueen et al., 2020).

Most of the nuclear power plants operating or under construction in East Asia, western Europe, and the U.S. are near sedimentary basins, and thus show potential for a “nuclear energy–sorbent-based DAC–sedimentary reservoirs” CDR system. Co-location of sedimentary basins and hydropower facilities currently under construction is variable. Regions with the best opportunities are Europe, Turkey, the Sichuan province in China, Southeast Asia, and along the coast of Brazil, but most hydropower facilities are not co-located with sedimentary basins. In the Himalayas, South America, and East Africa, hydropower might be better co-located with basalt or ultramafic rock formations.

Some geothermal plants are located over sedimentary basins, which might look ideal for the proposed potential CDR system. However, high-temperature geothermal resources tend to be incompatible with high-quality sedimentary reservoirs because the heat reduces the porosity and permeability of sedimentary rocks and thus their ability to sequester CO2. This geothermal energy–sorbent-based DAC–CO2 sequestration potential CDR system would therefore likely require transporting CO2 long distances from the capture location to injection locations in sedimentary basins. Alternatively, basalt and ultramafic rocks have high CO2 capture and mineralization potential and are better co-located with high-quality geothermal resources.

Figure

3.8

Figure 3.8. Global opportunities for a geothermal / nuclear / hydropower-sorbent-based DAC-sedimentary basins CDR system. For the same type of power plant, the color of the dots varies to highlight power plants that are located less than 100 km from sedimentary reservoirs. Note that the size of the dots corresponds to different capacities for the different types of plants. References to the data used to build this map can be found in the legends of Figures 3.4 to 3.6.

cdr primer map 3 8

Figure 3.8. Global opportunities for a geothermal / nuclear / hydropower-sorbent-based DAC-sedimentary basins CDR system. For the same type of power plant, the color of the dots varies to highlight power plants that are located less than 100 km from sedimentary reservoirs. Note that the size of the dots corresponds to different capacities for the different types of plants. References to the data used to build this map can be found in the legends of Figures 3.4 to 3.6.

 

3.4.2

Geothermal energy-sorbent-based DAC-CO2 sequestration in basalts

A California case study detailed opportunities for pairing DAC plants with geothermal energy onsite and transporting CO2 to injection points in geologic formations. Geothermal energy is widely available in California, with required temperatures of over 100o C necessary to regenerate the solid sorbents of a DAC plant. Suitable areas are located near the Salton Sea, the Geysers, Mammoth Lake, and in the northeast region of the state. Geothermal opportunities (geothermal wells with available data for warm-water flow rate and temperatures) have been shown in California along with sequestration options by Baker et al. (2019). While geological sequestration has a 50+ year safety record, CO2 is unlikely to be injected under the city of Los Angeles due to a lack of public acceptance in such a large population center. So the Sacramento and San Joaquin basins in the Central Valley are more likely to be used for CO2 sequestration. The geothermal area of the Geysers is thus ideally placed for setting up a geothermal–sorbent-based DAC–sedimentary reservoir system. Another geothermal hot spot is in Imperial County, south of the Salton Sea. The closest sedimentary basins are in Arizona, thus requiring collaboration between states.

Most global geothermal opportunities are actually closer to basalts or ultramafic rocks, but in California none of these formations has been assessed for CO2 sequestration. The CarbFix pilot project in Iceland has demonstrated success with this option, where CO2 is injected in basalt and permanently mineralized in less than a year (Aradóttir et al., 2011; Gislason et al., 2010; Gunnarsson et al., 2018; Matter et al., 2016; Snæbjörnsdóttir et al., 2017). The project developed a method for injecting CO2 and H2S from the geothermal plant (10,000 tCO2/yr and 6,000 tH2S/yr) and is now successfully injecting CO2 captured from air as well (about 50 tCO2/yr). This project shows that geothermal power plants have the potential to be carbon-neutral or even negative. This might help expand use of geothermal energy, which is renewable but has been criticized for its GHG emissions (as described in Section 3.3.2).

Since most geothermal plants are located over or close to basalts, the CarbFix project sets the precedent for the development of a geothermal energy–sorbent-based DAC–CO2 sequestration in basalts CDR system. The method requires large amounts of water and has been successfully tested on fresh water at pilot scale (Aradottir et al., 2011; Gislason., 2010; Matter et al., 2011, 2016; Snæbjörnsdóttir et al., 2017; Gunnarson et al., 2018) and tested in the laboratory (Wolff-Boenisch et al., 2011). Salt water has a different pH than freshwater, which might affect the carbon mineralization of CO2, but in areas where fresh water is not readily available, seawater could be an adequate alternative (Snæbjörnsdóttir et al., 2020). If seawater is proven usable in that system, it could, for instance, be implemented at a small scale on volcanic islands in the Pacific Ocean (Hawaii already has a geothermal power plant) and would have the combined advantage of reducing the plants’ dependence on energy imports and implementing CCS. When CO2 sequestration in basalts reaches the commercial scale, large-scale projects could take place in Iceland and Washington state (U.S.), where pilot projects are already operating. Local opportunities for CO2 sequestration in basalts coupled with geothermal energy also exist in most countries around the Pacific Ocean.

 

3.4.3

CSP + PV or wind–sorbent-based DAC–CO2 sequestration

Concentrated solar power (CSP) technology focuses heat from the sun’s rays in order to create a focused beam of energy, driving a generator producing steam, which in turn generates electricity. The thermal energy produced by CSP during the day can be stored for four to six hours in molten salts. This provides a shorter night gap compared to PV technologies, and CSP-powered plants can even run continuously (Ortega et al., 2008). As 80 percent of the total energy requirements of today’s leading DAC technologies are thermal (Beuttler et al., 2019), this technology could be a valuable source of energy for DAC. Some of the heat could be used directly for regenerating the solid sorbent of a DAC plant, and the remaining heat for electricity production. Here we examine the overlap of CSP with renewable electricity options such as PV and wind for the additional electricity generation required. Figures 3.9 to 3.12 show the co-location of wind and PV with CSP on converted lands and geological reservoirs for CO2 sequestration (within 100 kilometers for sedimentary reservoirs, as explained in the introduction of this section).

Figure

3.9

Figure 3.9. Global opportunities for CSP paired with PV or wind and co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). Countries hatched were not studied for assessing sedimentary reservoirs for CO2 sequestration. The legends of Figures 3.3, 3.5, and 3.6 reference the data used to build this map.

cdr primer map 3 9

Figure 3.9. Global opportunities for CSP paired with PV or wind and co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). Countries hatched were not studied for assessing sedimentary reservoirs for CO2 sequestration. The legends of Figures 3.3, 3.5, and 3.6 reference the data used to build this map.

CSP siting is restricted by operating conditions that require it to be within 45o of the equator. The combination that could be largely deployed on converted lands is a CSP + PV–sorbent-based DAC–sedimentary reservoirs CDR system. Regions around the world with the highest potential for such siting on converted lands are southern Africa, the Arabian Peninsula, the western U.S., Australia, Morocco, and Algeria (Figure 3.9). In Mexico and East Africa’s Rift Valley basalt is predominant for CO2 sequestration, and it could also be an alternative in countries where sedimentary reservoirs appear to be the best option. However, no assessment of available capacity has been made for basalts. In the Arabian Peninsula, Oman hosts the largest ophiolite (serpentine rock formation) in the world, which could be another potential option for CO2 sequestration in ultramafic rocks (Figure 3.11).

Figure

3.10

Figure 3.10. Opportunities for CSP + PV–sorbent-based DAC–sedimentary reservoir in southern Africa (A) and eastern Australia (B). References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

cdr primer map 3 10

Figure 3.10. Opportunities for CSP + PV–sorbent-based DAC–sedimentary reservoir in southern Africa (A) and eastern Australia (B). References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

Figure

3.11

Figure 3.11. Opportunities for the SCP + PV–sorbent-based DAC–CO2 sequestration CDR system in Saudi Arabia (A) and Oman (B). All CO2 sequestration options are shown for this system. Opportunities within 100 km of sedimentary reservoirs (in brown) are yellow, opportunities co-located with basalts (in purple) are orange, and opportunities co-located with ultramafic rocks (in green) are red. References to the data used to build this map can be found in the legends of Figure 3.3, 3.5, and 3.6.

cdr primer map 3 11

Figure 3.11. Opportunities for the SCP + PV–sorbent-based DAC–CO2 sequestration CDR system in Saudi Arabia (A) and Oman (B). All CO2 sequestration options are shown for this system. Opportunities within 100 km of sedimentary reservoirs (in brown) are yellow, opportunities co-located with basalts (in purple) are orange, and opportunities co-located with ultramafic rocks (in green) are red. References to the data used to build this map can be found in the legends of Figure 3.3, 3.5, and 3.6.

The potential for CSP is generally higher at lower latitudes, whereas the potential for onshore wind generation increases at higher latitudes. These two technologies are thus rather incompatible, except in the Great Plains region of the United States, which has one of the highest global concentrations of onshore wind opportunities on converted land. Here, opportunities are restricted to converted land to outline opportunities on lands already disturbed by human activities. Siting DAC plants in these locations would have two advantages: avoiding competition with human activities (food production or human settlements) and preserving pristine ecosystems. Other local opportunities might exist, but at a much lower scale. 

Figure

3.12

Figure 3.12. Opportunities for the SCP + wind–sorbent-based DAC–CO2 sequestration CDR system in the Great Plains (U.S.). References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

cdr primer map 3 12

Figure 3.12. Opportunities for the SCP + wind–sorbent-based DAC–CO2 sequestration CDR system in the Great Plains (U.S.). References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

 

3.4.4

PV or wind–electric DAC–CO2 sequestration

One of the most flexible CDR systems is carbon dioxide capture plants that rely solely on electricity provided by solar PV or wind turbines. This requires a DAC facility that needs only electricity, or another source of heat for a sorbent-based DAC plant. Electric DAC technology for CO2 capture, such as the faradaic electro-swing reactive adsorption technology, is still under development at the laboratory scale (Voskian and Hatton, 2019). Through experimentation, the process has demonstrated a high faradaic efficiency and low energy consumption (40 – 90 kJ/mol) (Voskian and Hatton, 2019). Additionally, this technology primarily utilizes electric energy, making it suitable for either continuous operation or operations requiring repeated start-up and shut-down. While this CDR system is not ready to be deployed, it may be in the future.

Other possible DAC systems requiring only electrical energy would require identifying another source of thermal energy for a sorbent-based DAC. Various opportunities may exist, depending on local context. For instance, many industrial sectors produce heat necessary for their processes (e.g., cement, iron and steel, refining, chemicals, food processing). This excess heat, if not consumed in some other way at the facility, could be used to regenerate the sorbent of a DAC plant installed onsite, while the plant’s electricity needs could be met by a nearby PV or wind farm. If no waste heat is available, industrial heat pumps powered by PV or wind electricity could be used. This new type of industrial heat pump can raise the temperature of the working fluid from warm ambient temperatures (30 – 40o C) to about 100C. This system would be implemented in warm climates.

Figure

3.13

Figure 3.13. Global opportunities for PV and wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). References to the data used to build this map can be found in the legends of Figures 3.3, 3.5, and 3.6.

cdr primer map 3 13

Figure 3.13. Global opportunities for PV and wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). References to the data used to build this map can be found in the legends of Figures 3.3, 3.5, and 3.6.

