Summary

As understanding of the risks and damages of climate change has improved, almost all nations have committed to limit total global warming to less than 2°C over preindustrial levels, with an aspirational target of 1.5°C. Meeting a 2°C target is becoming exceedingly challenging; the global mean temperature has already risen about 1°C over the 20th century. Most climate and integrated assessment models project that the concentration of atmospheric carbon dioxide (CO₂) would have to stop increasing (and perhaps start decreasing) by the second half of the century for there to be a reasonable chance of limiting warming and the associated dangerous climate impacts.

Fossil fuel consumption, agriculture, land-use change, and cement production are the dominant anthropogenic sources of CO₂ to the atmosphere. The focus of climate mitigation is to reduce energy sector emissions by 80-100 percent, requiring massive deployment of low-carbon technologies between now and 2050. Progress toward these targets could be made by deploying negative emissions technologies (NETs), which remove carbon from the atmosphere and sequester it. Under the present conditions, where fossil CO₂ is continuously added to the atmosphere, removing CO₂ from the atmosphere and storing it has exactly the same impact on the atmosphere and climate as simultaneously preventing emission of an equal amount of CO₂. NETs have been part of the portfolio to achieve net emissions reductions, at least since reforestation, afforestation, and soil sequestration were brought into the United Nations Framework Convention on Climate Change, albeit as mitigation options, more than two decades ago. Recent analyses found that deploying NETs may be less expensive and less disruptive than reducing some emissions, such as a substantial portion of agricultural and land-use emissions and some transportation emissions.

In 2015, the National Academies published Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration, which described and initially assessed NETs and sequestration technologies. This report acknowledged the relative paucity of research on NETs and recommended development of a research agenda that covers all aspects of NETs from fundamental science to full-scale deployment. To address this need, the National Academies convened the Committee on Developing a Research Agenda for Carbon Dioxide Removal and Sequestration to assess the benefits, risks, and “sustainable scale potential” for NETs and sequestration and to define the essential components of a research and development program, including its estimated costs and potential impact (Box S.1). The full Statement of Task is presented in Box 1.4. The

committee held a series of public workshops and meetings to inform its deliberations and the writing of this report.

MITIGATION IN A NET ZERO EMISSIONS SYSTEM

Studies using integrated assessment models that link greenhouse gas emissions, the economy, and climate conclude that the reductions in net anthropogenic emissions required to meet even the 2°C target will be difficult and expensive to achieve, even with foreseeable technological breakthroughs. In its Fifth Assessment Report (2014), the Intergovernmental Panel on Climate Change (IPCC) projects high costs for reducing the concentration of atmospheric CO₂, with costs in many scenarios reaching

more than $1,000 per ton of CO₂ emissions by 2100. Stopping the growth of atmospheric CO₂ requires that anthropogenic emissions are less than or equal to natural and anthropogenic carbon sinks—not that they cease altogether. As reported in IPCC (2014b), the lowest cost trajectories for achieving the 2°C target typically include massive deployment of one specific type of NET, bioenergy with carbon capture and sequestration (BECCS), to avoid the even steeper costs associated with relying on emissions cuts alone. However, BECCS at this scale requires more feedstock than is available from biomass waste. For example, 30 million to 43 million hectares is required to raise BECCS feedstocks per Gt/y CO₂ of negative emissions. Thus, 10 Gt/y CO₂ of negative emissions from BECCS requires hundreds of millions of hectares of land, which is almost 40 percent of global cropland according to some studies reviewed in IPCC (2014b).

The committee repeatedly encountered the view that NETs will primarily be deployed to reduce atmospheric CO₂ after fossil emissions are reduced to near zero. In contrast, because it will likely be very expensive to decrease anthropogenic emissions once they reach low levels, methods for reduced and negative emissions will probably be competitors for an extended period, even during a sustained period of net negative global emissions (see Figure S.1). For example, few alternatives to chemical fuels are likely to exist for commercial aviation. One option for zero net aviation emissions

would be deployment of $100/t CO₂ NETs to capture and store 2.5 kg of CO₂ for each liter of aviation fuel consumed.¹ This will add ~25 cents per liter of fuel. This is just one example of how NETs might be conceptually bundled with emissions sources that are difficult to eliminate.

