Chapter 4

Analysis and Quantification of Negative Emissions

Authors

Noah McQueen, Ben Kolosz, Pete Psarras, and Colin McCormick

To assess the potential of CDR technologies and systems, we must first define what constitutes negative emissions and how we evaluate the system boundaries, cost, and net carbon balance of such systems. According to a 2019 paper by Tanzer and Ramírez, there are four main criteria that qualify systems as carbon dioxide removal (Tanzer and Ramírez, 2019).

  1. Greenhouse gases are removed from the atmosphere. 

  2. The removed gases are stored out of the atmosphere in a manner intended to be permanent.

  3. Upstream and downstream greenhouse gas emissions associated with the removal and storage process, such as emissions from land use change, energy use, unintended emissions from industrial processes (fugitive emissions), gas fate, and co-product fate, are comprehensively estimated and included in the emission balance.

  4. The total quantity of atmospheric greenhouse gases removed and permanently stored is greater than the total quantity of greenhouse gases emitted to the atmosphere.

A CDR system must meet these four criteria. In addition to CDR systems, there are systems that either avoid emissions or reduce carbon dioxide emissions (Sutter et al., 2019). These systems are commonly confused with CDR systems. Technologies that avoid emissions may produce lower emissions than a reference system delivering an equivalent product or service. Take, for example, the application of carbon capture and sequestration (CCS) to an existing gas-fired power plant. In this scenario, carbon capture reduces the amount of CO2 emitted from the power plant per unit of electricity generated, but increases emissions upstream (due to increased fuel use) and downstream (through transport and storage of CO2) (Cuéllar-Franca and Azapagic, 2015). This results in net emissions reduction (or avoidance of emissions) since the power plant with CCS emits significantly less CO2 per unit of electricity generated, than the same plant without carbon capture. However, even with CCS, this system still results in net emissions to the atmosphere. Therefore, this application of CCS is not classified as a CDR system. 

Within the energy sector, there are technologies that result in essentially zero CO2 emissions during electricity generation. For example, solar and wind electricity do not emit CO2 to the atmosphere during use. However, these technologies do not remove CO2 or other greenhouse gases from the air. Additionally, PV cell and wind turbine manufacturing creates emissions, which must be accounted for. Therefore, these systems are not classified as CDR systems, either.

Finally, it is possible to implement net-zero CO2 emissions processes, which remove as much CO2 from the atmosphere as they emit over their lifetime (Hertwich et al., 2015). As an example, the production of a liquid fuel from biomass could be carbon-neutral with the implementation of CCS. The biomass feedstock inherently contains carbon dioxide removed from the atmosphere via photosynthesis. Converting the biomass to fuel could capture and store some of that carbon dioxide. The storage of atmospheric CO2 from the biomass feedstock has the potential to offset emissions associated with the energy required to convert the biomass to liquid fuel, or the CO2 emitted elsewhere in the supply chain (e.g., production of fuel conversion and processing facilities). To emphasize the importance of system boundaries on negative emissions, Section 4.1.5 contains an example of a liquid solvent-based approach used for direct air capture, where the captured CO2 is ultimately stored in an underground geologic formation.

Supplement

4.1

GWP and Time Horizons

While you are most likely to hear about CO2, other greenhouse gas emissions also cause climate change. Different greenhouse gases have different effects on Earth’s warming. The two primary ways that these gases differ from one another are their radiative forcing (or the difference between the adsorption and emission properties of the gas) and their lifetime (how long a gas stays in the atmosphere) (United States Environmental Protection Agency [EPA], 2017).

A gas’s global warming potential (GWP) allows us to compare the warming effects of greenhouse gases by relating how much energy 1 tonne of a greenhouse gas will absorb over a period of time to how much energy 1 tonne of CO2 absorbs. In technical terms, the GWP is the time-integrated radiative forcing due to a pulse emission of a given component relative to a pulse emission of an equal mass of CO2 (Intergovernmental Panel on Climate Change [IPCC], 1990). This means that the GWP of CO2 is always 1. The mathematical definition of GWP is provided in Equation 4.1. 

There are two time periods (or time horizons) commonly used to evaluate the GWP: 20 years (GWP20) and 100 years (GWP100). Since the GWP is time-integrated, the time horizon of the GWP impacts its value. Therefore, for compounds with shorter atmospheric lifetimes, the GWP will decrease with longer time horizons relative to CO2. For example, methane has a relative short lifetime in the atmosphere compared to CO2 (IPCC, 1990). Therefore, the GWP20 for methane of 84 is larger than the GWP100 of 28.

The GWP is used to adjust the emissions of different greenhouse gases to a basis of carbon dioxide equivalent, or CO2eq. This allows for a direct comparison of these emissions to CO2, providing a means to directly compare the climate impact. This impact can be seen by comparing the rate of emissions for non-CO2 greenhouse gases, to those adjusted to CO2eq (as shown in Figure 4.1).

Some critics of the conventional GWP method for determining CO2eq argue that the method misrepresents the global temperature impact of short-lived climate pollutants (or greenhouse gases that have much shorter lifetimes than CO2 in the atmosphere). Methods to address the issues associated with applying conventional GWP methods to these short-lived climate pollutants, as well as other methods to better quantify the climate impact of greenhouse gas emissions, is ongoing (Allen et al., 2018)

Equation

4.1

Here, TH is the time horizon; AGWP is the absolute global warming potential; a is the radiative efficiency of a given unit increase in greenhouse gas, i, or CO(in units of W/m2/s); xi(t) is the time-dependent decay of a given greenhouse gas, i, following an instantaneous release t = 0; and xCO2(t) is the time-dependent decay of COfollowing an instantaneous release at time t = 0.

cdr primer equation 4 1

Here, TH is the time horizon; AGWP is the absolute global warming potential; a is the radiative efficiency of a given unit increase in greenhouse gas, i, or CO(in units of W/m2/s); xi(t) is the time-dependent decay of a given greenhouse gas, i, following an instantaneous release t = 0; and xCO2(t) is the time-dependent decay of COfollowing an instantaneous release at time t = 0.

