Chapter 1

The Case for Carbon Dioxide Removal: From Science to Justice

Authors

Andrew Bergman and Anatoly Rinberg

Since the beginning of the pre-industrial era in 1850, humans have emitted approximately 2,400 metric gigatonnes of heat-trapping carbon dioxide (CO2) into the atmosphere, predominantly from the use of fossil fuels and land use change. About 55 percent of this total has been absorbed by land and ocean sinks – processes that remove carbon dioxide from the atmosphere – with the remainder accumulating in ever-higher concentrations in the global atmosphere (Friedlingstein et al., 2019). According to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), warming from CO2 and other greenhouse gas (GHG) emissions has raised Earth’s average temperature above pre-industrial levels by more than 1C. This warming has initiated extensive changes in the climate, which are already causing significant harm to populations around the globe (AR5, 2007; IPCC, 2018a). To dramatically reduce future societal harm, many nations, members of civil society, and scientists advise prioritizing efforts and technologies that, first and foremost, significantly reduce GHG emissions (IPCC AR5, 2015; IPCC, 2018b; UNEP, 2017).

Achieving deep reductions in GHG emissions is essential to managing the climate crisis. Decarbonization is the first step on the path to climate stabilization — one that is as enormous as it is necessary. We now know, however, that approaches that remove CO2 from the atmosphere will also need to be deployed at a large scale this century to keep warming below dangerous levels. 

Only when the planet reaches net-zero greenhouse gas emissions will global warming cease. Reaching net-zero emissions will require large-scale carbon dioxide removal (CDR) to offset continued emission sources that, because of physical constraints and social justice standards, are particularly hard to avoid (known collectively as “hard-to-avoid emissions”, see Section 1.4). These hard-to-avoid emissions will likely be between 1.5 and 3.1 gigatonnes of carbon dioxide equivalent (GtCO2eq) per year (Supplement 1.3). That is approximately 3.5 – 7.3 percent of 2019 global CO2 emission levels (Friedlingstein et al., 2019). Even if global decarbonization moves as quickly as possible and only these hard-to-avoid emissions are left over, keeping warming below 1.5° C will require removing gigatonnes of atmospheric CO2 per year by the end of the 21st century (IPCC, 2018b) — an enormous and daunting challenge. 

This primer considers the prospects of achieving this large scale of CDR in the current century and presents a framework for evaluating the feasibility of deploying CDR technologies and strategies. Non-CO2 GHG emissions contribute substantially to global warming and comprise a large portion of hard-to-avoid emissions. However, due to the relative abundance of CO2 compared to other greenhouse gases in the atmosphere and the long atmospheric lifetime of CO2 (explained in Section 1.3), we consider only CO2 as a target for removal in this primer. 

To chart a path for equitable CDR deployment that goes beyond addressing climate change to benefit society as a whole (IPCC, 2018c) and ensure justice for frontline, indigenous, and other marginalized communities, including those disproportionately harmed by climate change, we need to consider every policy option that is rooted in rigorous science. To that end, this primer uses science and engineering as guiding principles to assess what is possible and is deliberately agnostic about the costs of and policy supports in place for particular technologies. This primer does not ignore pathways for deploying CDR because they may currently be difficult or costly, nor does it focus on pathways because they may be cheaper or easier to implement under current policies (for example, subsidizing a particular carbon utilization practice). Thus, instead of limiting its scope to options that appear most tractable in the short term, our assessment seeks to accurately examine what is possible to achieve in the decades ahead – and beyond. At the same time, this primer acknowledges the moral hazards associated with justifying or planning for large-scale CDR without adopting a social justice-oriented framework for considering its deployment.

This approach is intended to provide the most complete menu of options to policymakers and the public so that they do not inadvertently dismiss pathways that rely on certain technologies or policy structures, and are well equipped to consider difficult political questions that arise when considering how to equitably address climate change. Although not the focus of this primer, these questions include, for example, how to weigh economic development against emissions reductions; how to consider the role of publicly-owned infrastructure versus for-profit CDR deployment; and whether to choose fossil fuel regulations that may be more expensive today but will reduce long-term environmental harm to frontline communities. By presenting the fullest set of scientifically plausible CDR deployment options, we hope to enable the discussion to move beyond the confines set by current policies, economic frameworks, and technologies, to an accessible and democratic discourse about how CDR deployment is governed in the future.

1.1

1.1 —

Why do we need a primer on CDR now?

As more nations announce climate neutrality legislation and set ambitious long-term goals (e.g., the United Kingdom, Sweden, New Zealand, and China), CDR has become a focus in the discussion about how to reach net-zero GHG emissions. This “net-zero” target means that the world must reach a balance of greenhouse gas sinks and sources that results in no additional warming (see Section 1.3 for further discussion of greenhouse gas properties), which is crucial for halting catastrophic climate change (IPCC, 2019b). Despite the growing conversation about climate neutrality and the need for CDR, however, misconceptions and uncertainties continue to hamper the design of equitable CDR implementation strategies that properly account for the risks and co-benefits of CDR deployment. 

This primer aims to frame the global CDR challenge, unifying shared concepts across different approaches (including defining what activities qualify as CDR) and acknowledging and explaining controversies. Part of the challenge lies in creating a universal set of terms and definitions to communicate about CDR to a growing and diverse community. While fields such as chemical engineering, agronomy, ocean biogeochemistry, geology, and social science will continue to have their own technical languages, this primer highlights and clarifies shared terminology that is often used (and sometimes misused) in the CDR conversation. (Section 1.2 and Supplement 1.1 offer CDR-related definitions and provide context for how terms have been applied differently in recent prominent reports). By providing a more consistent language foundation, this primer can guide future communication about new CDR research. This primer is also intended to help demonstrate how ethical, economic, and geographical considerations can bridge lab-scale work and theoretical analyses to achieve reliable gigatonne-scale deployment. Finally, we hope this primer will be used as a teaching tool to provide scientists, engineers, and policymakers with a guide for thinking about CDR so that they are better equipped to develop solutions.

Achieving gigatonne-scale CDR deployment by mid-century remains a daunting challenge. Some pathways will require extensive early-stage research, while other approaches that are already being tested outside the lab will benefit from substantial iteration on a demonstration scale.  Scaling up CDR approaches will take decades and will work only if the effectiveness of funding and investments are continuously monitored and assessed over time. It is therefore essential to establish a framework now for evaluating and comparing them. The risks of large-scale deployment, many of which are specific to each CDR approach, need to be understood and evaluated, and governance should be tailored accordingly to ensure just, effective, and equitable deployment. Carbon policies are underdeveloped in many regions, and, as policymakers develop new ones, we must understand the technical and resource trade-offs of different approaches and how to incentivize safe and scalable technology development.

CDR methods range from biological systems, such as forest or soil ecosystems that could store more carbon through alternative management practices, to technological processes that chemically separate CO2 directly from the air and compress, transport, and store it in subsurface geologic formations deep underground. Table 1.1 lists some of the main CDR approaches with corresponding prominent references, and Chapter 2 expands upon some of these CDR approaches in significant detail. While energy of capture per molecule of carbon dioxide is often the focus of technological approaches, other key metrics – such as land and material requirements, water usage, energy resource choice, and storage permanence – serve as performance indicators for evaluating trade-offs. Together, these metrics will ultimately aid those involved in deciding how and where to deploy CDR projects. 