Conclusion 1: Negative emissions technologies are best viewed as a component of the mitigation portfolio, rather than a way to decrease atmospheric concentrations of carbon dioxide only after anthropogenic emissions have been eliminated. The central question is “which is least expensive and least disruptive in terms of land and other impacts—an emission reduction or an equivalent amount of negative emission?” The committee recognizes that there is a possibility that large negative emissions in the future could result in a moral hazard, by reducing humanity’s will to cut emissions in the near term. Reducing emissions is vital to addressing the climate problem. However, the least expensive and least disruptive solution involves a broad portfolio of technologies, including those with positive, near-zero, and negative emissions. In addition, a broad portfolio of technologies (including multiple NETs) improves the ability to manage unexpected risks from nature and mitigation actions.

TECHNOLOGIES CONSIDERED

As directed by the Statement of Task, the committee focused on six major technical approaches to CO₂ removal and sequestration:

• Coastal blue carbon (Chapter 2)—Land use and management practices that increase the carbon stored in living plants or sediments in mangroves, tidal marshlands, seagrass beds, and other tidal or salt-water wetlands. These approaches are sometimes called “blue carbon” even though they refer to coastal ecosystems instead of the open ocean.
• Terrestrial carbon removal and sequestration (Chapter 3)—Land use and management practices such as afforestation/reforestation, changes in forest management, or changes in agricultural practices that enhance soil carbon storage (“agricultural soils”).
• Bioenergy with carbon capture and sequestration (Chapter 4)—Energy production using plant biomass to produce electricity, liquid fuels, and/or heat combined with capture and sequestration of any CO₂ produced when using the bioenergy and any remaining biomass carbon that is not in the liquid fuels.

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¹ Combustion of a gallon of gasoline releases approximately 10 kg of CO₂ to the atmosphere.

• Direct air capture (Chapter 5)—Chemical processes that capture CO₂ from ambient air and concentrate it, so that it can be injected into a storage reservoir.
• Carbon mineralization (Chapter 6)—Accelerated “weathering,” in which CO₂ from the atmosphere forms a chemical bond with a reactive mineral (particularly mantle peridotite, basaltic lava, and other reactive rocks), both at the surface (ex situ) where CO₂ in ambient air is mineralized on exposed rock and in the subsurface (in situ) where concentrated CO₂ streams are injected into ultramafic and basaltic rocks where it mineralizes in the pores.
• Geologic sequestration (Chapter 7)—CO₂ captured through BECCS or direct air capture is injected into a geologic formation, such as a saline aquifer, where it remains in the pore space of the rock for a long time. This is not a NET, but rather an option for the sequestration component of BECCS or direct air capture.

The exclusive focus of this report on terrestrial and near-shore coastal NETs reflects the Statement of Task. The committee recognizes that oceanic options for CO₂ removal and sequestration (e.g., iron fertilization and ocean alkalinization), which fall outside the scope of its task, could sequester an enormous amount of CO₂ and that the United States needs a research strategy to address them. The report also does not address mitigation options such as enhanced energy efficiency, renewable electricity, or reduced deforestation because they are not NETs. Their exclusion is not a statement about priorities. Reducing emissions is vital to addressing the climate problem. Policymakers should consider the broadest possible portfolio of technologies to find the most inexpensive and least disruptive solutions, including those with positive, near-zero, and negative emissions.

CARBON REMOVAL POTENTIAL AND NEED

The committee identified the potential rates of CO₂ removal and sequestration that could be achieved safely and economically, given our current knowledge and level of technological development. These results are summarized in Table S.1 and expanded in Tables 8.1 and 8.2. “Safe” means that the deployment would, with high confidence, not cause the large potential adverse societal, economic, and environmental impacts that are described in detail in each report chapter. “Economical” means that the deployment would cost² less than $100/t CO₂. Most of the NETs that meet that criterion actually cost less than $20/t CO₂. Differences among rows in the significant digits listed reflect heterogeneity in the state of knowledge. It is important to understand

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² The committee refers to direct costs of attaining negative emissions (e.g., operating costs, labor costs). It recognizes that all NETs have a full set of indirect costs (e.g., impact on land values) that may not be reflected in direct cost estimates.