Table

4.1

cdr primer table 4 1
Figure

4.1

Greenhouse gas emissions from 2017 broken into (a) share of emissions from different greenhouse gases and (b) CO2eq emissions from different greenhouse gases using a GWP of 100 (EPA, 2019). Both figures are in the mass percent of overall emissions.

cdr primer figure 4 1

Greenhouse gas emissions from 2017 broken into (a) share of emissions from different greenhouse gases and (b) CO2eq emissions from different greenhouse gases using a GWP of 100 (EPA, 2019). Both figures are in the mass percent of overall emissions.

4.1

4.1 —

LCA as an assessment tool

 

4.1.1

Life cycle analysis (LCA)

Life cycle analysis (LCA) is a compilation and evaluation of the inputs, outputs, and potential environmental impacts of a product system throughout its life cycle (The International Standards Organisation [ISO], 2006). LCA can provide insight into the resources that are necessary to create and maintain a functioning system, as well as the quantification of GHG emissions that are allocated to each resource (Hellweg and Canals, 2014; Sick et al., 2019). This includes energy resources, materials, water, and land use, amongst others. While the data needed for adequate LCA vary widely for different types of CDR systems, important examples are listed below

  • Carbon intensity or embodied emissions of the materials used to construct and maintain the system;

  • Energy resources consumed to construct and maintain the system;

  • Energy resources consumed to operate the system, including electricity and thermal energy. In both cases the ultimate source of that energy must be considered; for thermal energy, required temperature is an important parameter;

  • Water requirements to construct, maintain, and operate the system;

  • Land required to operate the system;

  • Mineral requirements to construct, maintain, and operate the system;

  • Chemical requirements to construct, maintain, and operate the system;

  • Carbon-intensity of materials or embodied emissions; 

  • Albedo effect (if any) from the construction, maintenance, and operation of the system; and

  • Impacts on CO2 storage and likely storage duration.

Life cycle analyses help us understand the potential trade-offs involving a system’s impacts and identify these elements of the system (the so-called hot spots) that contribute most to its environmental performance. For a CDR system, much of the emphasis of the LCA is on understanding the greenhouse gas impacts of the technology, as an LCA determines if the system successfully removes more CO2 from the atmosphere than it produces. In this case, the impact of interest is the greenhouse gas emissions, which is the focus of many greenhouse gas accounting protocols. However, a CDR system operates within a wider socioeconomic context, affecting resource, water, and energy consumption and land use. Therefore, it is necessary to consider system impacts holistically to understand CDR systems and trade-offs (e.g., greenhouse gas emissions; water consumption; and toxicity to animals, plants, and ecosystems). For example, one CDR system may remove the most CO2eq but have a major impact in another category, such as mineral, land, or water requirements. Given that preventing climate change will be balanced against other important goals (e.g., reducing poverty, protecting biodiversity), incentivizing CDR may not be limited to the effectiveness of CO2 removal (United Nations, 2020; see Section 1.6). Additionally, if two processes have similar impacts in terms of net CO2eq removed, other factors such as land or water intensity may determine that one system is preferable to the other, in addition to the impact assessment methods used to estimate such factors (Yang et al., 2020). 

Since LCA accounts for CO2eq emissions within chosen boundary conditions, suitable boundary conditions must be chosen to correctly evaluate CO2eq emissions over the technology life cycle. Potential inputs, outputs, and CO2eq emissions are outlined in Figure 4.2. Here, the inputs and outputs are generalized to provide a technology-agnostic overview. Not every input and output is present for each CDR approach. 

Figure

4.2

Potential inputs and outputs of a CDR approach. Innermost arrows indicate flows to and from the system. Arrows in either direction represent emissions into the atmosphere in CO2eq as a direct or indirect part of the process. For simplicity, only CO2eq flows are shown.

cdr primer figure 4 2

Potential inputs and outputs of a CDR approach. Innermost arrows indicate flows to and from the system. Arrows in either direction represent emissions into the atmosphere in CO2eq as a direct or indirect part of the process. For simplicity, only CO2eq flows are shown.

Within many greenhouse gas accounting protocols, emissions fall within three classes: direct emissions, indirect emissions, and embodied emissions. Direct emissions are those that are directly controllable and produced from within the system boundary (Section 4.1.2). For example, CO2eq emissions from an on-site combustion unit are considered direct emissions. Indirect emissions occur as a result of system activity but do not occur within the system boundary. For example, a system that uses grid electricity does not directly produce the emissions associated with electricity, but does contribute to the demand for electricity, resulting in emissions during power generation. Finally, embodied emissions are those that result from the production and use of any good or the provision of any service (e.g., the emissions embodied in materials and equipment used by the system, or any product resulting from the system). These emissions are incurred either once (for most capital equipment, excluding replacements) or repeatedly throughout the system’s lifetime (for most operating equipment or items that are repeatedly purchased). For example, the embodied emissions of steel used for a reactor include emissions associated with acquiring the raw materials, processing, manufacturing, transportation, and energy used in steel production. While common in greenhouse gas accounting, direct, indirect, and embodied emissions terminology is not frequently used in LCA literature because the meaning depends on the system boundaries for any given assessment.

The Greenhouse Gas Protocol is a commonly used method for reporting GHG emissions that was developed by the World Resources Institute (WRI) and the World Business Council for Sustainable Development (WBCSD). Within this framework, direct emissions are referred to as “Scope 1,” indirect emissions from the purchase of generated energy are referred to as “Scope 2,” and embodied emissions (as well as other related emissions) are referred to as “Scope 3.” Because different frameworks for an LCA are used by different analyses, this terminology sometimes causes confusion (Greenhouse Gas Protocol, 2020).

An LCA can be used to determine a system’s net emissions/removal of CO2eq from the atmosphere. Using Figure 4.2, this corresponds to the total CO2eq input minus the total CO2eq output and any other CO2eq emissions (orange arrows). This can also be represented as the ratio of all the emissions from a system to all of the CO2 removed from the atmosphere by the system. If this ratio is greater than 1, the system emits more CO2eq than it removes and is not considered a CDR system.