What would it take to deploy CDR at gigatonne scale? Table 1.2 provides back-of-the-envelope estimates for what is required to remove 1 GtCO2/yr, comparing direct air capture with carbon storage (DACCS) to afforestation and reforestation methods and detailing the complexity of the trade-offs between them. While biological systems may seem less expensive today, their efficacy depends on a wide variety of factors related to biogeochemical processes and Earth’s climate, such as soil conditions, precipitation levels, temperature, and the carbon cycle more generally – all of which can make them more difficult to control and quantify than technological systems. These factors can also limit the permanence or durability of carbon stored in such systems. Even more challenging, perhaps, is the sheer scale of land use changes needed to achieve climatologically relevant levels of CDR from terrestrial biological systems. While this scale of biological CDR may result in important co-benefits, such as helpful regulation of air and water quality and the possibility of protecting biodiversity (Smith et al., 2019a), these systems will most likely prove controversial if pursued in the years ahead due to conflicting imperatives for how land is used, especially for food production. Technological systems such as DACCS tend to have significant infrastructure, material, and energy requirements, and they will also require a substantial amount of land if renewable energy sources are used to power them. The differing harms and co-benefits associated with large-scale CDR are discussed further in Section 1.6, and the different ways these trade-offs can be considered from a climate justice perspective are discussed in Section 1.7.

Table

1.1

Examples of Carbon Dioxide Removal Approaches and Prominent Related References. Blue filled-in cells indicate that the given CDR approach is addressed in the corresponding report: NASEM, 2019, The Royal Society, 2018; IPCC, 2018.

cdr primer table 1 1

Examples of Carbon Dioxide Removal Approaches and Prominent Related References. Blue filled-in cells indicate that the given CDR approach is addressed in the corresponding report: NASEM, 2019, The Royal Society, 2018; IPCC, 2018.

Table

1.2

A gigatonne of CO2 removed per year is a massive scale to comprehend. Here we use some highly simplified assumptions to get a rough sense of approximately how much energy, land, water, and material resources are needed to achieve 1 GtCO2/yr of removal. To demonstrate the difference in the type and magnitude of underlying biophysical constraints, we compare biological CDR methods of afforestation and reforestation (AR) to a technological method, direct air capture with carbon storage (DACCS).

cdr primer table 1 2

A gigatonne of CO2 removed per year is a massive scale to comprehend. Here we use some highly simplified assumptions to get a rough sense of approximately how much energy, land, water, and material resources are needed to achieve 1 GtCO2/yr of removal. To demonstrate the difference in the type and magnitude of underlying biophysical constraints, we compare biological CDR methods of afforestation and reforestation (AR) to a technological method, direct air capture with carbon storage (DACCS).

1.2

1.2 —

How terms are used in this primer

“Carbon dioxide removal,” “greenhouse gas removal,” “negative emissions technologies,” and “drawdown” are just some of the terms that have emerged as scientists, engineers, and policymakers from different communities and with different expertise grapple with how best to remove heat-trapping gases from the atmosphere. In an effort to standardize terminology and avoid confusion, this primer adopts the terms used by the IPCC to refer to these concepts, adding supplementary context and framing. For context, we also provide a brief assessment of the historical usage of different terms across various recent reports (Supplement 1.1). 

The IPCC’s 1.5º C Report defines “carbon dioxide removal” (CDR) as:

“Anthropogenic activities removing CO2 from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural CO2 uptake not directly caused by human activities."

The IPCC further defines “negative emissions” as a particular sink – created or enhanced by human activity – where greenhouse gases are being removed. In this primer, only CO2 is considered. Such a sink could be an area where reforestation is taking place or a DACCS facility that is geologically storing the captured CO2. These negative emissions can be thought of as the inverse of “positive emissions,” such as exhaust from a car or emissions from a coal-fired power plant.

Using consistent language to describe CDR is more challenging than it might seem because achieving negative emissions requires rigorous information about an approach’s life cycle emissions and depends on the timeframe over which CO2 is intended to be stored outside the atmosphere. We describe each issue in turn. 

The first challenge arises in assessing whether a CDR approach actually achieves “net-negative” emissions. To determine whether a certain activity is net-negative (removes more emissions from the atmosphere than it contributes), we need a life cycle analysis (LCA) that includes all of the flows of CO2 and other greenhouse gases (along with impacts to other environmental or social impacts of concern). Chapter 4 of this primer explains the utility and challenges of LCAs in more detail. Although life cycle thinking is necessary to evaluate whether a proposed strategy achieves net-negative emissions in practice, the complexity and subjectivity involved in setting the boundaries of LCA for CDR approaches can frustrate anyone seeking clear answers. Reasonable differences of expert opinion over projects’ and programs’ life cycle boundaries can significantly impact the results of LCAs and, in some cases, reveal that CDR projects emit more CO2 than they remove.  

As a result, any given system that provides negative emissions under some combination of reasonable assumptions can, at best, be called a “potential CDR system,” meaning it has the capacity to generate net-negative emissions but is not guaranteed to do so under all conditions. Only when a system is shown to consistently yield net-negative emissions across a range of reasonable assumptions can we say conclusively that it operates as a CDR system. Other strategies that appear to achieve negative emissions under some, but not all, reasonable assumptions for LCA system boundaries may spark ongoing discussion and disagreement in the expert community and among experts, policymakers, and other stakeholders. Furthermore, a CDR system with the “net-negative” label that just barely achieves net-negative emissions could be misleading to policymakers and stakeholders who may not have sufficient context to determine if greater net-negative emissions could be achieved. A CDR system that can produce only a small quantity of net-negative emissions under ideal conditions might be of far less interest than a potential CDR system that does not currently achieve net-negative emissions but has a significant capacity to do so, either now or in the future. 

It is possible, even without a full LCA, to show that a CDR approach could never be net-negative, just through high-level energy and emissions accounting. For example, removing CO2 from the atmosphere through DACCS alone, while not necessarily net-negative, by definition does result in negative emissions (CO2 is removed from the atmosphere). If the energy source used to power the facility emits less CO2 than is captured, that means the DACCS and energy system as a whole also achieves net-negative emissions, and it would then be considered a CDR system. If the corresponding energy source emits more CO2 than is captured, however, the overall result is not net-negative emissions, even if the CO2 removed from the air is permanently sequestered deep underground.

The second challenge relates to the timeframe over which CDR benefits are achieved, an important concept referred to as “permanence” or “durability.” If carbon is removed from the atmosphere, it must go somewhere else. Both this primer and the research community use carbon “storage” and “sequestration” interchangeably to describe the ultimate destination of carbon removed from the atmosphere. For example, some CDR strategies store carbon in biological systems, such as forest or soil ecosystems, whereas others inject CO2 deep underground or chemically transform CO2 into stable, mineral forms. In all cases, the duration of carbon storage outside the atmosphere is a critical concern because the climate benefit of CDR depends not just on the volume of carbon removed, but also how long it is prevented from returning to the atmosphere. The significance of the storage duration and location (reservoir) is discussed in more detail in Section 1.5, which examines the relationship between CDR and the carbon cycle. Timing matters, but the timeframe of CDR is an input for life cycle assessments, not an output. The outcome of LCA analysis can change and even reverse, depending on whether one is looking at short or long timeframes. 

We close this section by addressing two terms we do not use in the primer because they are commonly used elsewhere and should be understood in relation to what we cover here. The term “negative emissions technologies” is often used interchangeably with the term CDR. In this primer, however, we opt to follow the IPCC and use CDR as much as possible since the term “technology” is often not appropriate for describing some CDR processes, including forest management, soil management, and some forms of carbon mineralization. Nearly all CDR applications involve critical social, economic, and political questions that go far beyond the narrow confines of technological feasibility. In addition, we avoid using NETs because of common confusion around the difference between “negative emissions” and “net-negative emissions” in a CDR approach. Rather, we consider the use of LCA and other information to identify when CDR achieves net-negative emissions. Often the answer is more contextual than categorical, which is why a “technology”-focused label can be misleading. 