TABLE S.1 Cost, Limiting Factors, and Impact Potential of NETs with Current Technology and Understanding. “Safe” maximum rate of CO₂ removal means that the deployment would not cause large potential adverse societal, economic, and environmental impacts. These estimated rates assume full adoption of agricultural soil conservation practices, forestry management practices, and waste biomass capture.

NOTE: Number of significant digits reflects state of knowledge among different NETs and between US and global estimates.

^a Global removal rate based on coastal wetland area lost since approximately 1980 and annual burial rate with restoration; does not include active management of existing areas or managed wetland transgression.

^b Cost for deployed air capture remains substantially above $100/t CO₂ (as high as $600/t CO₂).

that safe and economical limits to NET deployment are not necessarily achievable in a practical sense, because human behavior, logistical shortages, organizational capacity, and political factors can also limit deployment.

The collection of NETs in Table S.1 exhibit a wide range of technical maturity. Some approaches to carbon removal, such as reforestation, have been developed over many decades and are already deployed at large scale. Others, such as several types of enhanced carbon mineralization, are at the early stages of exploration by academic researchers and have never been tried in the field. In general, the cost estimates for technologies that have not been demonstrated are more speculative than those for technologies that have been deployed at scale. However, even NETs that are relatively mature will benefit from additional research to reduce costs and negative impacts and to increase co-benefits.

There are also fundamental differences among the primary factors that limit potential rates and capacities of NETs. Land-based NETs, especially afforestation/reforestation

and BECCS, are constrained by land availability, because of competing needs to produce food and preserve biodiversity and by the responses of landowners to incentives. Research on land-based options would help to ensure that the capacities listed in Table S.1 can be realized or increased as discussed in Chapter 8, but their potential will remain limited by available land. In contrast, the major barrier to large-scale direct air capture is the high current cost. If made less expensive, direct air capture technologies could be scaled up to remove very large amounts of carbon. Finally, carbon mineralization is currently constrained by many scientific unknowns, as well as uncertainty about environmental impacts and likely cost. However, like direct air capture, carbon mineralization technologies could have very large capacity if their costs and environmental impacts could be sufficiently reduced. The analysis in Table S.1 leads to the committee’s most important conclusions about the readiness of NETs:

Nonetheless, the potential global uptake from current technologies of significantly less than 10 Gt/y CO₂ is substantially lower that the negative emissions in most scenarios that would produce less than 2°C of warming, according to virtually every recent assessment (Chapter 8). For example, Figure S.1 shows net anthropogenic emissions of all greenhouse gases falling from more than 50 Gt CO₂e today to less than 20 Gt CO₂e at midcentury and to approximately zero by 2100. Approximately 10-20 Gt CO₂e of gross anthropogenic emissions come from sources that would be very difficult or expensive to eliminate, including a large fraction of agricultural methane and nitrous oxide. Most scenarios that meet the Paris agreement, such as the one in Figure S.1, thus rely on CO₂ removal and storage that ramps up rapidly before midcentury to reach approximately 20 Gt CO₂ by century’s end.

Conclusion 4: 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/y CO₂ globally by midcentury and ~20 Gt/y CO₂ globally by the century’s end.

FACTORS AFFECTING SCALE UP

The committee considered a range of factors that will affect the scale-up of NETs. Described in detail in Chapters 2-7, these factors include the availability of land given the competing needs for food and biodiversity preservation, other environmental constraints, energy requirements, high cost, practical barriers, permanence, monitoring and verification, governance, and insufficient scientific or technological understanding. They helped to inform development of the recommended research program for each NET. Collectively, Conclusions 2-4 are critical, because they demonstrate that all existing safe and economical NETs and mitigation options together do not have sufficient capacity to meet the Paris agreement. Uncertain research breakthroughs will be required before those NETs can provide even the minority share of the solution in Conclusion 6. Any argument to delay mitigation efforts because NETs will provide a backstop drastically misrepresents their current capacities and the likely pace of research progress.