Solar PV and wind opportunities on converted lands are available around the world, as are CO2 sequestration options (Figure 3.13). All CSP opportunity areas are also viable for PV, and PV can generate electricity at latitudes as high as 60o. This offers more opportunities than CDR systems using CSP, particularly in large GHG-emitting countries such as China, the European Union (member state emissions combined), India, Russia, and the U.S. (Figure 3.14). Sequestration options are mainly in sedimentary reservoirs, which could allow for faster implementation. In India, the Deccan Traps and the Rajmahal basalts are potential CO2 reservoirs and could be the sequestration choice in these areas. However, there are low prospects for DAC in India as the country suffers from regular power shortages. Since these basaltic provinces are located under a highly industrialized region in India, they could be used for CO2 sequestration from point-source capture at industrial facilities. 

A fully electric system would also favor onshore and offshore wind at higher latitudes. Here, offshore wind is displayed if it has potential generation of more than 20 GWh/yr. As previously mentioned, opportunities for onshore wind are widely available in the Great Plains of the United States. Numerous opportunities also lie in northern Europe, in particular on the coast of Norway, the UK, and the North Sea, where the wind speed is high and sedimentary reservoirs are widely available to create a wind–electric DAC–sedimentary reservoirs CDR system. For the same reasons, Cape Horn (Chile), the Cape of Good Hope (South Africa), New Zealand, and Tasmania are also good candidates (Figure 3.15).

Figure

3.14

Figure 3.14. Local opportunities in India (A) and northeast China (B) for PV and wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). References to the data used to build this map can be found in the legends of Figures 3.3, 3.5, and 3.6.

cdr primer map 3 14

Figure 3.14. Local opportunities in India (A) and northeast China (B) for PV and wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). References to the data used to build this map can be found in the legends of Figures 3.3, 3.5, and 3.6.

Figure

3.15

Figure 3.15. Local opportunities in the Great Plains (U.S.) (A), northern Europe (B), southern Australia (C), and New Zealand (D) for wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). Countries hatched were not studied for assessing sedimentary reservoirs for CO2 sequestration. References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

cdr primer map 3 15

Figure 3.15. Local opportunities in the Great Plains (U.S.) (A), northern Europe (B), southern Australia (C), and New Zealand (D) for wind co-located with sequestration reservoirs (within 100 km for sedimentary reservoirs). Countries hatched were not studied for assessing sedimentary reservoirs for CO2 sequestration. References to the data used to build this map can be found in the legends of Figures 3.3 and 3.5.

 

3.4.5

Considerations for responsible DAC siting

The above examples consider siting CSP, PV, and wind farms only on converted lands, in order to minimize the impact of such installations on ecosystems and human activities. For the same reason, marine protected areas were not considered opportunities for offshore wind farms. We focused on using low-carbon energy and co-locating the elements of a potential CDR system to optimize the amount of CO2 removed from the air. Other considerations are necessary to evaluate the impact of a potential CDR project, though the rapid reduction of the global atmospheric carbon dioxide stock is the ultimate goal. Appropriate uses of low-carbon energy, the transportation of CO2, and other resources for CO2 sequestration are discussed below. 

Carbon dioxide removal compliments CO2 reductions. Removal cannot replace reductions. DAC and other CDR methods are meant to offset sectors that are hard to avoid, in addition to removing legacy emissions. It is more efficient to avoid releasing CO2 in the atmosphere in the first place. For instance, building a PV or wind farm to replace a coal-fired power plant is more efficient than keeping the coal-fired power plant and using the PV or wind farm next to it to power a DAC plant. Figure 3.16 shows the global distribution of coal- and natural gas-fired power plants. Coal-fired power plants are concentrated in China (946), the U.S. (338), Europe (326), India (254), and Russia (96) (World Resources Institute [WRI], 2019). 

Figure

3.16

Figure 3.16. Global distribution of coal- and natural gas-fired power plants (WRI, 2019)

cdr primer map 3 16

Figure 3.16. Global distribution of coal- and natural gas-fired power plants (WRI, 2019)

The transition from fossil fuel energy to renewable energy is underway. For instance, the Public Service Company of Colorado in the U.S. (PSCo) is replacing 600 MW of coal-fired units with 1,800 MW of renewable energy (1,131 MW wind, 707 MW solar PV) and 275 MW of battery storage. This should reduce coal-generated electricity in Colorado’s energy grid from 44 percent in 2017 to 24 percent in 2026 and increase renewable energy from 28 percent to 53 percent (Pyper, 2018). This might lower Colorado’s GHG emissions by 60 percent for CO2 and 90 percent for nitrogen oxides and sulfur dioxide, compared to state emissions in 2005. Transitioning to low-carbon energy production is crucial for limiting carbon dioxide emissions and reduces reliance on CDR systems in the long term. However, the latest IPCC report (2018) stated that CDR systems will be necessary to reach the goal of preventing warming from increasing beyond 1.5o C or 2o C globally. To power these CDR systems, the world will need to produce more renewable energy than is needed to replace fossil power.

Social impacts like unemployment rates are also important considerations when transitioning from one technology to another. Coal miners generally do not migrate far when they lose their jobs (Danson, 2005; Gore and Hollywood, 2009; Hollywood, 2002), leading to local social disasters when coal mines are closed. Pai et al. (2020) analyzed the job-transition potential of shifting toward low-carbon energy and concluded that solar PV has more potential than wind in coal areas in India (Pai et al., 2020). Their study also included Australia, China, and the U.S., but they examined the total potential for each renewable technology (rather than constraining their analysis to converted areas already disturbed by human activity), which might overestimate the true renewable energy potential.

Other social parameters must also be considered for a successful CDR project. For example, public acceptance (Wong-Parodi and Ray, 2009), misunderstanding, and the perceived risks posed to communities related to CO2 injection in the subsurface might prevent CDR system development in densely populated areas.

In addition to technical, economic, and social considerations, local field studies of potentially suitable sites for DACCS or BECCS are essential. Indeed, global or regional datasets might lack important local characteristics. For instance, peatlands (Sections 2.5 and 3.2.3) are often poorly mapped, according to Chico et al. (2020). The authors analyzed mapping data for Europe, where peat bogs are particularly abundant in Ireland and the UK and also appear in France and Spain. A map issued by Xu et al. (2017) shows that peatlands are widely distributed in these areas. Preserving these environments is crucial because, though peatlands represent less than three percent of the Earth’s land surface, they store 20 percent of soil carbon (Chico et al., 2020). Installing wind turbines in these environments would disturb them and could cause large releases of CO2 into the atmosphere. Northern Europe has large wind resources, as shown on Figures 3.3, 3.13, 3.15B, and 3.17, and because peatlands are poorly mapped, peat bogs might still prove to be good places to build wind farms.

Western Europe is one of the most densely populated areas in the world and one of the biggest emitters of GHGs. Because of that density, offshore CO2 sequestration options are probably better than those on land. Figure 3.17 shows the sedimentary reservoirs selected by the CO2StoP study in the North Sea, which has large available resources and is where the Sleipner CO2 sequestration project has been operating since 1996.

Injecting CO2 captured onshore into offshore sedimentary reservoirs requires pipelines. Buffers of 100, 250, and 500 kilometers around these reservoirs are displayed in Figure 3.17 for a rough estimate of the length of these pipelines. Pipelines of about 500 kilometers can reach most of the northwestern countries, but reaching most of the European opportunity areas and implementing low-carbon energy–DAC–offshore sedimentary reservoir systems in Europe would require building a dense network of CO2 pipelines. Europe’s industrial and electricity sectors are still not carbon-neutral, and some of these facilities might choose to implement point-source capture in an effort to avoid releasing most of their CO2 into the atmosphere. An operating network of CO2 pipelines, such as the one proposed by Morbee et al. (2011) with booster stations and carbon hubs to collect the CO2 from major pipelines, might stimulate projects for CO2 capture from industrial streams or from the air and lead an effort toward true CDR systems and negative emissions. A similar project led by Bellona is looking into injecting CO2 in sedimentary formations off the coast of Norway after it is transported by a pipeline network or ships from several locations in northern Europe (Bellona Europa, 2020).

According to the CO2StoP study, other offshore CO2 sequestration reservoirs hold promise for avoiding very long-distance transportation of CO2. Southern Europe has opportunities off the coast of Portugal, in the Adriatic Sea, and in northeastern Europe in the Baltic Sea (Figure 3.5).

Figure

3.17

Figure 3.17. Distance to low-carbon sources of energy from sedimentary basins for CO2 sequestration under the North Sea. References to the data used to build this map can be found in the legends of Figure 3.3 to 3.5.

cdr primer map 3 17

Figure 3.17. Distance to low-carbon sources of energy from sedimentary basins for CO2 sequestration under the North Sea. References to the data used to build this map can be found in the legends of Figure 3.3 to 3.5.

As another example, a study in India by Garg et al. (2017) investigated pipeline trajectories for minimizing the transportation distances by linking several point sources of CO2 to potential sequestration fields. A similar study has been carried out in the U.S. for developing pipelines in the Midwest. These pipelines would link ethanol plants that produce high-purity CO2 as the result of the fermentation process with sequestration reservoirs, which are often not co-located with ethanol plants (Fry et al., 2017). Co-location efforts like these are beneficial for areas with great potential for CO2 removal but few opportunities for carbon sequestration.

 

3.4.6

BECCS

Another technological system that captures CO2 and sequesters carbon dioxide is BECCS (described in Section 2.7). These systems capture CO2 via photosynthesis when biomass grows and, when that biomass is used for bioenergy generation, a carbon-containing product such as CO2 or biochar is sequestered. The ability to both capture CO2 and produce energy makes BECCS attractive compared to other CDR technologies that require energy. However, BECCS requires significant land area, which can compete with the need to protect existing ecosystems and other CDR approaches, such as reforestation or afforestation (described in Sections 2.4 and 3.2). To avoid competing with food production and harming ecosystems, two approaches have been proposed: using waste biomass only (Baik et al., 2018) or managing biomass growth and production from land already disrupted by human activities (Daioglou et al. 2019). The second approach is similar to the one taken by Baruch-Mordo et al. (2019), described above in Section 3.3.2, which restricts opportunities for solar and wind to converted lands. Opinions on dedicated land use for bioenergy (whether in the form of production forests or dedicated energy crops) still differ, due to potential competition with other important land uses.

Moreover, the definition of waste biomass varies. Most definitions of waste include high-moisture biomass, agricultural wastes destined for landfills, the organic portion of municipal solid wastes, and industrial waste sludge from the food and beverage industries (Faaij, 2018). Broader classifications of waste biomass incorporate “residues,” which include woody biomass residues as a byproduct of timber or pulp and paper mills. Also, biomass growth varies throughout the year, so the composition and availability of wastes may be seasonally specific. 

BECCS deployment assessments will need to examine spatial co-location of suitable storage basins and biomass availability to minimize long-distance transport of biomass and CO2 or other carbon products. Biomass availability varies widely and is based on local ecosystem and development contexts. While global aggregated storage capacity is generally not considered a limiting factor for BECCS or CCS deployment, the capacities of storage sites vary widely and may lead to regional storage constraints. Consideration of the storage and injection rate capacity of storage formations at a fine spatial scale is crucial in determining potential storage sites suitable for near-term BECCS deployment. As discussed in Section 3.3.3, further investigations are needed to determine locations suitable for CO2 injection. Also, importing biomass from countries will require rigorous governance to ensure sustainable land management. Interregional cooperation (i.e., developing joint agreements for managing carbon, setting shared carbon sequestration goals, and trading negative emissions credits and biomass) is central to sustainably and affordably scaling up BECCS. This multilateralism in biomass and carbon credits trading provides important opportunities to create value for key providers of CO2 removal (Fajardy & McDowell, 2020).