 

4.1.2

System boundaries

System boundaries delineate what is included and what is excluded in a given analysis. The system boundary is not necessarily a geographical boundary (i.e., on-site vs. off-site), but rather a boundary designed to identify all components required to operate the CDR approach. Here, we discuss the three main types: (1) gate-to-gate, (2) cradle-to-gate, and (3) cradle-to-grave . These three system boundaries are displayed for a generic CDR approach in Figure 4.3. A gate-to-gate analysis assesses only one distinct phase (e.g., crude oil refining, copper smelting). A cradle-to-gate analysis includes all of the process steps and associated flows between extraction of a natural resource (e.g., crude oil, copper ore) and production of a finished product (e.g., diesel fuel, a microprocessor). Cradle-to-grave extends this analysis to include the use (e.g., combustion of a fuel, calculations by a computer) and disposal (e.g., landfill, recycling, combustion) of a finished product. Since carbon dioxide emissions can occur anywhere in the system (from extraction to production to final use), a cradle-to-grave analysis can determine if a defined system truly results in negative emissions. 

Figure

4.3

Process flow diagram for technology-agnostic CDR approach, illustrating the boundary conditions for cradle-to-gate, gate-to-gate, and cradle-to-grave analyses. For simplicity, only CO2eq flows are illustrated. In an LCA, other waste streams are also considered.

cdr primer figure 4 3

Process flow diagram for technology-agnostic CDR approach, illustrating the boundary conditions for cradle-to-gate, gate-to-gate, and cradle-to-grave analyses. For simplicity, only CO2eq flows are illustrated. In an LCA, other waste streams are also considered.

 

4.1.3

Allocation methods and decision making for life cycle analyses

Many product systems are multifunctional: They produce more than one product. This occurs with many carbon capture and utilization systems, including BECCS and CO2-to-fuels. Assessing multifunctional systems requires dividing up the environmental impacts of the process between the co-products based on their allocation. The International Standards Organisation (ISO) states: “Wherever possible, allocation should be avoided by: 1) dividing the unit process to be allocated into two or more sub-processes and collecting the input and output data related to these sub-processes, or 2) expanding the product system to include the additional functions related to the co-products….” (2006). The first point in the ISO quote indicates that what is viewed as one complete system may be composed of multiple subsystems. In some cases, these subsystems can operate independently of one another, allowing focus on the specific sub-process of interest to perform the LCA. However, multifunctional processes usually cannot be subdivided into independent sub-processes that eliminate the need for allocation. The second point in the ISO quote above refers to the process of system expansion, in which system boundaries are expanded until they encompass the impacts of what would otherwise be co-products. System expansion allows for allocating those impacts to one product, eliminating the need for allocation methods that divide the impacts of the process across multiple co-products. In other words, the system expansion method displaces emissions from its co-products compared with conventional energy production and credits them to the specified product (Nguyen and Hermansen, 2012). This method is used when separation is not possible or meaningful. An example is the co-production of electricity in a steam methane reforming hydrogen production plant. When comparing their performance to alternative hydrogen production systems, the boundary of the alternative system has to be expanded to include electricity production, e.g., by a natural gas-fired power plant (Antonini et al., 2020). 

Allocation methods distribute the environmental impacts of the system to products through the use of a functional unit (Kolosz et al., 2020). The functional unit provides a quantified performance of the product system for use as a reference unit (Rebitzer et al., 2014). This means a) that the functional unit provides the reference on which inlet and outlet flows of the system are based, and b) that the unit itself is relevant and specific to the type of system being analyzed. 

Allocation methods can include physical allocation (by mass or volume) (Guinée et al., 2002), energy allocation (Luo et al., 2009), and market or economic allocation (Guinée et al., 2002; Kolosz et al., 2020), among others. Each method partitions the impact of the system among the co-products in different ways.

  • Physical (or mass, or volume) allocation: The physical allocation method distributes inputs and outputs from the system to the co-products based on their mass or volume. In the literature, physical allocation is best applied when there are limited co-products. Additionally, if one co-product has a service in terms of energy (i.e., fuel) and the other product is valuable in terms of mass (i.e., plastic), it does not make sense to use physical allocation.

  • Energy Allocation: The energy allocation method assesses performance from the perspective of the effort required to produce a product. For a system meant to capture CO2, the effort may be measured in Megajoules (MJ) required per tonne of CO2 captured (Nguyen and Hermansen, 2012; Pradhan et al., 2011; Wang et al., 2004). If the main product of an LCA is mass-based only (e.g., tonnes of CO2 removed) energy content would not be used directly on CDR systems, and instead, mass or other allocation methods should be used.

  • Market (or economic) allocation: In market or economic allocation, the inputs and outputs are distributed to the co-products based on their market value (Zimmermann et al., 2018). All feedstocks have costs that fluctuate, depending on supply and demand (Luo et al., 2009). Market allocation essentially operates in two distinct modes: fixed and non-linear. Some parameters may or may not be constants but consist of marginal data that can be predicted through time.

  • Exergy allocation: The exergy allocation method distributes the impact of the system to the co-products based on their exergy (i.e., the maximum theoretical work as defined by the second law of thermodynamics). 

For example, take a polygeneration plant that (1) uses biomass to produce electricity, capturing the CO2 from the combustion process, (2) uses part of the captured CO2 for urea production, and (3) geologically stores the remainder of the CO2. This results in two co-products from the system for which the impacts of the process are divided: electricity and urea. This system has the additional benefit of geologically storing CO2, which can be credited to the two products. The electricity and urea production system cannot be broken up into subsystems that do not require allocation. System expansion could be used to compare this system to traditional energy systems, which would be the preferred choice. Since electricity is produced in units of energy (typically megawatts), while urea is commonly used as fertilizer and sold in units of mass, mass and energy allocation are not preferred. Market allocation could be used to allocate to the electricity and urea proportional to their market price. 

As seen above, different allocation methods can produce conflicting results, and care must be taken when selecting and applying them to CDR systems. If allocation is used, then it is advantageous to use more than one allocation method to evaluate the results. A related discussion of the carbon footprint of CO2 capture further illustrates the importance of choosing a suitable approach for the boundary conditions (Müller et al., 2020).