The Land Constraint

Because food demand is expected to double by mid-century, repurposing a significant amount of current agricultural land to produce feedstocks for BECCS or for afforestation/reforestation might have a significant effect on food availability and food prices, with far-reaching effects on national security and biodiversity. If afforestation/reforestation and BECCS could be extended to hundreds of millions of hectares of arable land without disrupting the food supply or causing the clearance of remaining tropical forests, then these options could deliver more than 10 Gt/y CO₂ of negative emissions. However, such an extension would require either a revolutionary breakthrough in agricultural productivity or revolutionary changes in diets (with greatly reduced meat consumption) and reduced food waste. Until research proves otherwise, it is prudent to view as impractical upper bounds for afforestation/reforestation and BECCS deployment of much greater than 10 Gt/y CO₂.

Other Environmental Constraints

Environmental concerns vary among the NETs. Because forests established at high latitudes decrease albedo, afforestation/reforestation at high latitudes would cause net warming despite the cooling caused by the forest’s CO₂ uptake. In addition, forests established in regions with limited rainfall would have adverse effects on streamflow, irrigation, and groundwater resources. Mining minerals that spontaneously bind CO₂ would create enormous volumes of waste rock that may contaminate water and/or air. Approaches that enhance carbon uptake and storage in agricultural soils generally have large positive side benefits, including increased productivity, water holding capacity, stability of yields, and nitrogen use efficiency, but sometimes increase nitrous oxide emissions. Afforestation/reforestation, BECCS, and potentially some direct air capture routes may have substantial water requirements. In particular, irrigated bioenergy crops may lead to a trade-off between land and water requirements, in addition to freshwater ecosystem degradation and biodiversity loss.

Energy Requirements

Direct air capture and some carbon mineralization options require a large energy input per ton of CO₂ captured, which increases costs. Direct air capture systems require between 5 and 10 GJ to capture a ton of atmospheric CO₂. To put this into perspective, combustion of 100 gallons of gasoline releases about 13 GJ of energy and emits about a ton of CO₂. Gt-scale direct air capture thus necessitates an enormous increase in

low- or zero-carbon energy to meet energy demands, which would compete with use of such energy sources to mitigate emissions from other sectors.

High Cost

The primary impediment to several NETs is high cost. Climeworks, which has the only existing commercial direct air capture machine, captures CO₂ at a reported cost of $600/t CO₂. The estimated cost of capture and sequestration for BECCS systems that produce electricity is $70/t CO₂, which is higher than costs for capture and sequestration from fossil fuel electricity. Although costs for direct air capture and BECCS may decline quickly, they are not currently competitive. The costs of carbon mineralization are uncertain because the fundamental understanding of the processes and engineering systems required for effective sequestration is insufficient.

Practical Barriers

Implementing NETs at the scale necessary to limit warming to 2°C presents practical barriers. For example, a 10 percent per year increase from the 65 Mt/y CO₂e that is currently injected underground for CO₂-enhanced oil recovery and saline aquifer sequestration is needed in order to scale up underground injection for geological sequestration. Comparable or greater rates will be required of every available NET. At these rates, scale-up could be limited by materials shortages, regulatory barriers, infrastructure development (i.e., CO₂ pipelines and renewable electricity), the availability of trained workers, and many other barriers.

Furthermore, humans often resist actions that appear to be in their economic interest. For example, historical adoption rates of agricultural soil conservation and forestry management practices that would save farmers and forest landowners money have been surprisingly low, as have dietary changes, such as reduced meat consumption, that would increase health while freeing agricultural land for forestry NETs and BECCS. These behaviors could limit deployment of NETs, as could public resistance to new local infrastructure, but they are not well represented in integrated assessment models.

Permanence

The terrestrial and coastal blue carbon options are reversible if the carbon sequestering practices are not maintained. For example, forested land could be cleared again, reverting to intensive tillage will eventually reverse soil carbon gains from reduced tillage in

agricultural soils, or a restored coastal wetland could be drained again. Although temporary CO₂ storage will have some climate benefit, scientific and economic requirements to ensure the permanence of storage within ecosystems are substantial. In contrast, BECCS, direct air capture, and carbon mineralization have comparatively minor issues of permanence. CO₂ that is geologically sequestered can leak from saline aquifers but at rates low and straightforward enough to remediate. Permanence for carbon mineralization would be very high for mineralization of basalt or peridotite.