Daioglou et al. (2019) provided a global map of potential opportunities in 2050 and 2100 for biomass cultivation on abandoned agricultural lands and on grassland, shrubland, savannah, and tundra environments. This information should be correlated with reliable geological sequestration in order to show opportunities for a full BECCS system. The choice among BECCS, biological CDR techniques, and other CDR approaches relies on multiple parameters, including the co-benefits of each CDR approach (e.g., electricity production and biodiversity increase), local energy supply, local incentives and regulations, and public opinion.

No global mapping of available waste biomass alongside geologic sequestration capacity exists, but Baik et al. (2018) studied waste biomass availability in the U.S. and correlated biomass sources with sequestration in sedimentary reservoirs. In addition to showing regions with the greatest waste biomass availability, the authors estimated potential CO2 capture using BECCS to be in the hundreds of MtCO2/yr (Baik et al., 2018). Similar studies in other countries would help determine their available waste biomass to implement BECCS without disturbing current ecosystems and competing with reforestation and afforestation.

 

3.4.7

Sources of alkalinity for CO2 sequestration

The above sections discussing DAC and BECCS siting focused on sequestering CO2 in sedimentary reservoirs, as this technology is currently the best understood and the most ready to be implemented at large scale in the next few years. Opportunities for CO2 injection in basalts and ultramafic rocks (peridotites and serpentinites) are also shown above, as all of these options might be viable for in-situ CO2 sequestration. CO2 sequestration options other than in-situ injection in subsurface pore space can be considered as well. As described in Section 3.3.8, mines and industrial facilities produce significant amounts of alkaline wastes that can be reacted with CO2 and offset part of the facility’s emissions.

Figure 3.18 shows the spatial distribution of carbon mineralization opportunities in the U.S. for asbestos, gold, copper, and nickel mine tailings, as well as for multiple industrial sources of alkalinity (CKD, lime, red mud, steel slag, and fly ash), along with in-situ opportunities for CO2 injection (Kirchofer et al., 2013; USGS, 2003 and 2010).

Sedimentary rocks react very slowly with CO2. Therefore, Figure 3.18 explores the total capacity of each basin in terms of CO2 sequestration. The EASiTool module was used to provide a rough estimate of the capacity that exists within the local area in addition to the thickness and porosity of each reservoir (Ganjdanesh and Hosseini, 2017; Hosseini et al., 2019). The fair category corresponds to roughly 0.1 Gt or less, the good category to 0.1 to 1 Gt, and the excellent category to 1 Gt or more. These estimates account for a maximum of 100 wells per basin, 10 km spacing between CO2 injection wells, and an injection duration of 20 years.

Figure

3.18

Figure 3.18. Opportunities in the U.S. for CO2 sequestration in mine tailings and industrial wastes rich in alkalinity along with in-situ injections in sedimentary reservoirs, basalts, and ultramafic rocks (peridotites and serpentinites) (Blondes et al., 2019; Van Gosen, 2011; USGS, 2018; Ganjdanesh and Hosseini, 2018; Hovorka et al., 2012; Krevor et al., 2009; U.S. Geological Survey [USGS], 2003, 2010, 2013; Kirchofer et al., 2013). 

cdr primer map 3 18

Figure 3.18. Opportunities in the U.S. for CO2 sequestration in mine tailings and industrial wastes rich in alkalinity along with in-situ injections in sedimentary reservoirs, basalts, and ultramafic rocks (peridotites and serpentinites) (Blondes et al., 2019; Van Gosen, 2011; USGS, 2018; Ganjdanesh and Hosseini, 2018; Hovorka et al., 2012; Krevor et al., 2009; U.S. Geological Survey [USGS], 2003, 2010, 2013; Kirchofer et al., 2013). 

Reservoir capacity is trickier to estimate for mafic and ultramafic rocks, as each rock behaves differently. Injection of CO2 in these rocks takes place at a lower TRL than sedimentary basins, but they react more readily with CO2. To remain in a supercritical state, CO2 should be injected deeper than 800 meters into basalt rock. The pilot project CarbFix has shown full mineralization of CO2 at depth in less than a year (Aradóttir et al., 2011; Gislason et al., 2010; Gunnarsson et al., 2018; Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Ultramafic rocks (peridotites and serpentinites) react even more quickly than basalts and, thus, CO2 might not have to be sent deeper than 800 meters to avoid release back into the atmosphere and to ensure full mineralization in subsurface pore space. Their TRL remains at an initial level, and only lab-scale experiments have taken place.

Finally, ultramafic rocks, particularly mine tailings, can also be used for surficial carbon mineralization. Asbestos crystals are needle-shaped, so they have a large surface area, which increases their reactivity with CO2. Two types of minerals take the form of asbestos: chrysotile (asbestiform serpentine) and crocidolite (asbestiform amphibole), the latter being more hazardous to human health. The ranking of asbestos mine tailings is associated with the following categories: “Good” indicates the presence of chrysotile, “fair” indicates a combination of chrysotile and crocidolite (implying a riskier access to the chrysotile resource), and “poor” indicates the presence of crocidolite only, which makes the mine tailings unsuitable for use. Using mine tailings has distinct advantages: It avoids mining additional alkalinity while sequestering CO2, some wastes can be upcycled into products, and it can help mitigate the risks of hazardous materials like asbestos..

All CO2 sequestration opportunities in the U.S. are displayed in Figure 3.18, showing a heterogeneous distribution and indicating that although distinct opportunities are regional, most regions have potential for storage. The same type of analysis could be done on a more granular level regionally to provide stakeholders with a comprehensive overview of CO2 sequestration opportunities.

3.5

3.5 —

Conclusions

Through a series of existing and original spatial mapping efforts, this chapter explored global opportunities for the deployment of CDR systems. Multiple CDR approaches are available in most regions of the world and depend on the availability of resources. For example, waste alkalinity sources from mine waste and industrial aggregate are produced in different volumes, depending on the region. Approaches with high TRL levels (as described in Chapter 2) are ready to be deployed immediately, and local regions can leverage their expertise and resources, ideally through global coordination, collaboration, and transparency.

Actual CDR deployment requires detailed local studies of the energy supply network and the sources of COemissions. Some CDR approaches are energy-intensive and may therefore be limited in their efficacy. In all cases, carbon dioxide removal must be deployed in tandem with reducing emissions by switching to low-carbon energy sources or avoiding emissions by implementing point source capture. Also, biological CDR solutions have additional environmental co-benefits compared to technological approaches, but technological approaches may result in more durable storage and remove more CO2 per land area. For any CDR approach, spatial analysis can guide deployments that maximize emission capture efficiency (discussed in Chapter 4). Ultimately, more accurate and granular data, further analysis, and a broad consideration of stakeholder and community interests and needs are all required to guide the deployment of future CDR projects. Geospatial analysis is simply a tool, not a determination.

1

In this section, enhanced weathering refers to the preparation (grinding, milling etc.) of substrates for the specific goal of accelerating natural weathering processes, in which CO2 is transformed into aqueous bicarbonate ions. It differs from COmineralization, which forms carbonate minerals Hence, enhanced weathering has an improved capture ratio of 1.5-1.8 moles CO2 per mole divalent cation (as opposed to 1:1 for carbon mineralization) (Renforth, 2019).

2

The platinum group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum. They all share similar chemical properties, indicating that they tend to be discovered at the same location. 

1

In this section, enhanced weathering refers to the preparation (grinding, milling etc.) of substrates for the specific goal of accelerating natural weathering processes, in which CO2 is transformed into aqueous bicarbonate ions. It differs from COmineralization, which forms carbonate minerals Hence, enhanced weathering has an improved capture ratio of 1.5-1.8 moles CO2 per mole divalent cation (as opposed to 1:1 for carbon mineralization) (Renforth, 2019).

2

The platinum group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum. They all share similar chemical properties, indicating that they tend to be discovered at the same location. 

45Q

45Q tax credit

AFOLU

agriculture, forestry, and other land use

ARPA-E

Advanced Research Projects Agency-Energy

ASU

air separation unit

bbl

barrel of oil

BECCS

bioenergy with carbon capture and sequestration/storage

BTU/mmBTU

British thermal unit/one million British thermal units

C

carbon

CAPEX

capital expenditures

CCS

carbon capture and storage

CCU

carbon capture and utilization

CCUS

carbon capture, utilization, and storage

CDL

cropland data layer

CDR

carbon dioxide removal

CI

carbon intensity

CKD

cement kiln dust

CLT

cross-laminated timber

CO2

carbon dioxide

CO2eq

CO2 equivalent

CRF

capital recovery factor

CRMS

Coastwide Reference Monitoring System

CSP

concentrated solar power

CWPPRA

Coastal Wetlands Planning, Protection and Restoration Act

DAC

direct air capture

DOE

United States Department of Energy

EIA

United States Energy Information Administration

EJ

exajoule

EOR

enhanced oil recovery

EPA

Environmental Protection Agency

EU

European Union

FOAK/Nth-OAK

first-of-a-kind (FOAK) or nth-of-a-kind

GCCC

Gulf Coast Carbon Center

GHG

greenhouse gas

GIS

geographic information system

Gt

gigatonne

GTM

global timber model

GWP

global warming potential

ha

hectare

HE

hard-to-avoid emissions

HHV

higher heating value

IAM

integrated assessment model

IEAGHG

International Energy Agency Greenhouse Gas Research and Development Programme

IFM

improved forest management

IPCC

Intergovernmental Panel on Climate Change

ISBL

inside battery limits

ISO

International Standards Organization

J

Joule

kJ

kilojoule

kt

kilotonne

kW

kilowatt

kWh

kilowatt-hour

LCA

life cycle analysis

LCFS

low-carbon fuel standard

LED

low energy demand scenario

LHV

lower heating value

LR

learning rate

MJ

megajoule

Mt

megatonne

MW

megawatt

NASEM

National Academies of Science, Engineering, and Medicine

NET

negative emissions technology

OSBL

outside battery limits

PGE

platinum group elements

pH

potential of hydrogen

ppm

parts per million

PSU

practical salinity units

PV

photovoltaics

RCP

representative concentration pathway

RD&D

research, development, and demonstration

SAU

storage assessment unit

SBSTA

Subsidiary Body for Scientific and Technological Advice

SDWA

Safe Drinking Water Act

SLCP

short-lived climate pollutant

SMR

steam methane reformation

SOC

soil organic carbon

SRM

solar radiation management

t

tonne

TEA

techno-economic analysis

TRL

technology readiness level

UN

United Nations

UNEP

United Nations Environment Programme

UNFCCC

United Nations Framework Convention on Climate Change

USDA

United States Department of Agriculture

USGS

United States Geological Survey

WACC

weighted average cost of capital

WBCSD

World Business Council for Sustainable Development

WGSR

water-gas shift reaction

WRI

World Resources Institute

wt

weight

Greenhouse gas (GHG)

Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and ozone (O3) are the primary GHGs in the Earth’s atmosphere. There are also a number of entirely human-made GHGs in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, managed under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol). Besides CO2, N2O and CH4, the Kyoto Protocol deals with the GHGs sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).