Finally, this primer avoids the term “geoengineering” because it usually is interpreted to refer to solar radiation management (SRM), a controversial strategy that involves reflecting a small amount of inbound sunlight back into space. If deployed, solar radiation management would not change concentrations of greenhouse gases, but instead would directly affect radiative forcing in an attempt to reduce warming – typically by injecting various substances into the atmosphere (NASEM, 2019). We emphasize that our choice reflects a focus on managing, rather than compensating for, a global pollution problem caused by the buildup of greenhouse gases in the atmosphere. In other words, unlike SRM (geoengineering), CDR addresses the root cause of climate change. Nevertheless, some CDR approaches are also considered controversial with significant impacts that might cause unintended consequences, especially when deployed at gigatonne scale. Those consequences must be transparently analyzed by the expert community and debated publicly.

Supplement

1.1

Historical Usage of Key Terminology

As the community working on the science, engineering, and policy related to carbon dioxide removal grows and diversifies, so too does the language used in this interdisciplinary field. The proliferation of definitions for related terms has increased recently, in part because, between 2018 and 2019, a substantial number of authoritative scientific and policy reports on climate change and CDR were published by various institutions, including the U.S. National Academies of Sciences, Engineering, and Medicine (NASEM), the U.K. Royal Society, the United Nations, and other university and nonprofit research groups. While nearly all of these analyses rely on the IPCC’s 1.5º C report, the IPCC’s 5th Assessment Report (AR5), and/or the 2017 UNEP Emissions Gap report as motivation for the need for gigatonne-scale CDR by the end of this century, they also significantly differ in scope and in the terminology used to define various approaches (IPCC AR5, 2015; IPCC, 2018b; NASEM, 2019b; The Royal Society, 2018; UNEP, 2017). Below are a few examples from three prominent publications that demonstrate these language differences:

The IPCC’s 1.5º C report (2018) defined “carbon dioxide removal” as “anthropogenic activities removing CO2 from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage but excludes natural CO2 uptake not directly caused by human activities.” The term “negative emissions technologies,” or “NETs,” was avoided, although the IPCC’s AR5 report notes that the term “can be used as an alternative to CDR.” The UN Environment Programme Emissions Gap Report 2017, which devotes a chapter to CDR, defines the term similarly to the IPCC and does not use the term “NETs.”

The Royal Society report (2018) uses the term “Greenhouse Gas Removal (GGR)”: “GGR methods involve two main steps: the removal of greenhouse gases from the atmosphere and their storage for long periods. The process is best established for carbon dioxide (CO2) removal. Removal is achieved through a wide variety of approaches, involving either biology, accelerating natural inorganic reactions with rocks, or engineered chemical processes. The carbon is then stored in land-based biomass, subsurface geological formations, the oceans, or the built environment.” This report also avoids the term “NETs.”

The National Academies report (2019) uses the terms NETs and CDR interchangeably. The report defines NETs as approaches “which remove carbon from the atmosphere and sequester it.” The term “sequestration” is often used loosely to refer to storage of carbon in any reservoir: terrestrial, oceanic, nearshore/coastal environments, or geologic. However, not all sequestration processes constitute a NET in and of themselves. Unlike in the case of afforestation or reforestation, which combine the CO2 removal and sequestration steps into a unified biological NET process, the report explicitly defines geological sequestration as: “not a NET, but rather an option for the sequestration component of BECCS or direct air capture.”

1.3

1.3 —

Why focus on carbon dioxide for climate stabilization?

If emissions of multiple greenhouse gases (carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons) are causing the climate crisis, why does this primer focus only on removing CO2 from the atmosphere? The answer lies in the properties of greenhouse gases once they reach the atmosphere as well as their relative atmospheric concentration.

Under a common measure of cumulative long-term warming impacts, carbon dioxide is the most important greenhouse gas emitted by human activity (Edenhofer et al., 2014). This measure takes into account the total emission rate of the gas, as well as its atmospheric lifetime and ability to absorb incoming solar radiation (Myhre et al., 2013). Carbon dioxide is a very long-lived gas, with carbon cycle impacts that can last centuries to millennia (Archer et al., 2009). By contrast, other important greenhouse gases, commonly referred to as short-lived climate pollutants (SLCPs), have much shorter atmospheric lifetimes closer to 10 to 100 years. While the atmospheric concentration of CO2 may already seem low at around 410 parts per million (ppm), its concentration is significantly larger than the next-most-abundant greenhouse gas, methane, which is around 2 ppm (Saunois et al., 2020). The relative abundance of CO2, its long atmospheric lifetime, and its chemical reactivity make CO2 an appealing candidate for removal. Furthermore, the global carbon cycle flux of CO2 (its rate of movement between reservoirs) is substantially larger than that of any other gas, which allows for more biological, geological, and chemical CDR interventions to be explored.

In terms of current warming contribution, the most important gas after carbon dioxide is methane (CH4), which is produced primarily by fossil fuel systems, agricultural activities, and some ecosystems, among other sources such as hydropower reservoirs (Saunois et al., 2020). Agriculture is also a critical driver of nitrous oxide (N2O) emissions, particularly from the use of nitrogen-based fertilizers (Tian et al., 2020). Finally, a suite of fluorinated gases (known as F-gases) contribute a small but rapidly growing share of warming, arising largely from emissions of hydrofluorocarbons (HFCs), which are used in a variety of industrial and refrigeration applications (Velders et al., 2015). Each of these gases has a significantly stronger effect on global warming per emitted molecule than CO2, but their lifetime in the atmosphere is significantly shorter.

Methane, for example, has an atmospheric lifetime on the order of about a decade (Saunois et al., 2020). This short-lived gas is particularly relevant because it is a potent substance in terms of its instantaneous and near-term warming impacts, but it degrades quickly in the atmosphere and eventually has little long-term warming impact (Myhre et al., 2013; Alvarez et al., 2012; Alvarez et al., 2018). Both CO2 and CH4 concentrations are rising in the atmosphere, but once humanity brings its emissions under control, the concentration of CH4 will fall over the course of a few decades whereas much of the historically emitted CO2 will remain in the atmosphere on an effectively permanent basis. This difference in atmospheric lifetime also means that, while CO2 emissions at a constant rate continue to contribute to warming, constant methane emissions are balanced by the rate of degradation of atmospheric methane on a timescale of a few decades and do not contribute to increased warming once a steady state is established (Allen et al., 2018). While some have argued for the need for methane removal strategies that are not considered in this primer (Jackson et al., 2019), others point out that a strategic focus on reducing SLCPs in lieu of reducing long-lived gases results in permanently higher rates of total warming (Pierrehumbert, 2014) and has profound equity considerations (Stohl et al., 2015).

Due to the different atmospheric dynamics of short- and long-lived gases, stabilizing temperature or stopping warming (often referred to as the net-zero greenhouse gas target) is a more complicated concept than it may first appear. If carbon dioxide were the only GHG being emitted, then a simple accounting of total sinks and sources would identify the net-zero point. Yet, when all greenhouse gases are accounted for, reductions in SLCPs (e.g., methane) can mean that there can be positive net CO2 emissions for some period of time while still not causing increases in net radiative forcing. Simple metrics, such as Global Warming Potential (GWP; explained in detail in Supplement 4.1, in Chapter 4), have been proposed as a way to compare the impacts of different greenhouse gases by normalizing to a CO2-equivalent measure. However, GWP, which asks how much energy the emission of a given gas will absorb over a given period (typically either 20 or 100 years) relative to CO2, has been criticized as too simplistic and misleading with respect to net-zero targets (Allen et al., 2018; Cain, 2018; Kleinberg, 2020). Earth system climate models that evaluate radiative forcing based on complete emissions trajectories account more accurately for the different physical properties of the greenhouse gases (Allen et al., 2018). Understanding these differences is crucial when determining the possible scale of CDR because a substantial portion of the emissions that will remain after deep decarbonization efforts will likely be non-CO2 GHGs.