 

4.1.4

Challenges of life cycle analysis

One of the biggest challenges associated with LCA is data availability and accuracy (Sick et al., 2019). Analyses typically use extensive amounts of data to gather and analyze information related to the carbon footprint, environmental impacts, and economic impacts of systems. Inaccurate data or poor data resolution can lead to greater LCA uncertainty, so it is imperative to complement any LCA with a sound and suitable uncertainty analysis (Fernández-Dacosta et al., 2017; Mendoza Beltran et al., 2018). Because there are several inputs in an LCA, ensuring that data are as accurate as possible is crucial to avoiding compounding uncertainty (Bamber et al., 2020; Lima et al., 2020; Heijungs and Huijbregts, 2004). Parameter uncertainty (also known as data uncertainty) can be quantified through established uncertainty methods, such as statistical methods or Monte-Carlo analysis (Baek et al., 2018; Heijungs, 1996; Maurice et al., 2000). However, other areas of uncertainty, such as scenario uncertainty and knowledge base uncertainty,  are more difficult to address, but methods are being developed to evaluate these as well (Fernández-Dacosta et al., 2017; Mendoza Beltran et al., 2018). Generally, the more mature a technology is, the more reliable the associated data for LCA.

Choosing a reference system is another challenge associated with LCAs. The reference system is the incumbent system compared with the new product system, on which the LCA is being performed. The reference system allows for comparison of environmental impacts. The choice of reference system plays a large role in assessing the environmental impact savings for the system, and therefore impacts the results of the LCA. For CDR, few of these approaches are deployed today. This provides a challenge when selecting a reference system, as there is not a similar reference system for comparison.

The permanence of CO2 removal is an additional and important consideration for life cycle analyses. CO2 can be safely stored deep underground for hundreds to millions of years and with no or little meaningful leakage – the result effectively being permanent storage (Alcalde et al., 2018). Because these systems remove excess CO2 from the atmosphere and store it “in a manner intended to be permanent,” they meet the second criterion for CDR presented at the beginning of this section. In other cases, especially those involving biological systems, the durability of carbon storage is limited, in some cases to decades or less, due to both physical risks and socioeconomic factors. The climate benefits of these systems will be limited compared to permanent storage, and these differences must be accounted for in LCA and any other analysis (Herzog et al., 2003; Levasseur et al., 2010, 2012). See Chapter 1 for a more complete discussion of permanence. 

Further, LCA for newly-proposed CDR systems or approaches with low technology readiness levels (TRL) also proves challenging (See Figure 4.4). Low-TRL approaches are unproven on an industrial scale and therefore carry large inherent uncertainty surrounding system impacts. This uncertainty is compounded by limited data on these approaches. While LCAs of low-TRL systems increase uncertainty, these analyses can be very helpful in determining areas for improved environmental impact (Moni et al., 2020). 

A final challenge for LCA analyses is the lack of common guidelines for such analyses, specifically with respect to low-TRL technologies. To compare the potential for negative emissions across technologies, LCAs must be performed with analogous system boundaries. This proves difficult when comparing, for example, technological and biological systems. Currently, various initiatives provide guidance in harmonizing the application of LCA. For example, the World Resource Institute and the Greenhouse Gas Protocol developed a set of guidelines on how to account for greenhouse gas emissions and carbon dioxide removal from land use, land use change, bioenergy, and related topics (Greenhouse Gas Protocol, 2020). In another example, a report by Zimmerman et al. (2018) provided recommendations on the type of scenario modelling and system boundaries that should be taken into account when CO2 is used within the system boundary.

Figure

4.4

Overview of technology readiness levels (TRLs) and their associated definitions. TRLs are a method for estimating the maturity of a given technology (Mai, 2017). There are nine TRLs, with TRL 1 the lowest and TRL 9 the highest. The technology stage on the left-hand side leads to the TRL corresponding to that technology stage. To evaluate the TRL of a CDR system, the technology is compared to a set of parameters at each TRL, and the specific parameters that dictate each TRL are technology-dependent. Note that TRL 2 has lines to both ‘Basic Technology Research’ and ‘Research to Prove Feasibility’ because it occupies both levels of technology development. (TRL definitions obtained from Frank, 2015).

cdr primer figure 4 4

Overview of technology readiness levels (TRLs) and their associated definitions. TRLs are a method for estimating the maturity of a given technology (Mai, 2017). There are nine TRLs, with TRL 1 the lowest and TRL 9 the highest. The technology stage on the left-hand side leads to the TRL corresponding to that technology stage. To evaluate the TRL of a CDR system, the technology is compared to a set of parameters at each TRL, and the specific parameters that dictate each TRL are technology-dependent. Note that TRL 2 has lines to both ‘Basic Technology Research’ and ‘Research to Prove Feasibility’ because it occupies both levels of technology development. (TRL definitions obtained from Frank, 2015).

 

4.1.5

Example of a CDR approach using solvent-based direct air capture with carbon storage (DACCS)

Solvent-based direct air capture (DAC) uses chemicals dissolved in water to contact and capture CO2. The process discussed uses a 1.0 – 2.0M potassium hydroxide solution (KOH(aq)) to capture CO2 from the air in the contactor unit, forming a potassium carbonate (K2CO3(aq)) solution (Keith et al., 2018; NASEM, 2019). The solution is then fed into a precipitation/solid separator unit. Here, anionic exchange occurs between calcium hydroxide (Ca(OH)2(aq)) and K2CO3(aq) reproducing the KOH(aq) solution and precipitate, calcium carbonate (CaCO3(s)). From here, the KOH can be fed back to the contactor. The CaCO3 is processed and sent to the calciner to produce CO2 and calcium oxide (CaO(s)) at temperatures around 900º C. The CaO is fed into the slaker, where it reacts with water to reproduce Ca(OH)2. The CO2 is dehydrated in the condenser, then compressed for pipeline transport to geologic sequestration. Figure 4.5 demonstrates this process.

Figure

4.5

Representative diagram of solvent-based direct air capture. Solid lines represent material flows. Dashed blue lines indicate system electricity demands, while dashed red lines represent thermal demands of the system.

cdr primer figure 4 5

Representative diagram of solvent-based direct air capture. Solid lines represent material flows. Dashed blue lines indicate system electricity demands, while dashed red lines represent thermal demands of the system.

This process has large power requirements, ranging from 300 – 500 MW at a scale of 1 million tonnes of CO2 removal per year (NASEM, 2019). These power requirements include both electrical and thermal energy, as outlined in Figure 4.3. To date, the thermal energy requirements have been evaluated exclusively by the use of an oxy-fired calcination unit fed with natural gas. Future systems may utilize electricity to meet these thermal energy demands and therefore could use low-carbon resources such as wind, geothermal, and solar energy. 