Monitoring and Verification

Monitoring and verification will be critical components of any large-scale deployment of NETs. Terrestrial carbon removal and sequestration approaches are sufficiently well understood that, in most cases, they could be deployed in the United States with remote monitoring and verification backed up by onsite measurements in a statistical sample. Effective monitoring and verification will require improved systems to monitor forest carbon and soil carbon on agricultural lands, expanded remote sensing of land use and management practices, better integration of existing data sets and models, and selected improvements in global monitoring to help address “leakage” (i.e., land-use changes in one location inducing land-use changes in other locations). Coastal blue carbon could be monitored and verified inexpensively through remote methods, although landscape heterogeneity would require finer resolution than terrestrial approaches, backed up by onsite measurements of a statistical sample. Because most carbon mineralization efforts would store CO₂ in inert and essentially permanent solid form, they might have the lowest monitoring and verification costs of all the NETs. This is particularly true for in situ carbon mineralization, whereas dispersal of reactive rock material in soil, along beaches, or into the shallow ocean could be as difficult, or more difficult, to verify than enhancements to agricultural soil carbon. Monitoring and verification of direct air capture would be straightforward because the amount of CO₂ can be directly measured. Monitoring and verification of sequestration in saline aquifers requires sophisticated methods such as seismic imaging, measuring pressure in and above the sequestration reservoir, and routine measurements of well integrity.

Governance

Appropriate governance of NETs and sequestration is critical because overly lax oversight would lead to ineffective CO₂ removal and loss of public confidence, while overly strict oversight would limit deployment. Governance is especially critical when large-scale deployment is imminent. Currently, governance is well established only in those

sectors covered by a national or international agreement, including afforestation/reforestation (under the United Nations Framework Convention on Climate Change) and saline aquifer storage (under the Underground Injection Protocol of the Safe Water Drinking Act). In addition, the United States and other governments have ample experience regulating agriculture and forestry for non-NET purposes. One way to maintain public confidence during rapid deployment of NETs is to invest in a substantial effort to educate the public during the research and development stage.

Insufficient Scientific/Technical Understanding

Significant scientific gaps remain for all NETs, but especially for carbon mineralization and coastal blue carbon. Regarding the former, there is limited understanding of the kinetics of CO₂ uptake, no inventory of appropriate geologic deposits and existing tailings of reactive but unreacted rock, and no technical expertise to manage tailings piles so that they effectively take up CO₂. In addition, negative feedbacks cannot be predicted, nor can the long-term consequences of depositing crushed reactive minerals in agricultural soils, along the coasts, or into the shallow ocean. For the latter, many of the critical processes that govern carbon burial and sequestration in coastal ecosystems lack a mechanistic understanding of how they may change under high rates of sea-level rise and other direct and indirect impacts of climate change, and few studies have been performed on transgression of coastal wetlands inland.

PROPOSED RESEARCH AGENDA

Scaling the capacity of NETs to address the expected needs for carbon removal will require a concerted research effort to address the constraints that currently limit deployment. The research agenda should address not only potential research gaps, but also other needs for scale-up of NETs including cost reductions, deployment, and monitoring/verification. Based on its review of the factors that will affect scale-up of NETs, the committee drew the following conclusions that motivate the selection of research priorities:

The committee developed a detailed research agenda that is divided into two categories: 1) projects that would exclusively advance NETs (Table S.2); and 2) NET-enabling research on biofuels and CO₂ sequestration that should be undertaken as part of an emissions mitigation research portfolio (Table S.3). Drawing on the expertise of certain members, the committee estimated the budget for each research effort, which could be funded across a range of agencies (described further in Chapters 2-7). Of course, these budget estimates contain some uncertainty, but there is value in assessing the relative levels of investment needed to advance each of the NETs and sequestration approaches. For example, the estimated research budget for afforestation/reforestation and forest management is lower than that for other NETs because these approaches are more mature. In contrast, the estimated research budgets for carbon mineralization and direct air capture are larger because these technologies are relatively new and underexplored. The committee also identified an appropriate timespan to fund and conduct each effort. In many cases, the research should be conducted in stages—that is, funding will continue if certain milestones are met.