Carbon dioxide equivalent (CO2eq)

Describes the impact of a given GHG (CO2, CO, CH4, N2O, etc.) by converting its mass to the equivalent mass of CO2 that would have the same global warming effect. The mass of a GHG is converted to the mass of CO2eq based on the GHG molecule’s potential to affect global warming, or its global warming potential (GWP). The GWP takes into account both the radiative forcing effect of the GHG and the gas’s lifetime in the atmosphere, and is dependent on the time horizon, which is most commonly 20 years (GWP20) or 100 years (GWP100). These values are different because the GWP is time-integrated and the GWP of CO2 is always 1, regardless of the time horizon. (See Chapter 4, Supplement 4.1.)

Montreal Protocol on Substances that Deplete the Ozone Layer

A 1987 international treaty that regulates the production and consumption of nearly 100 man-made chemicals referred to as ozone-depleting substances. (UNEP)

Kyoto Protocol

A 1997 agreement that operationalizes the United Nations Framework Convention on Climate Change (UNFCCC) committing industrialized countries and economies in transition to limit and reduce greenhouse gas emissions in accordance with agreed-upon individual targets. (UNFCCC)

Paris Agreement

A 2016 agreement formed by Parties to the UNFCCC to combat climate change and to accelerate and intensify the actions and investments needed for a sustainable low-carbon future. (UNFCCC)

Carbon dioxide removal (CDR)

Activities that remove CO2 from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. CDR includes enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO2 uptake not directly caused by human intervention.

CDR approach

Refers to methods, which may not be complete CDR systems, that can achieve carbon dioxide removal. For example, DAC in the absence of permanent storage is a CDR approach, but is not on its own a CDR system.

(Potential) CDR system

A system with the capacity to generate net-negative emissions, but which will not necessarily do so under all conditions. A comprehensive life cycle analysis (LCA) and other analyses are required to conclusively demonstrate that a “potential” CDR system is, indeed, a CDR system. For example, in the absence of an LCA, a CDR system that barely achieves net-negative emissions could look the same as one that achieves far greater net climate benefits per tonne of CO2 removed from the atmosphere.

Large-scale CDR

Carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

Gigatonne of carbon dioxide (GtCO2)

Refers to a billion metric tonnes (metric tons) of CO2, which is equivalent to 1015 g. It is also helpful to know that 1 GtCO2 is equivalent to 0.273 gigatonnes of carbon (GtC). This unit of measurement is used most frequently when discussing the scale of CDR required to prevent the worst impacts of climate change and to keep warming below 1.5° C (i.e., gigatonne-scale CDR).

Sink

Any process, activity or mechanism that removes a greenhouse gas, a precursor of a greenhouse gas, or an aerosol from the atmosphere.

Source

Any process, activity, or mechanism that creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

Positive emissions

Occur when a  particular source — created or enhanced by human activity — adds greenhouse gases to the atmosphere.

Negative emissions

Occur when a sink — created or enhanced by human activity — removes greenhouse gases from the atmosphere. (In this primer, only CO2 is considered.)

Net-negative emissions

Achieved when more greenhouse gases are removed from the atmosphere than are emitted into it.

Net-zero emissions

Achieved when the total emissions entering the atmosphere are balanced by the total removal of emissions from the atmosphere. It is sometimes used interchangeably with the term carbon-neutral.

Life cycle analysis (LCA)

An analysis of the balance of positive and negative emissions associated with a certain process, which includes all of the flows of CO2 and other greenhouse gases, along with impacts on other environmental or social impacts of concern. This analysis also includes greenhouse gas emissions that result from the materials used to construct a given process (commonly referred to as embodied emissions), as well as from the energy resources used to meet the energy demands of the process.

Embodied emissions

Emissions that result from the production or use of any good, or the provision of any service. For example, the embodied emissions of steel used for a reactor will include emissions associated with the acquisition of raw materials, processing, manufacturing, transportation, and the energy used in steel production.

Hard-to-avoid emissions

Emissions that are either physically extremely difficult to eliminate within a certain timeframe (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution, or because avoidance would require a technology that relies on a scarce resource) or which would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).

Avoided emissions

Permanently storing what would have been a CO2 emission in order to avoid an increase in atmospheric CO2. Note that this is distinct from CDR because the source of CO2 is a potential emission rather than the atmosphere. The climate benefit of such an intervention depends on the counterfactual scenario – what would have happened without our action? – which can only be estimated, not observed. If a project claims to avoid emissions that weren’t going to happen anyway, and other emissions are allowed to continue in exchange, then an avoided emission project can do more damage to the climate than doing nothing at all.

Emissions mitigation

A human intervention to reduce or avoid greenhouse gas emissions. Some publications have used the broad term “mitigation” to refer exclusively to emissions mitigation, whereas others have used mitigation (or “climate change mitigation”) to include emissions mitigation, CDR, and any other strategies for combating climate change.

Reservoir

Refers to the place where a greenhouse gas is stored. Examples include geological formations, alkaline-containing minerals, forest biomass, and soils.

Stock

A carbon reservoir that exchanges carbon with the atmosphere over relatively short timescales (e.g., less than 100 years), including the terrestrial biosphere and shallow oceans. By comparison, other reservoirs, such as geologic formations and the deep ocean, exchange carbon over much longer timescales (e.g., 1,000 years).

Storage (or Sequestration)

Two terms that can be used interchangeably to describe the addition of CO2 removed from the atmosphere into a reservoir, which serves as its ultimate destination. For example, some CDR strategies store carbon in biological systems, such as forests or soil ecosystems, whereas others inject CO2 deep underground or chemically transform CO2 into stable, mineral forms.

Permanence (or Durability)

The duration for which CO2 can be stored in a stable and safe manner. Storage duration can differ significantly, depending on the type of  reservoir. For example, concentrated CO2 stored in geologic formations deep underground is effectively permanent (thousands of years), whereas forest carbon stocks can release carbon back into the atmosphere due to wildfire or tree harvesting.

Leakage (physical)

Refers to initially-stored CO2 (or other GHG) that has left its storage state and returned to the atmosphere. Examples include combustion of a fuel made from CO2, burning of biomass, and migration of CO2 from underground storage.

Flux

The rate of movement of greenhouse gases between reservoirs (e.g., capturing CO2 through the chemical reactions of photosynthesis, resulting in carbon storage within a plant’s biomass.

Carbon cycle

The residence time and flux of carbon — in various chemical states — between the ocean, land, terrestrial biosphere, atmosphere, and geological formations in the Earth.

Additionality

Evaluates the degree to which an intervention (e.g., a CDR project) causes a climate benefit above and beyond what would have happened in a no-intervention baseline scenario. By definition, this counterfactual baseline scenario cannot be directly observed (because it did not happen), so can only be estimated or inferred based on contextual information. Additionality can be assessed at the level of individual projects or protocols that define categories of projects. In policy regimes such as cap-and-trade programs, where emissions are permitted in exchange for reduction or storage elsewhere, failures of additionality result in increased emissions.

Carbon offsets

Programs or policy regimes in which companies or individuals pay for activities that result in emissions reductions or CDR. In voluntary offset programs, individuals or companies pay project developers (or similar) directly to implement some activity that results in emissions reductions or CDR. In compliance offset programs, such as cap-and-trade programs, companies that are responsible for large amounts of emissions are allowed to continue to emit above a certain cap in exchange for projects taking place elsewhere that reduce emissions or remove carbon. In a compliance regime, an offset has no effect on total emissions in the best-case scenario and will result in more emissions than would have occurred otherwise if the project is ineffective in any way (e.g., due to failures of additionality or permanence).

Climate tipping points

Abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

Leakage (socioeconomic)

Occurs when CDR activities displace emissions to other locations, times, or forms. For example, leakage occurs in forest carbon offset credit programs when a reduction in timber harvesting at a project site, incentivized due to its potential for emissions reductions and/or CDR, causes timber harvesting to increase somewhere else to meet demand. Similarly, if an engineered CDR approach coupled to CO2 utilization results in higher costs, alternatives that emit more CO2 may become economically favorable.

Overshoot

Climate stabilization scenarios in which emissions trajectories exceed their concentration or temperature targets (e.g., 1.5º C or 2º C) early in the century, but use substantial amounts of CDR later to reduce atmospheric CO2 levels and achieve the original targets. See Chapter 1 for a discussion of moral hazards and other challenges associated with these scenarios.

Equity

The principle of fairness in access to opportunities, power-sharing, and burden-sharing. Equity is crucial to determining how to deploy strategies to address climate change, including CDR, that minimize harm to marginalized people and frontline communities. From the World Resources Institute: “Climate change poses the greatest threat to those that are the least responsible – generally people that are already vulnerable to deep-rooted challenges such as poverty. Conversely, those who have contributed the most to climate change have much better capacity to protect themselves from its impacts. As the effects of climate change mount, so does the urgency of addressing this equity challenge.”

Social justice

Just or fair relations within society that seek to address the inequitable distribution of wealth, access to resources, opportunity, and support and remove discriminatory systems and structures that block marginalized groups from accessing these benefits on the basis of race, gender, economic status, or any other factor.

Frontline communities

Some climate justice advocates have described these as communities that experience the “first and worst” consequences of climate change. These are often communities of color and those with low income levels that have insufficient infrastructure (such as well-maintained roads and up-to-date flood protection) and that will be increasingly harmed as our climate deteriorates. These may include (for example) Native communities whose resources have been exploited, or communities of laborers whose daily work or living environments are polluted or toxic.

Moral hazard

An ethical concern whereby skeptics of potential CDR systems worry that emission reduction will be reduced or delayed on the promise of future CDR deployment. Experts agree that most temperature-stabilization scenarios require ambitious efforts both on greenhouse gas mitigation and CDR, and that the potential for large-scale deployment of CDR does not justify slowing down the pace of mitigation.

Negative-emissions technologies

This term has been used as an alternative to CDR approaches in other publications.

Geoengineering

This term refers to the concept of manipulating the climate system in order to reduce the impacts of climate change. It usually refers to solar radiation management (SRM), a controversial strategy that involves the injection of material into the atmosphere, which directly affects radiative forcing to reduce warming, but does not change concentrations of greenhouse gases. CDR has been inaccurately conflated with geoengineering in some literature.

Storing C vs. storing CO2

Some CDR methods capture and store CO2 (carbon dioxide) directly, whereas others remove CO2 from the atmosphere but store only C (carbon). For example, in biological CDR approaches involving plants and trees, CO2 is fixed through the chemical reactions of photosynthesis, and is then broken down into C stored in the plant body and roots and O2 released back into the atmosphere. The relationship between stored C and stored CO2 is given as a ratio of the molecular weights (MW), i.e., GtCO2 = GtC×(MWCO2/MWC) = GtC×(44/12) = 3.67×GtC.

Active emissions vs. legacy emissions

The first term refers to the GHG being actively emitted by an entity/technology, while legacy emissions are emissions associated with previous emissions that contribute to the accumulated pool in the atmosphere.

CDR vs. geoengineering

Unlike geoengineering, CDR removes excess CO2 from the atmosphere, thereby reducing CO2’s warming effect on the Earth. Geoengineering such as SRM focuses only on the symptoms of climate change, without changing concentrations of greenhouse gases.