In this example, we compare the CO2eq emissions from using dedicated, stand-alone energy facilities to provide the steady 300 MW of power (work and heat) required for solvent-based DAC. The DAC facility is kept operational 90 percent of the time to prevent repeated start-up and shutdown costs associated with the calciner. To assess the negative emissions potential of the DAC approach, direct and embodied emissions for each part of the process should be considered when determining the net removal of CO2 from air. 

For the solvent-based DAC system, there are many different energy resources and configurations that can power the process, assuming the availability of electric calcination units. Figure 4.6 outlines five distinct cases in which different energy resources are used to power the DAC process. We consider two DAC configurations coupled to different energy resources: a natural gas-fired calciner directly heated with natural gas, and electric resistance calciners that use resistance heating and electricity from solar photovoltaic (PV), nuclear, wind, and geothermal power (McQueen et al., 2021). Each of these processes includes direct emissions, indirect emissions, and embodied emissions for the capture process, compression to pipeline pressure, transport via pipeline, and ultimate injection into geologic reservoirs deep underground for permanent storage. The direct emissions include emissions from combustion units, CO2 resulting from chemical reactions, or other emissions that result directly from chemical reactions. Here, emissions from energy production are included in direct emissions, as the system studied contains both the DAC system and the power system. However, in cases where the power source is outside of the system bounds, these would be considered indirect emissions associated with energy consumption. One example of the embodied emissions for this DAC system is the total amount of emissions resulting from manufacturing, transporting, and installing the components required to construct the system, such as the air contactor and calciner.

The natural gas pathway presented here utilizes an oxygen-fired calciner where the natural gas is combusted within the kiln, in direct contact with calcium carbonate. The primary direct emissions occur as a result of combustion of natural gas. In this case, all CO2 emissions from natural gas combustion inside the calciner are co-captured with the CO2 from air; however, only 90% of the emissions from natural gas-based electricity generation are captured. Additional emissions are embodied within the materials used to perform the process, as noted above. When evaluating natural gas-based systems, there are additional upstream embodied emissions considerations due to methane leakage (i.e., fugitive emissions). Estimates of methane leakage are uncertain but, here, values from Alvarez et al., (2018) are considered. Most embodied natural gas emissions come from recovering natural gas at extraction sites, accounting for 3.5 (Environmental Protection Agency, 2017) – 7.6 (Alvarez et al., 2018) Mt of methane emitted per year from the U.S. oil and natural gas supply chain. The second-largest source of embodied emissions is the transportation of unprocessed natural gas from the production site to the processing site, with 2.3 (Alvarez et al., 2018) Mt - 2.6 (Environmental Protection Agency, 2017) of methane emitted per year from the U.S. oil and natural gas supply chain. Within the embodied emissions, pneumatic controllers and equipment leaks make up the largest emissions sources in the oil and natural gas supply chain. Malfunctioning controllers are the major contributor to pneumatic controller emissions (Alvarez et al., 2018). Emissions also occur during processing, transmission and storage, distribution, and oil refining and transportation.

Embodied natural gas emission values are important because methane has a higher global warming potential (GWP) than CO2 with a GWP20 of 86 and a GWP100 of 28. The impact of methane emissions illustrates the importance of including non-CO2 greenhouse gas emissions in the analysis and demonstrates the impact these emissions can have on the overall emissions footprint of the process (Environmental Protection Agency, 2019). This impact can be seen in Figure 4.6 for natural gas, as it has the largest direct and indirect embodied emissions of any alternative energy resource coupled to DAC.  

Figure

4.6

Negative emissions potential of solvent-based direct air capture with geologic storage coupled to varying energy systems. These numbers describe the negative emissions potential of capturing COfrom the air, compressing the captured CO2 to pipeline pressure, transporting it via pipeline, and injecting it into permanent geologic storage. The numbers represent the following energy source scenarios: natural gas providing both the thermal and electrical requirements for the system; solar energy powering the system with a capacity factor between 24.1 percent and 32.5 percent; a system powered by nuclear electricity; wind power with an optimistic capacity factor of 52 percent; and geothermal power. Indirect emissions are not included in this figure, as electricity and power production is included as direct emissions. Bars that are above the axis or ‘+’ indicate emissions from the process into the atmosphere. Bars that are below the axis or ‘-’ indicate carbon dioxide removal from the atmosphere by the process. The figure measures emissions and removal as CO2eq, using the emissions calculated at a GWP of 100 and 20. Note that for non-natural gas pathways, GWP100 = GWP20, as there are only CO2 greenhouse gases emitted in these systems. 

cdr primer figure 4 6

Negative emissions potential of solvent-based direct air capture with geologic storage coupled to varying energy systems. These numbers describe the negative emissions potential of capturing COfrom the air, compressing the captured CO2 to pipeline pressure, transporting it via pipeline, and injecting it into permanent geologic storage. The numbers represent the following energy source scenarios: natural gas providing both the thermal and electrical requirements for the system; solar energy powering the system with a capacity factor between 24.1 percent and 32.5 percent; a system powered by nuclear electricity; wind power with an optimistic capacity factor of 52 percent; and geothermal power. Indirect emissions are not included in this figure, as electricity and power production is included as direct emissions. Bars that are above the axis or ‘+’ indicate emissions from the process into the atmosphere. Bars that are below the axis or ‘-’ indicate carbon dioxide removal from the atmosphere by the process. The figure measures emissions and removal as CO2eq, using the emissions calculated at a GWP of 100 and 20. Note that for non-natural gas pathways, GWP100 = GWP20, as there are only CO2 greenhouse gases emitted in these systems. 

The second pathway presented in the figure is solar energy. Here, there are no direct emissions caused by the energy source. Instead, all the emissions are embodied in the process materials, their production, and embodied energy. Solar is an intermittent energy resource with a capacity factor between 24.1 percent (seasonally adjusted) and 35.2 percent (best-in-class) (Honsberg and Bowden, 2014; NREL, 2019). This indicates that a significant amount of energy must be stored to keep the solvent-based DAC facility continuously operational (assuming it does not also use grid-supplied electricity). Conventional energy storage infrastructures (in this case, lithium-ion batteries) have large embodied emissions associated with their production. Therefore, while a solar-powered DAC system has no direct emissions, the embodied emissions of the energy system that must be coupled to the DAC plant may significantly increase the system’s emissions (Figure 4.6). Similarly, the wind scenario uses a best-in-class capacity factor of 52 percent (NREL, 2019). Since the goal of this analysis is to avoid intermittency in energy supply when operating the DAC facility with a dedicated energy facility, this system requires similar energy storage infrastructure, which contributes to the embodied emissions of the integrated system. 