1.4

1.4 —

The scale of hard-to-avoid emissions and the CDR needed to offset them

Permanently removing a tonne of CO2 from the atmosphere and storing it affects the climate system in the same way as preventing a tonne from being emitted. Thus, purely in terms of the climate system, in order to achieve a temperature target, more emissions reductions make CDR less necessary, and lower emissions reductions make CDR more critical.

However, these options come with different harms and benefits and are not interchangeable (as this simple framing might seem to imply). The least expensive option (expressed in terms of cost per tonne of CO2) is not necessarily the best one from a broader perspective that considers environmental impacts and social justice. 

Thus, a critical and controversial question facing climate science and policy communities is how much CDR is required to achieve climate goals. Many claims rely on the results of integrated assessment models (IAMs), where the scale of CDR is a model output. Here we will review those "top-down" methods and then propose a “bottom-up” approach to determine how much CDR is necessary by directly estimating “hard-to-avoid” emissions that will remain on a by-sector basis even with deep decarbonization.

IAMs use economic, ecological, and technological assumptions to model global emissions mitigation trajectories, and scenarios are typically compared on the basis of cost. IAMs include CDR – typically in the form of BECCS or reforestation and afforestation – as one of several mitigation tools that can be used in the model to reduce emissions. If a model determines that CDR is more cost-effective than pursuing other forms of emissions reduction as a result of its assumptions, the model will select more CDR – even if those other options are physically feasible and socially desirable under a broader set of considerations.

In an extensive analysis of the results of different published IAMs, Fuss et al. (2018) found that scenarios achieving 1.5° C included CDR deployment by 2050 ranging from 1.3 to 29 GtCO2/yr, with most falling between 5 and 15 GtCO2/yr. The range for 2° C scenarios was wider, from 0 to the higher end of the 1.5° C scenario range. Commonly-cited ranges based on a related 2° C analysis from the 2017 UNEP report require annual CDR of 10 GtCO2 by mid-century and 20 GtCO2 by 2100. As Minx et al. (2018) summarize, “Recent modelling features [CDR] at very large, sometimes staggering scales.”

The scale of CDR that these scenarios rely on, and imply we will need, presents distinct and important moral hazards and ethical considerations. 

First, many scenarios use massive amounts of CDR to avoid reducing emissions early in the century. Some even overshoot their concentration or temperature targets early on – sometimes by a substantial margin – and then use CDR later to achieve their original targets. As Anderson and Peters (2016) explain, these scenarios can make the promise of CDR “more politically appealing than the prospect of developing policies to deliver rapid and deep mitigation now,” thus avoiding the societal burden and political challenges of reducing anthropogenic emissions. These outcomes raise a moral hazard: Assuming the availability of large-scale CDR deployment could disincentivize emissions reductions in the present, significantly increasing the risk of catastrophic climate change by locking in a reliance on fossil fuels (Lenzi, 2018; Anderson and Peters, 2016).

The assumed large scale of potential future CDR may also reflect both unreasonable technological optimism and hubris in our ability to control complex natural systems (Lenzi, 2018). Questions have been raised about the feasibility of BECCS in particular at the scale assumed by models (Anderson and Peters, 2016). The moral hazard is additionally exacerbated because modelling CDR at massive scales, without understanding the associated social and environmental risks (described in Section 1.6), could overestimate the ability to deploy them effectively or justly. As Shue (2017) further points out, a failed gamble on CDR would harm future generations – especially the poorest and most vulnerable among them, who could not possibly consent.

Second, an ethical concern arises in how to interpret the specific amount of CDR derived from these modeling scenarios, which extends the moral hazard discussed above. Fuss et al. (2018) stress that these amounts reflect model dynamics and should not “be interpreted as requirements in a more formal sense.” IAMs do not include a measure of hard-to-avoid emissions as a model input. Some reports, however, have interpreted these numbers as proportional to emissions “sources that would be very difficult or expensive to eliminate” (NASEM, 2019) and thus use them as a basis for arguing that large-scale CDR is required for one temperature target or another. A report on CDR from the National Academies, for example, concludes that “if the goals for climate and economic growth are to be achieved, negative emissions technologies will likely need to play a large role in mitigating climate change by removing ~10 Gt/yr CO2 globally by midcentury and ~20 Gt/yr CO2 globally by the century’s end” (NASEM, 2019). But by construction, the emissions remaining in any particular IAM scenario, and the total amount of CDR required to compensate for them, are not based on an explicit analysis of which emissions will be deemed hard-to-avoid in the future. Calling them such, especially outside the context of the specific modeling approach used, unreasonably justifies a potentially excessive (and morally hazardous) amount of CDR.

Figure

1.1

Schematic of hard-to-avoid emissions and the CDR needed to offset them. Adapted from a figure produced by Glen Peters (2020)

cdr primer figure 1 1

Schematic of hard-to-avoid emissions and the CDR needed to offset them. Adapted from a figure produced by Glen Peters (2020)

An alternative “bottom-up” approach would scale the amount of required CDR to match a direct estimate of future hard-to-avoid emissions. Stabilizing global temperatures (or stopping increased warming) would require at least enough CDR to offset this amount.

For this purpose, we define hard-to-avoid emissions narrowly as emissions that will be either unacceptable to avoid from a social justice perspective or extremely physically difficult to eliminate within the given timeframe. Social justice considerations include instances, for example, where reducing emissions would be associated with depriving people of the means to satisfy their basic needs, like food security. Similarly, extreme physical difficulty considerations include biological or technological operating conditions (like specific temperature or humidity levels) that are not available on Earth at the scale required, or situations where avoidance would require a technology that relies on a globally scarce resource. We do not consider high cost alone as a reason for labeling a source of emissions hard-to-avoid. We further assume that hard-to-avoid emissions remain as a steady-state floor after transient political, economic, and technological constraints associated with decarbonization efforts have been overcome, resulting in a minimum estimate for annual future emissions (Figure 1.1).

Some of these emissions will come in the form of CO2 and can be directly offset by an equal magnitude of CDR. Other important greenhouse gas emissions, such as methane and nitrous oxide emissions in the agricultural sector, have different physical properties than CO2, relating to lifetime and energy absorption (discussed in Section 1.3), so the quantity of CDR necessary to stabilize temperature must be determined differently (discussed in detail in Supplement 1.3). 

Supplement 1.3 presents a simplified and conservative estimate of hard-to-avoid emissions, based on several studies that suggest possible lower limits in specific sectors under aggressive decarbonization strategies. The sum of these hard-to-avoid emissions (Table 1.3) ranges between 1.5 and 3.1 GtCO2eq/yr, coming mostly from the agriculture and transportation sectors, implying that between 1.5 and 3.1 GtCO2 of CDR/yr would be required to offset them. Note that this is substantially lower than the typical 5 – 15 GtCO2 of CDR/yr arrived at by many IAMs. Our analysis also yields lower values than a study by Davis et al. (2018), which defined "hard-to-decarbonize" emissions based on economic considerations rather than just physical and ethical feasibility. That study also included emissions from industry and electricity, whereas in our estimate these sectors are fully decarbonized. (See Supplement 1.3 for more details.) 

Limiting warming to 1.5° C, which would avoid the worst effects of climate change, requires reaching the net-zero greenhouse gas target by the end of this century (IPCC, 2018b). To stabilize temperature this century, the scale of hard-to-avoid emissions resulting from our analysis suggests the need for gigatonne-scale CDR to manage the climate crisis, even when maximizing emission reductions. Greater amounts may be justified by a determination that the ethical concerns discussed here are outweighed by other benefits. But, for the reasons described, we urge caution in interpreting the larger values resulting from IAM-based approaches, and we suggest adopting a broader CDR framework based on social justice and fully incorporating harms and benefits.