CDR vs. carbon capture

Unlike carbon capture, the siting of CDR is not fixed to a point source. Point-source carbon capture targets higher-purity streams of CO2, capturing them from emissions sources like smokestacks before they reach the atmosphere. By contrast, CDR approaches target the CO2 that is already in our atmosphere. While removing this diluted CO2 from the atmosphere is more challenging,  CDR has the potential to compensate for emissions from non-point sources (e.g., transportation) and hard-to-abate sectors like heavy industry and agriculture.

45Q tax credit (45Q)

Provides a progressive production tax credit for CCS (including DACCS, BECCS, and CCUS) reaching a maximum value of $50 USD per ton CO2 storage and $35 per ton for beneficial reuse in 2026 and adjusted for inflation thereafter. Under current provisions, qualified facilities can claim the credit for up to 12 years after being in service.

absorber

A unit operation used in the chemical industry to separate gases. In an absorber, the gas comes in contact with a specific solvent that will selectively scrub (remove) the target contaminant from the gas stream. For absorbers used in carbon dioxide (CO2) avoidance or removal, the target contaminant is CO2. Absorbers themselves can be configured in many ways, including spray columns, packed columns, or bubble columns. (See also: solvent)

absorption

When atoms, molecules, or ions enter the liquid or solid phase of a sorbent. (See also: solvent)

adsorption

The adhesion of atoms, molecules, or ions to a surface, creating a thin film of the adsorbate (the chemical species that is doing the adhering) on the surface of the adsorbent (the species that is being adhered to).

Advanced Research Projects Agency-Energy (ARPA-E)

United States federal agency which advances high-potential, high-impact energy technologies that are not yet ready for private sector investment. ARPA-E awardees are unique in that they are developing novel ways to generate, store, and use energy.

aerobic ecosystem

An aerobic environment is one characterized by the presence of free oxygen (O2), in contrast to an anaerobic environment, which is one devoid of O2. (Elgamal, 2016).

Additionality

Evaluates the degree to which an intervention – for example, a CDR project –  causes a climate benefit above and beyond what would have happened in a no-intervention baseline scenario. By definition, this counterfactual baseline scenario cannot be directly observed (because it did not happen), so can only be estimated or inferred based on contextual information. Additionality can be assessed at the level of individual projects or protocols that define categories of projects. In policy regimes such as cap-and-trade programs, where emissions are permitted in exchange for reduction or storage elsewhere, failures of additionality result in increased emissions.

afforestation

The creation of a new forest in an area that was not previously forested. (IPCC SR 1.5 Glossary).

agricultural byproduct/agricultural residues

Low-value biomass resources that are derived from the cultivation of primary cash crops and do not typically have widespread commercial applications. Examples include rice straw, wheat straw, rice husks, and corn stover, which in many places are left on the fields after harvests, used for fodder and landfill material, or burnt. (See also: stover). (Adhikari et al., 2018).

agriculture, forestry and other land use (AFOLU)

A term used in the 2006 Intergovernmental Panel on Climate Change (IPCC) guidelines, which describes the anthropogenic greenhouse gas emissions from two distinct sectors: Agriculture and LULUCF (Land Use, Land Use Change and Forestry).

agroforestry

The intentional integration of trees and shrubs into crop and animal farming systems to create environmental, economic, and social benefits. Agroforestry has been practiced in the United States and around the world for centuries. (USDA, 2019).

air contactor (contactor)

A structural component of direct air capture systems that provides a means for the capture media, e.g. basic solvents or sorbents, to make contact with and chemically bind carbon dioxide from ambient air.

air separation unit

Technology used to isolate and produce nitrogen or oxygen, and often co-produce argon

albedo

The proportion of the incident light or radiation that is reflected at the Earth's surface.

alkalinity

The capacity of a solution to resist acidification. Differs from basicity, which is an absolute measurement on the pH scale. For carbon mineralization, alkalinity (or alkaline potential) is measured by the presence of alkaline divalent cations (e.g. calcium and magnesium ions). 

allocation method

A collection of methods used in life cycle analysis that partition the input and/or output flows of a process to the product system that is under study. Allocation methods are used when a system is multifunctional (or produces more than one product). These methods provide a framework to divide the impacts of a product system across multiple co-products. (See also: co-product)

ambient weathering

Weathering that occurs naturally under normal environmental conditions. 

anaerobic ecosystem

An ecosystem or environment characterized by its lack of free oxygen (O2) but, which may contain atomic oxygen bound in compounds such as nitrate (NO3), nitrite (NO2), and sulfites (SO3). (Antonietti, 1999)

annualized cost

The cost paid annually on a capital cost loan, determined by the loan amount, loan period, and interest rate. 

anthropogenic emissions (anthropogenic carbon emissions)

Human activities, such as burning fossil fuel, deforestation, and livestock, that result in an overall increase in carbon dioxide emissions. 

artificial upwelling

Pumping nutrient-rich water to the surface to increase phytoplankton activity. This increases the uptake of carbon dioxide into the ocean. 

asbestiform

Fibrous, crystalline, durable material that can be toxic and highly dangerous to the human body. Asbestos is the common name for asbestiform minerals.

avoided emissions

Permanently storing what would have been a CO2 emission in order to avoid an increase in atmospheric CO2. Note that this is distinct from CDR because the source of CO2 is an existing emission rather than the atmosphere. The climate benefit of such an intervention depends on the counterfactual scenario — what would have happened without our action? — which can only be estimated, not observed. If a project claims to avoid emissions that weren’t going to happen anyway, and other emissions are allowed to continue in exchange, then an avoided emission project can do more damage to the climate than doing nothing at all.

bioenergy with carbon capture and storage (or sequestration) (BECCS)

The process of extracting bioenergy from biomass and capturing and storing the emitted carbon, thereby removing it from the atmosphere.

biochar

A stable solid rich in carbon that is made from organic waste material or biomass that is partially combusted in the presence of limited oxygen. The qualities that make up biochar vary depending upon the material from which it is made (feedstocks, i.e., timber slash, corn stalks, manure, etc.) and the temperature at which combustion occurs. (USDA, 2020)

biological carbon stock (biological stock)

Carbon that is fixed and mediated through biological processes, such as photosynthesis or respiration. For example, these stocks would include soil organic carbon and carbon stored in biomass. 

bio-oil

Pyrolysis oil, sometimes known as bio-crude or bio-oil, is a synthetic fuel under investigation as a substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500° C with subsequent cooling. (David et al., 2010)

biomass

Plant (or animal) material used as fuel. Examples include wood, agricultural residues such as stover, or crops grown for the purpose of bioenergy production, such as switchgrass.

biomass storage (or sequestration)

The conversion of biomass into derived materials with more durable storage than the source biomass, including the use of pyrolysis to convert biomass into bio-oil (fast pyrolysis) or biochar (slow pyrolysis).

blue hydrogen

Hydrogen produced from natural gas through steam methane reforming coupled to carbon capture and storage.

brecciated rock

Rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, which can be similar to or different from the composition of the fragments.

British thermal units (BTU)

A measure of the energy content in fuel, used in the power, steam generation, heating, and air conditioning industries. 1 mmBTU refers to one million British thermal units.

burial/carbon burial

Burial in sediments removes organic carbon from the short-term biosphere-atmosphere carbon cycle, and therefore prevents greenhouse gas production in natural systems.

calciner

A reaction vessel in which calcination occurs. Calcination is a process by which ore is heated to  temperatures high enough to decompose, but not melt, the ore. For example, calcium carbonate is calcined near 900º C to produce carbon dioxide and calcium oxide. 

capacity factor

The ratio of the actual operating capacity of a facility divided by the maximum operating capacity of the facility over a given period. for example, if a facility operates 24 hours a day, 7 days a week for 47 weeks out of a year, the capacity factor is (24x7x47)/(24x7x52.14) = 90 percent.

cap-and-trade-system

A market-based policy which establishes a ‘cap’ on the total allowable (permitted) emissions for entities covered under the policy. The policy allows covered entities to either reduce emissions or purchase permits (through auctions or from other covered entities) to achieve compliance. The cap declines over time to reduce emissions from covered entities. 

capillary trapping (residual gas trapping)

A secondary carbon dioxide trapping mechanism in subsurface reservoirs that renders the carbon dioxide gas immobile in the pore space via capillary forces. These forces are governed by fluid and interfacial physics and act at the spatial scale of rock pores. (See also: pore space)

capital expenditures

Monetary start-up costs used to implement an asset.

capital recovery factor

The ratio of a constant annuity to the present value of receiving that annuity for a given length of time.

caprock

An impermeable rock type overlying a porous rock type that acts as a reservoir. The pores of the porous rock are commonly filled with brine or hydrocarbons, and the caprock prevents fluids and gas from rising to the surface.

captive carbon dioxide (captive CO2)

CO2 that has been captured as a byproduct of industrial production for the purpose of internal reuse.

carbon accounting

The process of quantifying carbon dioxide and other greenhouse gas emissions throughout a system or product's lifecycle.

carbon capture and storage (CCS)

The process of capturing carbon dioxide formed during power generation and industrial processes and storing it so that it is not emitted into the atmosphere. Carbon capture and storage technologies have significant potential to reduce carbon dioxide emissions in energy systems.

carbon capture and utilization (CCU)

The process of capturing carbon dioxide to be recycled for further usage.

carbon capture, utilization and storage (CCUS)

A term encompassing methods and technologies to remove carbon dioxide from flue gas and from the atmosphere, followed by recycling the carbon dioxide for utilization and determining safe and permanent storage options.

carbon dioxide removal (CDR)

Activities that remove CO2 from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. CDR includes enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO2 uptake not directly caused by human intervention.

CDR approach

Refers to methods, which may not necessarily be complete CDR systems, that can achieve carbon dioxide removal. For example, DAC in the absence of permanent storage is a CDR approach, but is not on its own a CDR system.

carbon cycle

The carbon cycle concerns the residence time and flux of carbon – in various chemical states – between the ocean, land, terrestrial biosphere, atmosphere, and geological formations in the Earth.

carbon dioxide equivalent (CO2eq)

Describes the impact of a given GHG (CO2, CO, CH4, N2O, etc.) by converting its mass to the equivalent mass of CO2 that would have the same global warming effect. The mass of a GHG is converted to the mass of CO2eq based on the GHG molecule’s potential to affect global warming, or its global warming potential (GWP). The GWP takes into account both the radiative forcing effect of the GHG and the gas’ lifetime in the atmosphere, and is dependent on the time horizon, which is most commonly 20 years (GWP20) or 100 years (GWP100). These values are different because the GWP is time-integrated and the GWP of CO2 is always 1, regardless of the time horizon. (See Chapter 4, Supplement 4.1.)

carbon flux

The amount of carbon exchanged between the land, ocean, living things (i.e. plants, animals, fungi, bacteria, etc.), and the atmosphere. 

carbon intensity

The amount of carbon or carbon equivalent emissions – typically measured over the entire lifecycle – related to the unit production of a service or good.

carbon offsets

Programs or policy regimes in which companies or individuals pay for activities that result in emissions reductions or CDR. In voluntary offset programs, individuals or companies pay project developers (or similar) directly to implement some activity that results in emissions reductions or CDR. In compliance offset programs, such as cap-and-trade programs, companies that are responsible for large amounts of emissions are allowed to continue to emit above a certain cap in exchange for projects taking place elsewhere that reduce emissions or remove carbon. In a compliance regime, an offset has no effect on total emissions in the best-case scenario, and will result in more emissions than would have occurred otherwise if the project is ineffective in any way (e.g. due to failures of additionality, permanence, etc.).

carbon sink

A reservoir, natural or engineered, that has the capacity to store more carbon than it releases over a certain period of time.

carbon uptake

The flux of carbon from the atmosphere to other terrestrial subsystems.

catalytic hydrogenation

The addition of hydrogen over a reactive pi-bond (double or triple bond) in hydrocarbons. Typically performed over a transition metal catalyst to improve kinetics. 

cement hydration

A chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.

chemical species

Describes a group of atoms or molecules which are identical to one another. For example, a cylinder of pure carbon dioxide only contains molecules of the same species. 

chemiophysical

Both chemical and physical; relating to both the chemistry and the physics of a substance or organism.

clarifier

Settling tank used to continuously remove solids from a liquid.The collected solids are either discharged from the bottom of the tank (if they are concentrated and denser than the liquid) or removed from the top (if the particles float on the surface). 

climate tipping points

abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

clinker

A solid material produced in the manufacture of Portland cement as an intermediary product. It is the primary contributor to carbon dioxide emissions in concrete production.

Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA)

United States federal legislation  passed by Congress in 1990 to fund wetland enhancement. 

Coastwide Reference Monitoring System (CRMS)

A system designed to monitor the effectiveness of restoration actions at multiple spatial scales – from individual projects to the influence of projects on the entire coastal landscape. 

co-product

In life cycle analysis (LCA), a co-product is sometimes produced alongside the primary product of investigation. Co-product allocation is performed to distribute emission burden across each co-product based upon the scope of the LCA. (See also: allocation method)

carbon mineralization (carbon dioxide mineralization, CO2 mineralization, mineral carbonation or enhanced weathering)

A process in which minerals in rocks react with carbon dioxide, resulting in the formation of new carbonate minerals and permanent storage of carbon dioxide.

coastal blue carbon

The carbon captured by living coastal and marine organisms and stored in coastal ecosystems ((such as mangroves, tidal marshes, seagrass meadows, and other coastal habitat) that facilitate the long-term confinement of carbon in plant materials or sediment. Also refers to carbon dioxide removal techniques that utilize such ecosystems to increase carbon-removing biomass and, in particular, soil carbon. (NOAA, 2020)

compliance carbon market

Compliance carbon markets are marketplaces through which regulated entities obtain and surrender emissions permits (allowances) or offsets in order to meet predetermined regulatory targets.

concentrated solar power (CSP)

Concentrated solar power (also known as concentrating solar power or  concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight onto a receiver.

concentrated solar power on demand

A system is designed to absorb concentrated solar radiation in a molten eutectic salt.

converted lands

Terrestrial landscapes or freshwater systems already impacted by human activity (e.g. human settlements, agricultural lands, roads, and dams). (Baruch-Mordo et al (2019))

cracking

A catalytic process whereby large, complex organic molecules such as kerogens or long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons through the addition of hydrogen at high temperature and pressure.

cradle-to-gate

A life cycle assessment boundary describing those emissions generated as a result of upstream extraction and transport of raw materials through direct processing. This does not include contributions from downstream transport, consumption and/or end-of-life. (See also: life cycle analysis, cradle-to-grave, gate-to-gate)

cradle-to-grave

A life cycle assessment boundary describing all emissions and impacts that result over the entire system or product life cycle. This includes upstream extraction and transport of raw materials, direct processing, downstream transport, consumption, and/or end-of-life. (See also: life cycle analysis, cradle-to-gate, gate-to-gate)

cropland data layer (CDL)

A raster, geo-referenced, crop-specific land cover map for the continental United States, hosted on CropScape.

cross-laminated timber (CLT)

A wood panel product made from gluing together layers of solid-sawn lumber, i.e., lumber cut from a single log.

deep saline formations

Porous rock formations located from near the surface to several kilometers in depth, whose porosity is filled with brine.

degraded land

Land negatively impacted (e.g. by loss of biodiversity or the decrease of organic carbon in soils) by human activity and natural processes (e.g. fires, drought, land use changes, or peatland drainage).

deoxygenation

A chemical reaction where oxygen atoms are removed from a molecule. This can also refer to the removal of molecular oxygen from a gas or liquid solvent stream. 

United States Department of Agriculture (USDA)

The United States federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food.

United States Department of Energy (DOE)

A cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material. 

direct air capture (DAC)

A chemical process that removes carbon dioxide from ambient air. When paired with carbon storage strategies, sometimes referred to as direct air carbon capture and storage (DACCS).

direct emissions

Emissions that are directly controllable and produced from within the system boundary. 

discount rate (bank rate)

Rate of interest charged by banks on loans.

electrolysis

Using an input of electric current to drive an otherwise non-spontaneous redox reaction (the transfer of electrons between chemical species).

embodied emissions

Emissions that result from the production or use of any good, or the provision of any service. For example, the embodied emissions of steel used for a reactor will include emissions associated with the acquisition of raw materials, processing, manufacturing, transportation, and the energy used in steel production.

emissions mitigation

A human intervention to reduce or avoid greenhouse gas emissions. Some publications have used the broad term “mitigation” to refer exclusively to emissions mitigation (CITE), whereas others have used mitigation (or “climate change mitigation”) to include missions mitigation, CDR, and any other strategies for combating climate change. (IPCC).

energy crop

Crops that are cultivated primarily for the purpose of producing energy feedstocks. 

United States Energy Information Administration (EIA)

A principal agency of the U.S. Federal Statistical System responsible for collecting, analyzing, and disseminating energy information to promote sound policymaking, efficient markets, and public understanding of energy and its interaction with the economy and the environment.

enhanced oil recovery (EOR)

Injection of supercritical carbon dioxide into oil reservoirs for the purpose of increasing the amount of oil recovered beyond primary (conventional) extraction.

United States Environmental Protection Agency (EPA)

An independent executive agency of the United States federal government tasked with environmental protection matters. 

equity

The principle of fairness in access to opportunities, power-sharing, and burden-sharing. Equity is crucial to determining how to deploy strategies to address climate change – including CDR – that minimize harm to marginalized people and frontline communities. From the World Resources Institute: “Climate change poses the greatest threat to those that are the least responsible – generally people that are already vulnerable to deep-rooted challenges such as poverty. Conversely, those who have contributed the most to climate change have much better capacity to protect themselves from its impacts. As the effects of climate change mount, so does the urgency of addressing this equity challenge.” (IPCC).

evapotranspiration

The process by which water is transferred from land to the atmosphere by evaporation from soil and other surfaces and by transpiration from plants. (USGS, 2020).

European Union (EU)

A political and economic union of 27 member states that are located primarily in Europe. 

ex-situ carbon mineralization

Carbon mineralization performed with alkaline feedstock that has been removed from its original location and state (i.e., the Earth). 

exergy

The measure of useful work that can be extracted from a system as it comes into equilibrium with its surroundings. 

fault

A crack in the Earth’s crust.

faulting

Creation of a fault or a system of faults due to mechanical stresses in the Earth's crust. Faulting in geologic formations is observed at several spatial scales, from micrometric to pluri-kilometric. (See also: fault)

feedstock

A raw material that can be converted to a good of higher value (i.e. fuel).

first-of-a-kind (FOAK)

A set of acronyms used in engineering economics where the first item or generation of items using a new technology or design can cost significantly more than later items or generations.

fold

A geological term referring to undulation or waves in a stack of originally planar surfaces of the Earth, such as sedimentary strata.

fold belts

A series of mountainous foothills made of a series of folds and adjacent to an orogenic belt (or mountain range). (See also: fold, folding)

folding

Creation of one or several folds due to mechanical stresses in the Earth's crust. Like faulthing, folding in geologic formations is observed at a variety of spatial scales.

flux

the rate of movement of greenhouse gases between reservoirs – for example, capturing CO2 through the chemical reactions of photosynthesis, resulting in carbon storage within a plant’s biomass.

frontline communities

some climate justice advocates have described these as communities that experience the “first and worst” consequences of climate change. These are often communities of color and those with low income levels that have insufficient infrastructure (such as well-maintained roads and up-to-date flood protection), and which will be increasingly impacted as our climate deteriorates. These may include Native communities whose resources have been exploited, or communities of laborers whose daily work or living environments are polluted or toxic.

gasification

The conversion of any carbon-based feedstock into synthesis gas or syngas (such as carbon monoxide and hydrogen gas) through reaction with a controlled amount of oxygen, typically at high temperature and pressure.

gate-to-gate

A life cycle assessment boundary describing only those emissions and impacts generated as a result of direct processing and excluding contributions from the upstream extraction and transport of raw materials as well as downstream transport, consumption, and/or end-of-life. (See also: life cycle analysis, cradle-to-gate, cradle-to-grave)

geoengineering

This term refers to the concept of deliberately manipulating the climate system in order to reduce the impacts of climate change. It is usually used in reference to solar radiation management (SRM), a controversial strategy that involves the injection of  material into the atmosphere that directly affects radiative forcing to reduce warming, but does not change concentrations of greenhouse gases. CDR has been inaccurately conflated with geoengineering in some literature.

geologic formation 

A body of rock having a consistent set of physical characteristics that distinguish it from adjacent bodies of rock. This body of rock must be distinctive enough to be identifiable, and thick and extensive enough to be plotted on geological maps.

geological sequestration (or storage) of carbon dioxide

Injection of carbon dioxide into porous geologic formations deeper than 800 meters (and preferably 1000 meters) for long-time storage (>1000 years). The carbon dioxide is kept at depth by various trapping mechanisms: mechanical trapping (impermeable caprock over the reservoir), residual gas trapping (capillary forces), solubility trapping (dissolution in brines or hydrocarbons), and mineralization (formation of carbonates). It is the last step of engineered carbon dioxide removal approaches that capture carbon from flue gas (bioenergy with carbon capture and storage) or from the air (direct air capture and storage).

Gigatonne of carbon dioxide

(GtCO2) – refers to a billion metric tonnes (metric tons) of CO2, which is equivalent to 1015 g. 1 GtCO2 is equivalent to 0.273 gigatonnes of carbon (GtC). This unit of measurement is used most frequently when discussing the scale of CDR required to prevent the worst impacts of climate change and keep warming below 1.5° C (i.e., gigatonne-scale CDR).

United States Geological Survey (USGS)

The nation's largest water, earth, and biological science and civilian mapping agency. It collects, monitors, analyzes, and provides scientific understanding of natural resource conditions, issues, and problems.

grain size

The diameter of individual grains of sediment. Grain size can vary within a sample of sediment, and can be expressed as d(0.8), or the diameter which is larger than 80 percent of the grains in a sample. 

greenhouse gas (GHG)

Those gaseous constituents of the atmosphere, both natural and anthropogenic (human-caused), that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. This property causes the greenhouse effect, whereby heat is trapped in Earth’s atmosphere. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3) are the primary GHGs in the Earth’s atmosphere. There are also a number of entirely human-made GHGs in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, managed under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol). Besides CO2, N2O and CH4, the Kyoto Protocol deals with the GHGs sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).

geographical information system

A computer system for capturing, storing, checking, and displaying geographic data related to positions on the Earth's surface.

global timber model (GTM)

An economic model capable of examining global forestry land-use, management, and trade responses to policies.

global warming potential (GWP)

The heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide.