Like the previous scenarios, a nuclear energy-powered DAC facility does not have direct emissions associated with capturing carbon dioxide. This energy resource also has a capacity factor that can keep the DAC facility continuously operational without requiring energy storage (U.S. Energy Information Administration, 2019). This further reduces the embodied emissions associated with the system, resulting in the lowest-emissions option presented here. This is primarily because nuclear facilities can supply energy to DAC operations 90 percent of the year with no direct emissions. Despite these benefits, nuclear energy opponents cite high costs, long construction lead times, hazardous nuclear waste, and the risk of nuclear weapon proliferation as reasons to avoid this energy source.

The final energy scenario uses geothermal energy to power a solvent-based DAC facility. Like nuclear facilities, geothermal facilities can supply continuous energy to the DAC facility without the need for energy storage infrastructure, reducing the embodied emissions. However, the geothermal scenario includes CO2 emissions associated with the recovery of geothermal energy, which vary by location (Section 3.3.2 in Chapter 3). Direct emissions from geothermal energy production occur as a result of the working fluid produced from the Earth. These working fluids supply heat to the geothermal facility that is used to turn a turbine and create energy. However, geothermal working fluids contain dissolved gases, primarily CO2, producing average emissions of 7.71 kgCO2/mmBTU for unmitigated operations (U.S. Energy Information Agency, 2016).

After it is captured, the CO2 is compressed for transportation (via pipeline, truck, or ship). For the DAC process to result in negative emissions, the CO2 must be removed from the atmosphere in a manner intended to be permanent. Geologic storage of CO2 in sedimentary formations deep underground, such as saline aquifers and depleted oil and gas reservoirs, means the DAC facility is part of a DACCS system. As indicated by the hashed bars in Figure 4.6, the amount of CO2 permanently stored is defined as that removed from the air, minus the direct and embodied emissions from the individual steps required to remove, compress, transport, and inject CO2 deep underground. Of the five scenarios, DAC powered by nuclear energy coupled to geologic storage results in the greatest net CO2 removal from the air. Chapter 3 explores worldwide opportunities to site these DACCS systems in a responsible way. Chapter 5 examines alternate fates for DAC-derived CO2 that provide net carbon dioxide removal from the atmosphere (e.g., beneficial reuse or utilization).

4.2

4.2 —

Techno-economic analysis (TEA) as an assessment tool

 

4.2.1

TEA for CDR approaches

Another relevant consideration for CDR approaches is the cost of the technology. While there are multiple ways to estimate cost, they have much in common (n Rubin et al., 2013). This section focuses on the key components of cost estimates and discusses how, based on these estimates, full techno-economic analysis is undertaken, making use of suitable economic and financial parameters. Many existing analyses focus on the cost, particularly the cost per tonne of CO2eq removed. However, costs cannot be evaluated in a vacuum, as many environmental and social factors must be considered when evaluating a CDR system, among other conditions that limit the validity of TEA results for developing technologies. 

As in LCA, setting system boundaries is the first step for successful analysis. The TEA and LCA  of a CDR system often have different goals and scopes, which lead to different boundary conditions. But, in some cases, LCA and TEA can be performed together, sharing the same system boundaries and criteria. But as previously noted, the system boundary is not necessarily solely geographic. 

The next step is to estimate the cost of building the CDR system or, if the CDR approach involves agriculture, any upfront investments required to change practices. These costs are colloquially called the capital expenditure (CAPEX) of the technology, and represent all fixed, one-time expenses. For engineering solutions, the capital cost of the system includes any initial investment in core equipment and components (inside battery limits, or ISBL), as well as engineering fees and the supporting buildings or other infrastructure required to connect the plant to the outside world. Handbooks are available for the capital costing of processing plants (e.g., Gerrard, 2000; Peters et al., 2003; Towler and Sinnot, 2013), as well as dedicated works and guidelines from, for example, the CO2 capture and storage literature (DOE/NETL, 2015; Sintef energy research, 2017; Van der Spek et al., 2019; van der Spek et al., 2017a, 2017b). The sum of all capital investment may be called the Total Capital Requirement (TCR), which represents the total initial investment that will be necessary to deploy the technology, including the cost of financing the technology and any contingencies during development and construction (Rubin et al., 2013). 

Many costing studies, however, do not aim to present the absolute capital cost, but rather the cost on a per-tonne-CO2-stored basis (the so-called specific capital costs). This makes it possible to determine the most “carbon-efficient” use of a limited amount of capital: in other words, the optimal allocation of capital to maximize CO2 removed. To determine the capital cost on a per-tonne-CO2-captured basis, the capital cost of the system must be annualized. There are multiple ways to do this, but a simple initial methodology involves using a capital recovery factor (CRF): the amount of capital cost “recovered” per year (Rubin et al., 2013). This can also be thought of as repayment of capital borrowed to build the project. The CRF is based on the economic lifetime of the project (in years) and the discount rate, both of which have a large impact on the resulting annualized capital costs. The discount rate reflects the risk of the investment, represented as a cost of capital, including both the interest rate on borrowed funds and the returns expected by investors. In this case, the discount rate is the weighted average cost of capital (WACC). From this information, the CRF can be determined via the following equation.

Equation

4.2

cdr primer equation 4 2

In this equation, the symbol i represents the discount rate (between 0 and 1) and the symbol n is the CDR plant’s economic lifetime in years . Since the discount rate depends on the riskiness of a project, it may vary substantially among investments. For example, the discount rate charged for capital for an innovative, first-of-a-kind (FOAK) technology will be much higher than the discount rate for a widely deployed, lower-risk technology. The total capital cost of the system multiplied by the CRF gives the annualized capital cost of the system. Once the annualized cost of the system is determined, it can be divided by the annual amount of CO2 captured to determine the capital cost of the system on a per-tonne-CO2-captured basis.  