Supplement

1.2

An estimate of the scale of hard-to-avoid emissions

The following is a by-sector analysis based on multiple studies to estimate a range of values for global hard-to-avoid emissions. For each type of emission, the higher end of the range is based on the lowest emissions values of a set of socioeconomic model trajectories; the lower end is based on a direct sector-specific feasibility assessment. The exception is the lower end of agriculture and waste N2O emissions, which is based on a limiting model trajectory. This is because agricultural output is predominantly a social justice, not physical, constraint, relying on society-wide assumptions that cannot be calculated purely on a feasibility basis. Whenever more detail was available, we rounded results from analyses we used to the nearest 0.1 GtCO2eq. A measure of “CO2eq hard-to-avoid emissions” is used to compare across the different greenhouse gas emission sources and normalize to an equivalent warming from CO2. A large part of our analysis is based on the IPCC’s Low Energy Demand (LED) scenario (Grübler et al., 2018), which we evaluate because it estimates an upper bound for hard-to-avoid emissions by minimizing CDR use while limiting warming to 1.5º C. To meet these conditions, this model makes a case for the feasibility of decarbonizing the electricity and industrial sectors. Despite a massive 40% reduction of energy consumption compared to today, LED suggests significant hard-to-avoid emissions will remain, mainly in the agriculture and transportation sectors. The IEA 2020 Energy Technology Perspectives report is used to further justify decarbonization feasibility assessments.

Agriculture and waste nitrous oxide: The partial evaporation of fertilizer applied to soils and manure left on pasture, necessary for maintaining food security, are the largest contributors to global anthropogenic nitrous oxide (N2O) emissions (Tian et al., 2020). While fossil fuel and industrial sources of N2O could be decreased, given necessary waste processing practices and the massive area of global farmland and pasture, it is not feasible to prevent these emissions from reaching the atmosphere (e.g., through domes or other technological improvements). The lifetime of N2O is greater than a century, so its global warming potential at 100 years is used to normalize to CO2eq.

Why won’t constant methane emissions need continuous CDR offsetting? Substantial emissions of methane (on the order of tens of MtCH4/yr), including from livestock production, rice cultivation, and landfills, will also likely remain throughout this century (Saunois et al., 2020). Over a long timescale (longer than methane’s ~12-year lifetime), constant methane emissions are balanced by atmospheric methane degradation and do not accumulate in the atmosphere or contribute to increasing warming (Cain, 2018). For this reason, while these constant methane emissions can be considered hard to avoid, they do not factor into our estimate of the hard-to-avoid CO2eq emissions that require ongoing CDR (Allen et al., 2018). Note, however, that offsetting this constant level of methane emissions through a one-time “pulse” of CDR would reduce global temperature. 

Transportation: The movement of materials and people around the globe is critical for the world economy and for social justice. In the transportation sector, there is no indication that an alternative energy source for long-distance air travel other than hydrocarbon fuel will be feasible (IEA, 2020). While there are studies that point to using alternative fuels (such as hydrogen) for shipping, decarbonization trajectories continue to rely significantly on biofuels and synfuels (OECD, 2018). 

Buildings: Heating, cooling, and operating appliances constitute basic human needs. In global regions with more extreme climates, the clean technologies (especially heat pumps) needed to decarbonize buildings will not be effective given physical constraints (specifically, temperature) (IEA, 2020).

Which sectors have no hard-to-avoid emissions? We assume the electricity and industrial sectors can be fully decarbonized without any remaining hard-to-avoid emissions. While this is a tremendous challenge to achieve globally, and becomes increasingly difficult with each step closer to the 100 percent decarbonization target, this analysis relies on the IPCC’s Low Energy Demand scenario for these feasibility assumptions. All sectors contain significant deep decarbonization that is not explained in detail here but can be explored in the supplementary material of the IPCC’s 1.5º C report (2SM).

How are biofuels and synfuels treated? Low- or net-zero carbon alternatives, such as biofuels or synfuels (e.g., DAC-to-fuel) could displace fossil fuels in the transportation and building sectors. However, as long as hydrocarbons are combusted and positive point sources remain, this analysis considers them hard-to-avoid emissions.

Table

1.3

Estimates of hard-to-avoid emissions arising from different sectors (see Supplement 1.3)

cdr primer table 1 3

Estimates of hard-to-avoid emissions arising from different sectors (see Supplement 1.3)

1.5

1.5 —

Carbon dioxide removal and the carbon cycle

To understand the relevance of CDR to climate change, it is necessary to put CDR in the context of the global carbon cycle (Keller et al., 2018). The carbon cycle concerns the amount and flux of carbon – in various chemical states – between the ocean, terrestrial biosphere (or “land”), atmosphere, and geologic formations in the Earth (Figure 1.2a; Friedlingstein et al., 2019). Large-scale CDR deployment will directly affect levels of atmospheric carbon, but also create feedback loops that alter fluxes among other carbon reservoirs. For this reason, removing 1 GtCO2 from the atmosphere will ultimately reduce atmospheric CO2 concentrations by less than 1 Gt. To understand how CDR perturbs the carbon cycle, we need to characterize its effects on fluxes between reservoirs as well as how carbon is stored in reservoirs. Moreover, even if net-zero emissions are achieved by the end of this century through the use of CDR to offset hard-to-avoid emissions, the particular emission and CDR pathways may leave long-lasting harmful imprints on parts of the global climate system, such as ocean acidity or ecosystem health (Mathesius et al., 2015).

Figure

1.2

Schematic representation of the main carbon fluxes among the atmosphere, land, ocean, and geologic reservoirs, depicting: (a) unperturbed carbon cycle fluxes, (b) industrial-era carbon cycle perturbations, (c) net-positive emissions with CDR perturbations, and (d) global net-negative emissions perturbation. Reproduced from Keller et al., (2018).

cdr primer figure 1 2

Schematic representation of the main carbon fluxes among the atmosphere, land, ocean, and geologic reservoirs, depicting: (a) unperturbed carbon cycle fluxes, (b) industrial-era carbon cycle perturbations, (c) net-positive emissions with CDR perturbations, and (d) global net-negative emissions perturbation. Reproduced from Keller et al., (2018).

To simplify, the carbon cycle has fast (less than 100 years) and slow (greater than 1,000 years) processes. The fast processes include the exchanges among the atmosphere, terrestrial biosphere, and shallow ocean reservoirs. We refer to these carbon reservoirs as “stocks” to indicate their more transient and mutable nature. The slow carbon cycle processes can take thousands to hundreds of thousands of years and concern geological reactions, deep ocean dynamics, and even some organic processes. 

While there is substantial exchange among the atmosphere, terrestrial biosphere, and shallow ocean stocks on an annual basis – with the oceans currently releasing and taking up approximately 330 GtCO2/yr and vegetation photosynthesizing and respiring about 440 GtCO2 per year – these flows in the carbon cycle are relatively well-balanced. This means that on shorter timescales and in the absence of human-driven emissions, the total carbon stock (or total carbon stored in the atmosphere, terrestrial biosphere, and shallow ocean) does not change. 

Human-driven combustion of fossil fuels creates a rapid acceleration of the release of carbon from geological storage into the atmosphere, where it affects the coupling among the atmosphere, terrestrial biosphere, and shallow oceans (Figure 1.2b). The total carbon stock is currently around 15,000 GtCO2. Annual emissions, which are currently about 40 GtCO2, may seem small compared to the total carbon content of the fast-cycling carbon stocks. However, what matters to the planetary balance of radiative energy, and therefore the impact on the extent of warming, is the long-term impact of that seemingly small annual flux on total atmospheric concentrations. Since pre-industrial times, almost one sixth (655 GtC or 2,400 GtCO2) of the current fast-cycling carbon stocks is the result of human activity, increasing atmospheric CO2 concentration from approximately 280 ppm to 410 ppm in 2019 (Friedlingstein et al., 2019). Anthropogenic CO2 emissions can be expected to remain in the atmosphere for hundreds to thousands of years (Archer et al., 2009), effectively making this a permanent problem from the standpoint of human civilization – one that can be addressed only by radically reducing emissions and embarking on a program of large-scale CDR.