Gulf Coast Carbon Center (GCCC)

A research center associated with the University of Texas that studies geological sequestration of carbon dioxide.

hard-to-avoid emissions

Emissions which are either physically extremely difficult to eliminate within a certain timeframe (e.g., because of dependence on a particular infrastructure with a long lead time for carbon-free substitution, or because avoidance would require a technology that relies on a scarce resource) or which would be unacceptable to avoid from a social justice perspective (e.g., if mitigation would deprive people of the means to satisfy their basic needs, like food security).

higher heating value (HHV)

A measure of the total energy released upon combustion of a material plus that which is released through the heat of condensation of any combustion byproducts. Also known as gross energy, upper heating value, gross calorific value, or higher calorific value.

highly-deformed sedimentary basin

Sedimentary rock layers deformed by numerous faults and folds.

hydrocarbon field

Porous rock (reservoir rock) containing oil and/or natural gas that can be commercially recovered, with at least one common reservoir for the entire area.

hydrologic cycle (or water cycle)

The continuous circulation of water in the Earth-atmosphere system.

hydrology

Branch of science that studies the movement, distribution, and management of water in the atmosphere, on the surface, and in the subsurface of the Earth and other planets. This includes the water cycle, water resources, and watersheds.

igneous rocks

Any rock that has resulted from the solidification of a molten or partly molten material (i.e. magma).(Blatt et al., 2006)

improved forest management (IFM)

An agriculture, forestry and land use project category in the Verified Carbon Standard (VCS), improved forest management (IFM) refers to land management practices designed to increase the quantity of carbon stored in forests relative to baseline conditions (e.g., by modifying harvest schedules).

in-situ carbon mineralization

Carbon mineralization performed with feedstock that remains in its original place (i.e., the Earth). Typically, aqueous carbon dioxide is circulated into a deep geologic formation, where it mineralizes to form solid carbonate minerals.

indirect emissions

Emissions that occur as a result of the system activity, but do not occur within the system boundary.

injection

Physical means by which concentrated carbon dioxide (usually supercritical CO2) is transported deep into the Earth's subsurface for the purpose of storage or EOR.

inside battery limits (ISBL)

Includes all equipment and associated components and instrumentation of a given operation. The battery limit outlines the limit of responsibility for the plant operators. (See also: outside battery limits).

integrated assessment model (IAM)

An approach that integrates social, economic, and physical models for the purpose of understanding how different decisions drive emissions and emission mitigation

Intergovernmental Panel on Climate Change (IPCC)

An intergovernmental body of the United Nations that is dedicated to providing the world with objective scientific information relevant to understanding the scientific basis of the risk of human-induced climate change, its natural, political, and economic impacts and risks, and possible response options.

International Energy Agency Greenhouse Gas Research and Development Programme (IEAGHG)

An international research body established in 1991, evaluating technologies to reduce greenhouse gases from the use of fossil fuels, with a focus on carbon capture and storage.

International Standards Organization (ISO)

An international standard-setting body composed of representatives from various national standards organizations.

kilowatt-hour (kWh)

A standardized measurement of energy over time.

knowledge base uncertainty

A knowledge base is the underlying data or assumptions that make up the knowledge in a certain analysis or, more broadly, a given field of study. Knowledge base uncertainty refers to the uncertainty that is built into these assumptions, on account of lack of knowledge, bias data, or similar causes.

Kyoto Protocol

A 1997 agreement that operationalizes the United Nations Framework Convention on Climate Change (UNFCCC) committing industrialized countries and economies in transition to limit and reduce greenhouse gas emissions in accordance with agreed-upon individual targets. (UNFCCC).

Large-scale CDR

Carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

latent heat of vaporization

Energy released (upon condensation) or absorbed (upon evaporation) by a body or a thermodynamic system.

leakage (physical)

Refers to initially-stored CO2 (or other GHG) that has left its storage state and returned to the atmosphere. Examples may include combustion of a fuel made from CO2, burning of biomass, or migration of CO2 from underground storage.

leakage (socioeconomic)

occurs when CDR activities displace emissions to other locations, times, or forms. For example, leakage occurs in forest carbon offset credit programs when a reduction in timber harvesting at a project site, incentivized due to its potential for emissions reductions and/or CDR, causes timber harvesting to increase somewhere else to meet demand. Similarly, if an engineered CDR approach coupled to CO2 utilization results in higher costs, alternatives that emit more CO2 may become economically favorable by comparison.

learning rate

The rate at which a technology becomes more efficient in use of resources (both physical and monetary) as the number of produced units, or installed capacity increases. 

life cycle analysis (LCA)

An analysis of the balance of positive and negative emissions associated with a certain process, which includes all of the flows of CO2 and other greenhouse gases, along with impacts to other environmental or social impacts of concern. This analysis also includes greenhouse gas emissions that result from the materials used to construct a given process (commonly referred to as embodied emissions), as well as from the energy resource used to meet the energy demands of the process.

lignin

A complex organic polymer found in the cell walls of many plants, making them rigid and woody.

lignocellulose/lignocellulosic crops

Refers to plant dry matter (biomass) such as wood.  It is the most abundantly available raw material on Earth for the production of biofuels, mainly bioethanol.

low-carbon fuel standard (LCFS)

A rule enacted to reduce carbon intensity in transportation fuels as compared to conventional petroleum fuels, such as gasoline and diesel. 

low energy demand scenario (LED)

Refers to scenarios that limit global warming to 1.5° C, describing major transformations in energy supply and ever-rising energy demand.

lower heating/calorific value (LCV)

A calculation assuming that the water component of a combustion process is in vapor state at the end of combustion.

mafic rock

Igneous rock that contains relatively high amounts of magnesium and iron and is silica-poor (approximately 45 – 52 percent SiO2) – i.e., basalts and gabbros.

megajoule

A unit of energy equal to 1 million joules.

megawatt

A unit for measuring power that is equivalent to one million watts. One megawatt is equivalent to the energy produced by 10 automobile engines.

merchant CO2

carbon dioxide that is captured for the purpose of offsite distribution to other industries. 

metamorphic rock

Rock transformed by metamorphism (i.e. mineralogic or textural change that occurs in a rock in the solid state as a response to changes in environmental variables, especially temperature and pressure). (Blatt et al., 2006).

methanogenesis

Production of methane by archaea (microbial organisms distinct from eukaryotes and bacteria) known as methanogens.

mine tailings

Unwanted rock material that has been processed and rejected by a mining operation. Tailings are typically stored at a mine in ponds or piles, where they may remain for years or decades.

mineral dissolution

The rearrangement or destruction of a mineral's molecular lattice structure upon prolonged contact with a solvent, allowing ions to leave the lattice structure and dissolve into solution.

monoculture

The practice of cultivating only one agricultural crop or livestock at a time in a given area.

Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol)

A 1987 international treaty that regulates the production and consumption of nearly 100 man-made chemicals referred to as ozone-depleting substances. (UNEP).

moral hazard

An ethical concern whereby skeptics of potential CDR systems worry that emission reduction will be reduced or delayed on the promise of future CDR deployment. Experts agree that most temperature stabilization scenarios require ambitious efforts both on greenhouse gas mitigation and CDR, and that the potential for large-scale deployment of CDR is not an acceptable basis to justify slowing down the pace of mitigation.

National Academies of Science, Engineering and Technology (NASEM)

A United States-based non-governmental organization that provides independent, objective advice to inform policy with evidence, spark progress and innovation, and confront challenging issues for the benefit of society.

Negative emissions

occurs when a sink  – created or enhanced by human activity – removes greenhouse gases from the atmosphere (in this primer, only CO2 is considered).

negative emissions technologies (NETs)

This term is not used in the primer, but has been used as an alternate term for CDR approaches in other publications.

net-carbon balance

The net carbon emissions or reductions resulting from the sum of carbon uptake and emissions.

net-zero emissions

achieved when more greenhouse gases are removed from the atmosphere than are emitted into it.

non-highly-deformed sedimentary basins

An ideally non-deformed sedimentary basin is made of a series of overlapping horizontal layers of sedimentary rocks. In a non-highly-deformed basin, these sedimentary layers underwent slight deformation by faulting and folding.

ocean alkalinity enhancement

Processes that increase the amount of alkalinity (or charge balance of ions) in the ocean, resulting in a carbon dioxide flux from the air to the ocean and an increase in dissolved inorganic carbon concentration.

ocean carbon

Carbon that is in ocean waters, namely dissolved carbon dioxide, bicarbonate and carbonate ions, dissolved or particulate organic carbon, and particulate inorganic carbon.

ocean fertilization 

Promoting the growth of photosynthetic organisms (i.e. macroalgae) for increased carbon uptake  by enhancing ocean conditions for uptake with iron, nitrates or phosphates.

offsets for emissions trading systems

A certifiable reduction, removal, or avoidance of greenhouse gases in one process that can be sold or traded for the purpose of lowering the overall greenhouse gas footprint of another process. 

olivine

A magnesium iron silicate that occurs naturally in basalt, peridotite, and other rocks. It has two endmember compositions, an iron silicate called fayalite and a magnesium silicate forsterite.

outside battery limits (OSBL)

Includes all equipment and associated components and instrumentation of a given operation. The battery limit outlines the limit of responsibility for the plant operators. (See also: inside battery limits)

operating cost

The total cost of operation, including payroll, maintenance, overhead, and other day-to-day costs associated with maintaining a facility or business, including energy costs and chemicals. These are the costs repeatedly incurred by a facility over its lifetime.

ophiolite

Ultramafic rock that originates from the upper mantle of the Earth’s crust, consisting largely of serpentine.

organic carbon

A measure of the carbon content in soils derived from plant and animal matter. Organic carbon is most abundant at the surface of soils and sediments where detritus is deposited, most of which is derived from aerobic photosynthesis.

overshoot

climate stabilization scenarios in which emissions trajectories exceed their concentration or temperature targets (e.g. 1.5º C or 2º C) early in the century, but use substantial amounts of CDR later to reduce atmospheric CO2 levels and achieve the original targets. See Chapter 1 for a discussion of moral hazards and other challenges associated with these scenarios.

oxy-fired kiln or calcination unit

A kiln or calcination unit that is fired with high purity of oxygen instead of air, where oxygen makes up roughly 21 percent of the air stream. 