As an example of the impact of the discount rate and plant lifetime, consider a generic CDR system that costs $10 million to build and is designed to capture 10,000 tCO2/yr. In Figure 4.7, those two parameters are varied sequentially to show the impact on the annualized capital cost ($/year) on a per-tonne-CO2-captured basis ($/tCO2) as these are the preferred cost units. Looking at Figure 4.7, the same capital cost and capture rate can result in annualized capital costs ranging from $50 to $300/tCO2 over the assumed parameters. Therefore, it is important to carefully consider which parameters are used throughout the analysis. 

Figure

4.7

Annualized capital cost as a function of the discount rate and plant life for a $10 million plant designed to capture 10,000 tonnes CO2/yr.

cdr primer figure 4 7

Annualized capital cost as a function of the discount rate and plant life for a $10 million plant designed to capture 10,000 tonnes CO2/yr.

The last common aspect of CDR system costs is the inclusion of operating costs. Operating costs are those incurred during the operation of a system, including energy, chemicals, water, labor, maintenance, and other repeated purchases. These costs vary widely but can be calculated based on the individual requirements of each CDR system. Once the operating costs are determined on a per-year basis, they can also be estimated on a per-tonne-CO2-captured basis.

 

4.2.2

Challenges with techno-economic analyses for CDR approaches

There are many challenges when performing techno-economic analyses on CDR approaches. One of the more common challenges is estimating the costs for new or low-TRL approaches. Low-TRL approaches are unproven on an industrial scale and, therefore, carry large inherent uncertainty surrounding system requirements, such as infrastructure, materials, and equipment requirements outside battery limits. Thus, estimating these costs becomes challenging in comparison to existing, high-TRL approaches. Rubin (2019) put forward a method for calculating the cost of advanced CCS technologies that can also be useful in the context of CDR approaches. He applied a hybrid method of cost escalation from low-TRL approaches to FOAK plants, with subsequent cost reduction due to technological learning by repeated manufacturing, deployment, and operation (Section 2.2.4). However, the cost increase from a low-TRL technology to a FOAK plant, and to a lesser extent the technological learning rate after the first plant, are difficult to quantify, leading to high error margins of the final hybrid cost estimate. Van der Spek et al. (2017a) showed error margins of -60 percent to +100 percent for a TRL 3 CO2 capture technology when using Rubin’s methodologically suitable hybrid costing approach. That was double the uncertainty margins of the standard, but methodologically unsuitable, bottom-up costing approach (-30 percent to +50 percent). Although cost changes are difficult to estimate, it has been empirically observed that costs go up between early, low-TRL estimates and actual, as-built first-of-a-kind costs. Performing TEAs on low-TRL technologies necessarily increases uncertainty in the analysis. This is normal, and we advise embracing and communicating these uncertainties is important, as the results might help guide research and/or policy directions and indicate areas that are most likely to lead to more accurate estimates. An international working group under the auspices of the IEAGHG Cost Network recently published a good account of uncertainty analysis methods and guidance on their selection and use (Van der Spek et al., 2020).

Additionally, the cost of materials and energy resources is dependent on location (IEA Greenhouse Gas R&D Programme, 2018). This leads to the creation of location-constrained scenarios where the system cost is specific to a certain location. This is also true of analyses that incorporate tax incentives among other forms of governmental incentives, limiting the validity of these cost estimates to specific geographies.

Finally, system boundaries heavily influence the overall cost of the system. Inclusion or exclusion of various operations in the capital cost and the operating cost may have a large influence on the cost viability of the entire system. In many techno-economic analyses, the technology used to capture CO2 from the atmosphere is separated from post-processing of CO2 or acquisition of raw materials. Separating the capture system from post-processing systems can give the cost of CO2 removed by the capture system, but does not account for downstream costs associated with either storing or utilizing the captured CO2. This illustrates, again, why is it important to define robust and appropriate boundary conditions. 

 

4.2.3

Technology learning and experience curves

In conjunction with the techno-economic analysis methods described above, there are cases during deployment where the concept of “technological learning” may provide important insights for CDR approaches. This concept is based on the empirical observation that many emerging technologies get significantly cheaper per unit over time (Rubin et al., 2007). One of the best-known examples is solar photovoltaic modules, which have fallen in cost by two orders of magnitude over 40 years (Kavlak et al., 2018). Dozens of other technologies in sectors such as chemicals, electronics hardware, and energy have also shown substantial cost declines (Nagy et al., 2013). Notable exceptions do exist, such as nuclear power, where increased scale increases complexity, driving up the costs of the system (Grubler, 2012). It is particularly important to consider potential future cost changes when analyzing policy or investment choices over the medium or long term. 

Examining historical cost declines has shown that many technologies follow an “experience curve” (sometimes called a “learning curve”) pattern, in which the capital cost of producing the next unit of the technology falls in relation to the total (cumulative) stock of the technology that has been produced (Nagy et al., 2013).

Equation

4.3

cdr primer equation 4 3

Here, y(t) is the inflation-adjusted cost of producing the next unit of the technology at time t, B is the unit cost of the first deployment, x(t) is the total cumulative stock at time t, and w is a (usually positive) exponent related to the rate at which the costs change. The “unit cost” is normalized to a relevant parameter of technology’s capacity, such as MW for electricity generation technology or MWh for energy storage technology. The fractional cost reduction for one doubling of the cumulative production is known as the “learning rate” (LR) (Rubin et al., 2015).

Equation

4.4

cdr primer equation 4 4

Observed learning rates are usually expressed as a percentage, and they generally range between 5 percent (slower learning) and 30 percent (very fast learning). 

The reasons behind the observed patterns of learning are complex and continually debated. They appear to combine a variety of effects, including simple economies of scale, streamlined and standardized supply chains (e.g., replacing specialty parts with commodity parts, harmonizing and standardizing component sizes and interconnections), and true “learning” by technology designers, manufacturers, installers, and designers about lower-cost approaches to product design, manufacture, installation, and operation. The central conceptual insight of the experience curve approach is that as society gains experience with a technology (using the cumulative production as a proxy for “experience,”) it is typically able to lower production costs. The overall effect is known as “learning-by-doing,” and technologies are said to “move down the learning curve”.