Carbon dioxide emissions from human activity (notably, burning fossil fuels and deforestation) have significantly perturbed the balance between global sinks and sources of CO2. The net fluxes between the carbon stocks – that is, the differences between the incoming sink and outgoing source rates – are much smaller than the overall exchanges among the atmosphere, terrestrial biosphere, and shallow oceans, and are predominantly the consequence of human-driven emissions. The ocean is currently absorbing more than it is emitting, making it a net sink that takes up around 9 GtCO2/yr. The land, which is also a net sink, takes up close to 12 GtCO2/yr. Land and ocean sinks take up roughly half of annual emissions, while the other half of humanity’s carbon emissions end up in the atmosphere. As a result of these positive, sustained fluxes, all three stocks are growing. Since pre-industrial times, about 24 percent of human-driven emissions have ended up in the ocean, 30 percent in the terrestrial sink, and 40 percent in the atmosphere (with 6 percent of emissions not entirely accounted for). If it were not for the ocean and land sinks, the climate impact from emissions would be roughly two times greater at this point.

The climate system’s slow timescale for removing CO2 from the atmosphere means that unless drawdown is enhanced, atmospheric COconcentration will continue to increase – even if only hard-to-avoid emissions continue and all other emissions are abated. Even if we stopped emitting today, a substantial portion of the anthropogenic CO2 would remain in the atmosphere over the coming century and beyond. This sobering conclusion was established by a model intercomparison project that yielded consistent agreement that following a roughly 3,600- or 18,000-GtCO2 spike in released CO2, reaching steady state between the atmosphere and the ocean would take between 200 and 2,000 years, with 20 – 35 percent of that CO2 remaining in the atmosphere even after steady state is reached (Archer et al., 2009). Another study explored the behavior of Earth system models where emissions reach net-zero CO2 after 3,667 GtCO2 are added to the atmosphere (MacDougall et al., 2020). The most likely outcome is that after the initial warming associated with the emissions, there will be essentially no additional change in temperature on a timescale covering several decades. Warming is expected to stay fixed because, even though land and ocean sinks will continue to reduce atmospheric carbon, the capacity of the ocean to uptake heat in the decades after emissions cease will also decrease – as it turns out, in a roughly balanced fashion. Some models, however, continue to warm for decades to millennia after emissions end, while others cool substantially. This slow timescale is largely due to the deep ocean taking more than 1,000 years to achieve a new steady state with respect to both temperature and carbon concentrations after changes in atmospheric CO2 concentrations (AR5, 2013). The slow timescale of the carbon cycle is a strong motivator for offsetting hard-to-avoid emissions through CDR. 

CDR offers the possibility of accelerating CO2 drawdown into carbon reservoirs such as vegetation, soils, oceans, geologic formations, and building materials. Sufficiently fast CDR rates for addressing climate change may be possible, but are limited by the amount of energy, land, and resources that society is willing to devote, rather than fundamental biophysical constraints (Smith et al., 2015). (See Table 1.2 for estimates of scale.) However, CDR’s perturbation of the carbon cycle must be properly evaluated to understand its potential impact on the climate system. We highlight three important categories of impacts here.

First, just as land and ocean sinks take up a portion of the carbon that humans emit, removing CO2 from the atmosphere initiates its own feedback processes (Figure 1.2c-d; Keller et al., 2018). Following large-scale CDR that reduces atmospheric concentrations of CO2, some amount of CO2 will return to the atmosphere from the ocean and terrestrial biosphere, dampening the efficiency of CDR (Jones et al., 2016; Tokarsa et al., 2015). For example, one modelling study examining the Earth’s climate system in a pre-industrial steady state found that instantly removing 367 GtCO2 from the atmosphere would reduce atmospheric CO2 concentrations by only 92 GtCO2 (or about 25 percent of the initial magnitude) after 100 years (Figure 4 in Keller et al., 2017). In general, these effects depend on many factors, including temperature, atmospheric CO2 levels, and the general allocation of carbon throughout the Earth system. 

Second, the choice of CDR storage reservoir also has significant impacts on the carbon cycle (Keller et al., 2018). The “permanence” or “durability” (i.e., the length of time that CO2 stored in a particular manner is expected to remain out of the atmosphere) of different CDR approaches is discussed in detail in Table 1.4. On the less-permanent end, one option for CDR is to redistribute carbon stock from the atmosphere to the biosphere or the shallow ocean. Redistribution could occur in different ways, including by reducing the magnitude of certain carbon sources (e.g., reducing deforestation) and by increasing land sinks (e.g., by reforestation using natural regeneration). These different approaches could lead to similar outcomes for atmospheric COlevels but would require different governance models. Although stock perturbations provide wide-ranging opportunities for significant co-benefits – such as improvements in agricultural soil quality and general ecosystem health – they have relatively short-term carbon storage outcomes and are more susceptible to reversal since stored carbon remains in transient stocks. This observation does not mean that biologically-based CDR approaches are not sensible. Shorter-duration strategies could be justifiable on the basis of co-benefits, low costs relative to more permanent approaches, and the potential to sequence removal efforts and/or a transition to more permanent removals over time (Allen et al. 2020). Alternatively, CO2 could be sequestered in geologic formations deep underground that have the physical capacity to store the amount of CO2 necessary for large-scale CDR. Geological storage effectively removes CO2 from the atmosphere, terrestrial biosphere, and shallow ocean stocks on a permanent timeframe. Similarly, long-term storage can be achieved through carbonation to produce building materials and other carbon-based items that have greater permanence than biological stocks but are significantly more energetically, environmentally, and technically challenging. Conceptually, the large difference in permanence characteristics reveals that some CDR strategies are contingent on future socioeconomic conditions and decisions (such as ongoing implementation of soil management practices), whereas other strategies create outcomes that are largely independent of future decisions (such as CO2 mineralization).         

Third, the effectiveness of carbon sinks can change as humans continue to emit greenhouse gases. For instance, as oceans get warmer, their capacity to store dissolved CO2 decreases. There is also evidence that land sink uptake may decrease in proportion to rising emissions from fossil fuels and deforestation, in part due to climate effects on plant physiology and ecosystem disturbances (Fung et al., 2005; Hewitt et al., 2016; Anderegg et al., 2020). In general, if the terrestrial or ocean sinks weaken, then the emission of the same unit of CO2 would have a relatively stronger effect on global warming (AR5, 2014). These uncertainties and predicted harms contribute to the growing body of evidence that prioritizing ambitious reductions in both carbon dioxide and short-lived climate pollutants is the most effective strategy to avert the worst harms of climate change in both the near term and long term. 

Table

1.4

Here, we categorize the permanence of different types of CDR approaches. Rather than describe the maximum or minimum possible duration, each category features a representative permanence range that describes typical outcomes seen in practice and/or described in the literature. This range is meant to assess what is likely if a given approach is executed without error, and therefore any concerns about project implementation need to be considered as factors that would modify these ranges. We include a list of physical and socioeconomic risks to permanence. These risks concern factors that threaten to reduce the permanence of specific mechanisms, not potential physical or social impacts of CDR approaches. These risks also exclude environmental and economic co-benefits associated with each strategy, and thus are meant only to help understand the permanence of CDR approaches – not their comprehensive social appeal.

cdr primer table 1 4

Here, we categorize the permanence of different types of CDR approaches. Rather than describe the maximum or minimum possible duration, each category features a representative permanence range that describes typical outcomes seen in practice and/or described in the literature. This range is meant to assess what is likely if a given approach is executed without error, and therefore any concerns about project implementation need to be considered as factors that would modify these ranges. We include a list of physical and socioeconomic risks to permanence. These risks concern factors that threaten to reduce the permanence of specific mechanisms, not potential physical or social impacts of CDR approaches. These risks also exclude environmental and economic co-benefits associated with each strategy, and thus are meant only to help understand the permanence of CDR approaches – not their comprehensive social appeal.