Paris Agreement

A 2016 agreement formed by Parties to the UNFCCC to combat climate change and to accelerate and intensify the actions and investments needed for a sustainable low carbon future. (UNFCCC)

parts per million (ppm)

The number of units of mass of a contaminant per million units of total mass. As an example, the concentration of carbon dioxide in the atmosphere is approximately 410 ppm.

passivation

A process by which a material becomes more passive or is less prone to corrosion or reaction. For mineral carbonation purposes, this refers to the formation of a silica-rich outer layer as the carbonation reaction progresses. This layer is deficient in alkali cations and is therefore not reactive for the carbonation process and presents a barrier to diffusion. This layer is typically referred to as a "passivation layer."

peatland

A type of wetland that occurs in almost every country on Earth, currently covering 3 percent of the global land surface. The term ‘peatland’ refers to peat soil and the wetland habitat growing on its surface. In these areas, year-round waterlogged conditions slow the process of plant decomposition to such an extent that dead plants accumulate to form peat. (IUCN, 2020)

perfluorocarbons (PFCs)

Human-made chemicals that consist entirely of carbon and fluorine. Perfluorocarbons are powerful greenhouse gases. 

peridotite

A type of ultramafic rock that contains large amounts of olivine. (See also: olivine)

Permanence (or Durability)

The duration for which CO2 can be stored in a stable and safe manner. Storage dDuration can differ significantly between  reservoirs. For example, storage concentrated CO2 stored in geologic formations deep underground is effectively permanent for thousands of years, whereas forest carbon stocks can release carbon back into the atmosphere due to wildfire or tree harvesting.

permeability

The ability of a membrane to allow a certain gas to pass through it.

phenotype

The observable physical properties of an organism; these include the organism's appearance, development, and behavior. An organism's phenotype is determined by its genotype, which is the set of genes the organism carries, as well as by environmental influences upon these genes. (Nature, 2020)

photovoltaics (PV)

Refers to the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

platinum group elements (PGEs)

Six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum.

pneumatic controller

A mechanical device that measures temperature or pressure, determines any deviation in temperature or pressure from a given setpoint, and transmits an air signal to a final control element to correct for the deviation.

point source

A localized, stationary and concentrated source of pollution, such as a smokestack.

polygeneration plant

A facility that produces multiple outputs or products from a single source, e.g., a coal plant that yields electricity, heat, syngas, and chemicals.

pore space

A characteristic of the porosity of a material, described as the total amount of empty or void space between material grains or particles.

Positive emissions

Occurs when a  particular source – created or enhanced by human activity – adds greenhouse gases to the atmosphere.

(Potential) CDR system

A system with the capacity to generate net-negative emissions, but which will not necessarily do so under all conditions. A comprehensive LCA and other analyses are required to conclusively demonstrate that a “potential” CDR System is, indeed, a CDR System. For example, in the absence of an life cycle analysis (LCA), a CDR system that barely achieves net negative emissions could look the same as one that achieves far greater net climate benefits per tonne of CO2 removed from the atmosphere.

potential of hydrogen

A scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of H+ ions) are measured to have lower pH values than basic or alkaline solutions.

practical salinity units

The number of parts per thousand of sodium chloride (salt) in a given solution (i.e. 1 PSU = 1 gram per kilogram). This unit is based on the properties of sea water conductivity.

pyrolysis

Thermal decomposition of a material in the absence of oxygen.

radiative forcing/radiative effects

Refers to the difference between solar irradiance (sunlight) absorbed by the Earth and the energy radiated back to space. 

rangeland

Lands on which the indigenous vegetation is predominantly grasses, grass-like plants, forbs, and possibly shrubs or dispersed trees. Existing plant communities can include both native and introduced plants. (USDA)

reforestation

The natural or managed restocking of degraded lands that were or naturally would reach a climax community as a forest ecosystem. 

regional seal

Relatively impermeable rock (e.g. shale, anhydrite, or salt), that forms a barrier or cap above a reservoir rock (e.g. sandstone or limestone) such that fluids cannot migrate beyond the reservoir.

representative concentration pathway (RCP)

A greenhouse gas concentration (not emissions) trajectory adopted by the Intergovernmental Panel on Climate Change.

research, development, and demonstration (RD&D)

The set of innovative activities undertaken by corporations or governments in developing new services or products and improving existing ones. Also referred to as research and development (R&D, R+D), and known in Europe as research and technological development (RTD).

residence time

The average length of time that a substance will remain in a given location, condition, or control volume. For example, in reactor design, the residence time is the average time that a species will spend inside the physical reaction vessel. 

Reservoir

Refers to wherever a greenhouse gas is stored. Examples include geological formations, alkaline-containing minerals, forest biomass, and soils.

Safe Drinking Water Act (SDWA)

The principal federal law in the United States intended to ensure safe drinking water for the public.

second law of thermodynamics

Law of physics which states that the total entropy (disorder) of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. (Wilcox, 2012)

sedimentary formation

Geologic formation made of a succession of layers of sedimentary rocks. Sedimentary formations are mapped by geological age.

sedimentary rock

Any rock that has resulted from the accumulation, induration (hardening), and/or cementation of sediments of any grain size (e.g. sandstones, mud rocks, and carbonate rocks).

sequestration (carbon dioxide sequestration)

See geological storage of carbon dioxide.

serpentine

A mineral subgroup formed when peridotite, dunite, and/or other ultramafic rocks undergo hydrothermal metamorphism. The most common serpentine minerals are magnesium-dominant and comprise three species: lizardite, antigorite, and chrysotile. 

shapefile

A geospatial vector data format or file type for geographic information system (GIS) software.

short-lived climate pollutant (SLCP)

Refers to a greenhouse gas that has a relatively short atmospheric lifetime, including methane and hydrofluorocarbons.

silicate

Refers to a type of mineral which makes up about 90 percent of the Earth's crust. Silicates contain varying amounts of silica tetrahedra, where one silicon atom is surrounded by four oxygen atoms. The tetrahedra can be isolated or combined in chains, double chains, sheets, or frameworks, which defines the different types of silicates.

Sink

Any process, activity or mechanism which removes a greenhouse gas, a precursor of a greenhouse gas, or an aerosol from the atmosphere.

slaker

The equipment in which the slaking reaction occurs. The slaking reaction produces calcium hydroxide by mixing solid lime (i.e., calcium oxide) and water (or steam). 

slurry

A mixture of water and fine particles. 

social justice

just or fair relations within society that seek to address the inequitable distribution of wealth, access to resources, opportunity, and support and remove discriminatory systems and structures that block marginalized groups from accessing these benefits on the basis of race, gender, economic status, or any other factor.

soil amendment

Any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration, and structure.

soil carbon dynamics

Changes in the fluxes, forms, and stocks of carbon within soil pools as a result of natural and anthropogenic inputs. 

soil carbon stock

Carbon trapped within soil.

soil carbon storage (or sequestration)

The accrual of carbon within soils (organic and inorganic forms), either naturally or through improved management.

soil organic carbon (SOC)

A measurable component of soil organic matter. Organic matter makes up 2 – 10 percent of most soil's mass and has an important role in the physical, chemical, and biological function of agricultural soils.

soil respiration

The production of carbon dioxide from the respiration of organisms within the soil.

solar radiation management (SRM)

Strategies that aim to reflect the sun’s rays back into space, characterized as geoengineering.

solvent

A substance that dissolves a solute. For example, potassium hydroxide in water is a solvent that dissolves carbon dioxide, a solute. (See also: absorption)

sorbent

A catch-all term for a solid that is used to absorb (absorbent) or adsorb (adsorbent) a given liquid or gas. For example, solid sorbents for direct air carbon capture are typically amine grafted solids which adsorb carbon dioxide on their surfaces. 

sorption isotherm

Represents the amount of material adsorbed as a function of the relative pressure at a constant temperature. There are six International Union of Pure and Applied Chemistry classifications for adsorption isotherms that can indicate certain pore sizes and adsorption potential in the adsorbent material. These isotherms are typically shown graphically. (Sing et al., 1985)

Source

Any process, activity, or mechanism which creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

sparging

The method of using a diffuser to bubble gas into a liquid solution. This allows for a higher surface area of exposed gas that, dependent on the gas solubility, can increase dissolution into the liquid. 

steady state

An unvarying condition in a physical process.

steam methane reformation (SMR)

A method for producing syngas (hydrogen and carbon monoxide) by reacting hydrocarbons with water. 

Stock

A carbon reservoir that exchanges carbon with the atmosphere over relatively fast timescales (e.g. less than 100 years), including the terrestrial biosphere and shallow oceans. By comparison, other reservoirs, such as geologic formations and the deep ocean, exchange carbon over much slower timescales (e.g. 1,000 years).

Storage (or Sequestration)

Two terms that can be used interchangeably to describe the addition of CO2 removed from the atmosphere to a reservoir, which serves as its ultimate destination. For example, some CDR strategies store carbon in biological systems, such as forests or soil ecosystems, whereas others inject CO2 underground or chemically transform CO2 into stable, mineral forms.

storage assessment unit (SAU)

Mappable volume of rock that consists of a porous storage formation (reservoir rock) and an overlying regional seal formation with low permeability. (USGS)

stover

The leaves and stalks of field crops, such as corn (maize), sorghum, or soybeans, commonly left in a field after harvesting the crop.

Subsidiary Body for Scientific and Technological Advice (SBSTA)

A subsidiary body of the United Nations Framework Convention on Climate Change Conference of the Parties.

supercritical

The state of being beyond supercritical temperature and pressure, i.e. above the thermodynamic critical point. 

surface ocean anoxia

The depletion of oxygen in the ocean.

surficial carbon mineralization

Carbon mineralization performed on the surface of a feedstock with a high reactive surface area. Carbon dioxide-bearing fluid or gas is reacted with mine tailings, alkaline industrial wastes, or sedimentary formations rich in reactive rock fragments.

system boundaries

The divide between what is included in and what is excluded from a system of study.

techno-economic analysis (TEA)

A methodology framework to analyze the technical and economic performance of a process, product or service.

technology readiness level (TRL)

A method developed by the National Aeronautics and Space Administration (NASA) used to determine the level of readiness for deployment of a technology, ranging in scale from 0 – 9.

thermodynamic minimum work of separation

The absolute thermodynamic minimum work or energy that must be provided to a system for a reversible, constant-temperature and constant-pressure separation to occur. The minimum work for a separation system is directly correlated to the inlet and outlet purities of carbon dioxide in addition to the percent capture. (Wilcox, 2012; Wilcox et al., 2017).

turbidity

Measure of the level of particles (e.g. sediment, plankton, or organic by-products), in a body of water. (OSNOAA)

ultramafic rock

Any rock that is composed of ≥90 percent ferromagnesian minerals. They typically have high MgO content (>18 percent), and are ideal feedstock for carbon mineralization. Examples include peridotite, kimberlite, and dunite.

United Nations (UN)

An  intergovernmental organization that aims to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a center for harmonizing the actions of nations.

United Nations Environment Programme (UNEP)

A program responsible for coordinating the United Nations’ environmental activities and assisting developing countries in having environmentally sound policies and practices.

voluntary carbon market

Avoided or removed carbon dioxide emissions (quantified as offset credits) that are exchanged through marketplaces that are not created or utilized for policy compliance. Individuals and companies procure carbon credits through these markets on an entirely voluntary basis.

water-gas shift reaction

Describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen.

weighted average cost of capital

The projected return rate for investors while financing a company or project.

wetland

Areas where water covers the soil or is present either at or near the surface of the soil either all year or for varying periods of time during the year, including during the growing season. (EPA)

wollastonite

Calcium silicate mineral (CaSiO3) that is a product of a reaction commonly at the boundary of granitic intrusions and limestone.

working fluid

Gas or liquid that primarily transfers force, motion, heat, or mechanical energy.

World Business Council for Sustainable Development (WBCSD)

A CEO-led organization of over 200 international companies that support sustainable business practices.

World Resources Institute (WRI)

A global research non-profit organization established in 1982 with funding from the MacArthur Foundation under the leadership of James Gustave Speth. WRI's activities are focused on seven areas: food, forests, water, energy, cities, climate, and ocean.