Technology learning theory can be applied to CDR approaches. The relevant units by which to normalize capital costs are tonnes of CO2 (tCO2). However, there is very little historical data on the costs of CDR approaches, and no CDR approaches have been produced in a large enough amount to have increased the total cumulative production through several doublings, which is roughly the amount needed to begin reliably observing learning effects. The main challenge, then, is to find representative estimates of the learning rate for various CDR approaches. Despite this, it is still possible to estimate future cost reductions for individual CDR approaches by (a) assuming CDR approach learning rates are likely to fall within the range generally seen with other technologies (proxies), and (b) projecting a future schedule of production (Baker et al., 2020), although these estimates have high uncertainty. 

It is not possible to predict precisely what improvements will occur in production processes and technology designs to lower costs. While history strongly suggests that there will be improvements, with the learning rate representing the net impact of these as-yet-unknown improvements on future costs, over-reliance on technology learning can also be disadvantageous, since this learning is not guaranteed. Because technology learning carries inherent uncertainty in the cost projections of a given technology, using sensitivity analysis can help quantify the uncertainty in a given rate of technological learning, thus increasing the applicability of a given learning analysis (Rubin et al., 2015). An additional challenge to learning theory is the ability to project the true production cost trend of a given technology, as opposed to the market cost, which is non-equilibrium and can be influenced by many market conditions (Rubin et al., 2015; Wene, 2008). 

4.3

4.3 —

Net removed cost: an integrated techno-economic and life cycle analysis

To determine the cost of net-negative CO2eq emissions, the base cost of the process can be adjusted to account for life cycle CO2eq emissions. This means that the cost per-tonne-CO2eq is converted to the cost per-tonne-CO2eq net removed using the following expression:

Equation

4.5

cdr primer equation 4 5

Here, we define variable x as the ratio of CO2eq emitted to CO2eq removed over the system life cycle. For this cost to reflect a CDR system, the CO2eq emissions must be presented throughout the system life cycle, including subsequent storage or utilization. The denominator represents the net amount of CO2eq removed from air per-tonne-CO2eq captured from the air, if any. The numerator represents the total annualized cost of building and operating the CDR system on a per-tonne-CO2eq-captured-from-air basis. Collectively, this expression gives the cost of the system per tonne of CO2eq net removed from the air. The net removed cost is the estimated cost of reducing atmospheric CO2eq concentration by 1 tonne. Note that as the amount of CO2eq emitted approaches the amount of the CO2eq removed, the net removed cost approaches infinity. For negative values of 1 – x, or for x values greater than 1, the system does not reduce emissions and, therefore, is not a CDR system. 

4.4

4.4 —

Conclusions

To develop and deploy CDR approaches at the scale necessary to meet climate goals will require insight into the environmental impact of those approaches, as well as their technical and economic feasibility. If used correctly, both LCA and TEA can provide systematic methods for evaluating existing and emerging CDR approaches, while playing a large role in how we define net-negative emissions for CDR systems.

We also stress that LCA and TEA are just components of the decision process around CDR deployment. Careful consideration of impacts outside of carbon accounting (environmental, social, economic, and political) are both critical and rarely as straightforward to quantify as LCA and TEA. This challenge creates an opportunity to integrate the social sciences more fully into technical assessments.

1

In this primer, we distinguish between a CDR approach and a CDR system. A CDR approach can be said to be a CDR system if it results in net-negative emissions. Therefore, we define these criteria to be applicable to CDR systems. See Section 1.2 for further distinctions between a CDR approach and CDR system. 

2

Some fuel production pathways (such as biofuel production) use alternate terminology when referring to system boundaries. For example, cradle-to-grave is known as well-to-wake for aviation (Kolosz et al., 2020) and well-to-wheel for land transport.

3

Scenario uncertainty refers to uncertainty related to the choices made in constructing the scenario, including time horizon, functional units, geographical location, and scale. Knowledge base uncertainty describes uncertainty in the broader knowledge of a specific scenario: for example, incomplete knowledge. 

4

In this case, the capacity factor refers to the percentage of time in a day that the given energy resource is generating electricity. For solar, this is equivalent to the number of hours the sun is shining. The seasonally adjusted capacity factor accounts for seasonal variation in the number of sunlight hours per day, whereas the best-in-class capacity factor is an annual average number of sunlight hours per day at an optimal (equatorial) location.

5

One important distinction is that TEA is different from Life Cycle Costing (LCC) because it integrates technical criteria into cost and generally focuses on the production phase of the LCA only. LCC covers the entire system boundary and therefore is not part of this discussion.

6

This is an important distinction because, although a plant could still be operational, the technology may become obsolete due to the introduction of a more efficient technology or process; therefore, it may become uneconomical to continue running the plant.

1

In this primer, we distinguish between a CDR approach and a CDR system. A CDR approach can be said to be a CDR system if it results in net-negative emissions. Therefore, we define these criteria to be applicable to CDR systems. See Section 1.2 for further distinctions between a CDR approach and CDR system. 

2

Some fuel production pathways (such as biofuel production) use alternate terminology when referring to system boundaries. For example, cradle-to-grave is known as well-to-wake for aviation (Kolosz et al., 2020) and well-to-wheel for land transport.

3

Scenario uncertainty refers to uncertainty related to the choices made in constructing the scenario, including time horizon, functional units, geographical location, and scale. Knowledge base uncertainty describes uncertainty in the broader knowledge of a specific scenario: for example, incomplete knowledge. 

4

In this case, the capacity factor refers to the percentage of time in a day that the given energy resource is generating electricity. For solar, this is equivalent to the number of hours the sun is shining. The seasonally adjusted capacity factor accounts for seasonal variation in the number of sunlight hours per day, whereas the best-in-class capacity factor is an annual average number of sunlight hours per day at an optimal (equatorial) location.

5

One important distinction is that TEA is different from Life Cycle Costing (LCC) because it integrates technical criteria into cost and generally focuses on the production phase of the LCA only. LCC covers the entire system boundary and therefore is not part of this discussion.

6

This is an important distinction because, although a plant could still be operational, the technology may become obsolete due to the introduction of a more efficient technology or process; therefore, it may become uneconomical to continue running the plant.

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.