The relationship between CDR and the carbon cycle is complicated, as are the relative impacts of different carbon cycle interventions on the rate and extent of planetary warming. A more complete climate system assessment would include sinks and sources from all greenhouse gases, and in particular methane (Saunois et al., 2020) and nitrous oxide (Tian et al., 2020), but we will not undertake such an assessment in this section. Given the complexity of the interaction between CDR and the climate system, different communities and stakeholders are likely to reach different conclusions as to what actions are required, based on geographical, economic, social, and ethical considerations. We turn next to identifying some of the harms and co-benefits associated with CDR, and close the chapter with a discussion of social justice concerns.

1.6

1.6 —

Harms and co-benefits of large-scale CDR deployment

The deployment of gigatonne-scale CDR will require decision makers to simultaneously balance societal, economic, environmental, and technological considerations. The CDR approaches discussed in this primer would significantly increase demand for global energy production and land requirements, potentially straining the electricity and food production sectors. Decision makers will need to consider a wide range of qualitative and quantitative parameters – including societal value-based judgements, as well as options for reducing CO2 and non-CO2 greenhouse gas emissions – that will actively evolve with new information and experience. Regardless of the specific policies and approaches chosen for gigatonne-scale CDR deployment, some combination of materials, energy, land, and resources will need to be allocated accordingly at massive scales. (See Table 1.2 for an estimate of scale.) Applying frameworks of equity, as described below in Section 1.7, can ensure that the most vulnerable communities are economically and socially bolstered as CDR is deployed. To do that, however, we must first review some of the harms and co-benefits associated with CDR projects.

While there are significant risks and costs associated with all CDR methods, certain approaches also confer societal benefits. A systematic literature review found evidence that forest-based strategies, soil carbon management, terrestrial enhanced weathering, and biochar (all of which are reviewed in this primer) may contribute to soil quality, nutrient retention, and water cycling under appropriate management regimes (Fuss et al., 2018). There may also be local or regional socioeconomic benefits. For many ecosystem-level metrics, however, the literature is ambiguous, and more research is needed to understand possible harms. For example, trace GHG emissions could potentially be mitigated or intensified by changes in land management practices, although in-depth analysis is needed to clarify net effects over time.

Even the decision to implement well-understood CDR approaches may have anticipated negative consequences, which will depend on the mode of implementation and its scale. The choice to grow monocultures of eucalyptus, for example, minimizes required land area necessary for a given rate of CDR. However, this practice also conflicts with other social and environmental goals, including protecting biodiversity and preventing wildfires (Mac Dowell et al., 2017; Smith et al., 2019c). The effect on the Earth’s radiative forcing through albedo modification from large-scale land use change is another complicating factor. For example, growing forests over significant new areas of the planet both removes CO2 from the atmosphere and reduces the land’s ability to reflect incoming solar radiation, significantly reducing the climate benefits of CDR in higher latitudes (Smith et al., 2016). A vast literature testifies to the increased demand for land that would be likely if large-scale BECCS and afforestation programs are initiated, which would threaten food production and biodiversity. If powered by renewable energy sources, technological systems such as DACCS will also require a substantial amount of land (Table 1.2). Some of these risks can be mitigated by resorting to land that is currently not supporting communities or providing high-quality natural habitat, or by using waste biomass feedstocks. Yet, if CDR approaches a scale of double-digit gigatonnes of removal per year, fewer opportunities for careful implementation will remain, resulting in increasingly difficult trade-offs among different land uses and divergent community interests.

CDR strategies also carry the potential for serious harm to the environment, climate, and frontline communities. Harms can result from intentional behaviors, including fraudulent claims of CO2 sequestration to earn tax credits (Sylvan and Allen, 2020) and utilities knowingly neglecting infrastructure that causes catastrophic forest fires (Penn et al., 2019). Harms also can stem from inadvertent behaviors, ranging from accidentally releasing a toxic CO2 capture material to unknowingly misevaluating the economic or agricultural impacts of a proposed policy. While everything we choose to do as a society carries the potential for serious harms, thoughtful policies and frameworks can be put in place to try to foresee and account for both malice and error, and to prioritize equitable outcomes when people or future generations are harmed as a result of our collective action or inaction.

There is a significant range of capacity and permanence estimates across CDR approaches, which impact how that storage should be compared to other reservoirs and how to account for uncertainty (Table 1.4). The main distinction around permanence stems from the difference between sequestration reservoirs. Reservoir permanence ranges widely: Storage of CO2 in geologic formations deep underground can be near-permanent, while terrestrial carbon stocks can rapidly release carbon back into the atmosphere if management practices or external factors change. For example, forests are grown to sequester carbon in plant material over decades or hundreds of years, but a wildfire can return some portion of the carbon stored in plant tissues to the atmosphere as carbon dioxide in just days or weeks (Minx et al., 2018). Trees will grow back after a wildfire, but there is less carbon stored in the forest while they do. In contrast, when proper planning identifies reliable sequestration sites and establishes sound monitoring protocols, CO2 injected into geologic formations is unlikely to leak at significant scale over thousands of years. This difference in the sequestration timescale and the potential for rapid release highlights the importance of establishing robust frameworks to quantify, compare, and manage a portfolio of CDR approaches responsibly. The potential for intentionally malicious behavior, as well as simple mistakes, means that we will always need comprehensive planning, monitoring, and accountability frameworks.

Another crucial challenge is properly assessing additionality, or determining the impact of an intervention as compared to a baseline in order to avoid perverse incentives that increase emissions. Accounting for the CDR benefit from a given project requires comparison to a counterfactual scenario – what would have happened otherwise – which can only be estimated, not directly observed. If a single project claims credit for an emissions-reducing action that would have happened anyway (and if someone else gets credit to continue to emit what they are regulatorily required to reduce or eliminate, as is the case with some governments’ carbon offset programs) the project would result in net increases in greenhouse gas emissions and therefore damage to the climate (Haya, 2010; Haya et al., 2020; Warnecke et al., 2019; Bento et al., 2016; Cullenward and Victor, 2020). While always important to consider, additionality is easier to reason about in the case of very expensive projects, which likely would not occur without a strong policy driver. On the other hand, cheaper offsets, which tend to involve biological systems, are more likely to be non-additional.

Given that direct air capture, enhanced weathering, and ocean fertilization have more recently entered the discussion, their associated harms and co-benefits are under-researched relative to better-established CDR approaches. There are no strict biophysical constraints on the feasibility of building large-scale direct air capture infrastructure. But extensive deployment will require large amounts of materials and associated infrastructure that can cause associated greenhouse gas emissions and other negative impacts, as well as access to large amounts of low-carbon energy that might otherwise be used to decarbonize existing energy demand. The ecological impacts could be more pronounced for non-biological CDR approaches with transboundary impacts like enhanced weathering, which could result in air and water pollution at the mineral extraction site or changes in the physical and chemical properties of soils (Fuss et al., 2018a).

To ensure that we avoid the worst harms of climate change, it is important to acknowledge that CDR cannot completely undo the damage caused by accumulated greenhouse gas emissions (Keller et al., 2017). There are many societal factors that affect CDR deployment, and the particular greenhouse gas emission trajectory that humanity settles on in the coming decades matters. Abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse (Lenton et al., 2019) – not to mention climate tipping points scientists do not yet know about – could be triggered if greenhouse gas emissions are not reduced quickly. Enabling higher emissions on the assumption that late-century deployment of CDR can be relied upon to lower atmospheric carbon dioxide concentrations is harmful because peak CO2 concentrations can have significant and irreversible negative impacts on sensitive ecosystems and vulnerable populations (IPCC, 2018a). For example, there is evidence to indicate that marine ecosystems will take centuries to recover from the effects of ocean acidification, even if peak ocean acidity is subsequently reduced through the deployment of large-scale CDR (Mathesius et al., 2015).

More generally, the risk of “moral hazard”, in which emission reduction activities are reduced or delayed on the promise of future CDR deployment, is an ethical concern for large-scale CDR deployment (Lenzi, 2018). As detailed in Section 1.4, matching the scale of CDR to hard-to-avoid emissions calls for gigatonne-scale removal – a large endeavor, though significantly less than the massive scale suggested by many IAM studies. Section 1.4 also presents the distinct moral hazards and ethical considerations associated with the scale of CDR in these IAMs. The rate of reduction required for 1.5 and 2° C stabilization scenarios is so great as to refute any argument that the potential for successful CDR at scale in the future justifies slowing down the pace of emissions reductions (Minx et al., 2018; Morrow et al., 2020). Related moral hazard concerns arise when considering emission reduction pathways. Some technologies, like using carbon capture and sequestration (CCS) to avoid power plant emissions, may reinforce our current reliance on burning fossil fuels, which has significant environmental, equity, and economic consequences beyond climate change (Hamilton et al., 2013).

More sophisticated policy strategy could help alleviate moral hazard concerns with respect to CDR. Mac Dowell et al. (2017) argue that effective mitigation policy should target direct reductions of existing emissions, which achieves the most certain and permanent climate benefits of all – not CO2 utilization, which may or may not result in reduced overall emissions and could, in the worst case, distract investments from their most effective use and continue fossil fuel dependence and deforestation. Another option is to set climate goals with separate targets for existing emissions reductions and CDR (McLaren et al., 2019).

With so many unknowns in how society will manage the economy, the climate system, and global emissions, the impact of the future trajectory of GHG levels on the climate is uncertain. Ultimately, it is preferable to prevent harms before they occur, rather than attempt to reverse them (Schneider, 2014). Over the long term, reducing greenhouse gas emissions whenever feasible will be economically and socially preferable to undertaking large-scale CDR.

1.7

1.7 —

Imagining CDR deployment that centers social justice

Whether or not we operate within the United Nations’ articulated sustainable development paradigm (United Nations, 2020), or adopt a more holistic “just transition” approach for equitable societal transformation like those of climate justice advocates (Climate Justice Alliance, 2016), there is broad agreement that social justice must be at the center of global climate – and therefore CDR – policy and governance. The IPCC makes this clear in its 1.5 ° C Report (IPCC, 2018c):

“Social justice and equity are core aspects of climate-resilient development pathways for transformational social change. Addressing challenges and widening opportunities between and within countries and communities would be necessary to achieve sustainable development and limit warming to 1.5°C, without making the poor and disadvantaged worse off (high confidence). Identifying and navigating inclusive and socially acceptable pathways towards low-carbon, climate-resilient futures is a challenging yet important endeavor, fraught with moral, practical and political difficulties and inevitable trade-offs (very high confidence).”

In addition to the challenges in addressing the trade-offs themselves, the uncertainties inherent in trying to manage these trade-offs (described at the end of Section 1.5) require deep consideration when seeking to deploy CDR equitably. Because any harms are likely to fall disproportionately on communities that are already most susceptible to the perils of climate change, a social justice-oriented approach must seek to avoid harms in the first place, not to reverse damage.

A path for equitable CDR deployment, then, requires that we consider an expanded set of policy and technology options that are best able to incorporate the principles of social justice. Approaches that center social justice seek to establish just relations within society to equitably distribute wealth, access to resources, opportunities, and privileges in order to combat ongoing and historically-based inequities, including those founded in white supremacy, sexism, ableism, and economic factors.

While current policies, such as the 45Q tax credit in the U.S., encourage both fossil fuel and new CDR companies to deploy for-profit CDR, some leading advocates have questioned whether the oil and gas industry should have any role in a climate justice-oriented approach for CDR scale-up, suggesting “they may slow progress to protect business-as-usual operations or delay decarbonizing on the premise that they will use direct air capture in the future to do so” (Deich and Reali, 2019). Some have suggested that nationalization policies, transforming oil companies into “publicly run carbon removal entities,” may be the best of an imperfect menu of options, especially when made part of an approach that centers rural organizations, frontline communities, and workers, providing work for those who lose fossil fuel industry jobs (Buck, 2018; Buck, 2019).

In addition to considering public ownership of infrastructure, policymakers have the option to rethink the way that society does and does not govern large-scale land use and technological deployment. It will be critically important to include the public in the creation of regulatory frameworks that put a price on carbon and incentivize or devalue certain behaviors, relating, for example, to emissions, land use, and CO2 sequestration, if these efforts are to be accepted and retain legitimacy over the decades required to mitigate climate change. In addition to enabling negotiation of “diverse interests and preferences,” the IPCC says, with high agreement, that “inclusive governance processes … have been shown to serve the interests of diverse groups of people and enhance empowerment of often-excluded stakeholders, notably women and youth. They also enhance social- and co-learning which, in turn, … provides opportunities to blend indigenous, local, and scientific knowledge” (IPCC, 2018c). These inclusive governance processes can be imagined at several scales, from local to regional to global. Policymakers can also interface directly with the public by redirecting subsidies for CDR away from fossil fuel companies and toward “people who have suffered from environmental injustice, and to alleviate inequality while transitioning to a carbon-negative society” (Buck, 2018).

Policymakers and the public will need to rely on social science data and learnings if they hope to chart a path to equitably transform the socioeconomic systems required for large-scale deployment of CDR, described in Section 1.6. In a review of existing literature on the social implications of CDR scale-up, however, Buck (2015) found essentially no analysis of non-biological CDR approaches, such as DACCS and enhanced weathering. But studies related to biological CDR have yielded useful insights: One analysis on the social impact of bioenergy deployment reveals the shortcomings of integrated assessment models, which rely on economic metrics while leaving out holistic evaluations of human well-being and health, including the on-the-ground impacts of deployed projects on frontline communities (Creutzig et al., 2013). As Buck (2016) suggests, “By integrating empirical research on public and producer perceptions, barriers to adoption, conditions driving new technologies, and social impacts, projections about CDR can become more realistic and more useful to climate change policymaking.” 

To realize equitable global CDR deployment, we will need to broaden the conversation beyond the current approaches and policies being pursued. A broad effort to understand the social science aspects of CDR deployment and develop equitable governance models will guide that conversation. In the coming decades, if we hope to effectively and equitably address the climate crisis that humanity currently faces, we will need to consider technologies and activities that do not fit well within current global governmental or economic frameworks, but which may be achievable from a scientific and engineering standpoint.

1

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

2

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).

3

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.

4

Source – any process, activity, or mechanism which creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

5

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, Box 4.1.)

6

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).

7

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.

8

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

8

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).

9

(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.

10

Large-scale CDR – carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

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. 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 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. 

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).

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). 

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 CO2removed 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. 

Permanence (or Durability) – the duration for which CO2 can be stored in a stable and safe manner. Storage duration 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.

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

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.

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.

Climate tipping points – abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

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.” (World Resources Institute)

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 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. 

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.

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. 

1

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

2

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).

3

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.

4

Source – any process, activity, or mechanism which creates a greenhouse gas, a precursor of a greenhouse gas, or an aerosol.

5

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, Box 4.1.)

6

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).

7

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.

8

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

8

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).

9

(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.

10

Large-scale CDR – carbon dioxide removal on the order of or approaching gigatonnes of CO2 removed per year.

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. 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 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. 

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).

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). 

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 CO2removed 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. 

Permanence (or Durability) – the duration for which CO2 can be stored in a stable and safe manner. Storage duration 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.

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

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.

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.

Climate tipping points – abrupt and irreversible climate events, such as ice sheet loss and ecosystem collapse.

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.” (World Resources Institute)

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 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. 

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.

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. 

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.