The scale of these research costs is commensurate with the size of the carbon and climate problem. A solution such as the one depicted in Figure S.1 requires electricity

TABLE S.2 Research Plan and Budget for Negative Emissions Technologies Plus Sequestration (abbreviated from Table 8.3)

TABLE S.3 Research Plan and Budget for NET-Enabling Research (abbreviated from Table 8.4)

NOTE: All projects listed above not only support the advancement of NETs, but should also be part of any comprehensive research effort on carbon mitigation.

generated by wind, solar, biomass, nuclear, and natural gas with carbon capture and sequestration; biofuels and electricity for transportation; and some form of energy storage. Every one of these technologies has already received large government investment. For example, a recent Congressional Research Service report Renewable Energy R&D Funding History: A Comparison with Funding for Nuclear Energy, Fossil Energy, and Energy Efficiency R&D estimates that the federal government spent more than $22 billion on renewable energy research and development from 1978 to 2013. NETs have not received comparable public investment despite expectations that they might provide ~30 percent of the net emissions reductions required this century (i.e., maxima of 20 Gt/y CO₂ of negative emissions and 50 Gt/y CO₂ of mitigation in Figure S.1). NETs are essential to offset greenhouse gas emissions that cannot be eliminated, such as a large fraction of agricultural nitrous oxide and methane emissions. Inexpensive direct air capture or carbon mineralization could allow for some continued use of fossil fuels without impacting climate. Recent regulatory proposals to freeze light-duty fuel economy standards over the 2021-2025 period and changes to the Clean Power Plan may further the need for NETs. Table S.3 contains a proposed budget for improving CO₂ sequestration in saline aquifers and for enhanced oil recovery. This information is included because CO₂ sequestration is required by direct air capture and BECCS, but the size of the budget reflects the larger need for sequestration of CO₂ captured at other energy sources such as fossil fuel electricity plants. The scale of the recommended research budget is consistent with the need for NETs that can solve a substantial fraction of the climate problem.

Recommendation: The nation should launch a substantial research initiative to advance negative emissions technologies (NETs) as soon as practicable.

A substantial investment would (1) improve existing NETs (i.e., coastal blue carbon, afforestation/reforestation, changes in forest management, uptake and storage by agricultural soils, and bioenergy with carbon capture and sequestration) to increase the capacity and to reduce their negative impacts and costs; (2) make rapid progress on direct air capture and carbon mineralization technologies, which are underexplored but would have essentially unlimited capacity if the high costs and many unknowns could be overcome; and (3) advance NET-enabling research on biofuels and carbon sequestration that should be undertaken anyway as part of an emissions mitigation research portfolio.

CONCLUDING REMARKS

Recent analyses of economically optimal solutions to the climate problem have concluded that NETs will play as significant a role as any mitigation technology, with perhaps 10 Gt/y CO₂ of negative emissions needed approximately at midcentury and 20 Gt/y CO₂ by the century’s end. Several Gt/y CO₂ of negative emissions at less than $20/t CO₂ are already available. Nonetheless, existing options (coastal blue carbon, afforestation/reforestation, forest management, agricultural soils and BECCS) cannot yet provide enough negative emissions at reasonable cost, without substantial unintended harm. A substantial research investment is needed to improve existing NETs and to reduce their negative impacts and costs. In addition, direct air capture and carbon mineralization have essentially unlimited capacity and are almost unexplored.

The committee recognizes that the federal government has many other research priorities, including others in mitigation and adaptation to climate change. Multiple reasons exist to pursue research on NETs. First, states, local governments, corporations, and countries around the world are making substantial investments to reduce their net carbon emissions and plan to increase these investments. Some of these efforts already include negative emissions. This means that advances in NETs will benefit the U.S. economy if the intellectual property is held by U.S. companies. Second, as climate damages mount, the United States will inevitably take increased action to limit climate change in the future. Third, the United States is already making a substantial effort, including the new 45Q rule³ that provides a tax credit of $50/t CO₂ for capture and sequestration in saline aquifers and $35/t CO₂ for oil and gas reservoirs, which would leverage the value of new investments in NET research. Thus, though climate mitigation remains the motivation for global investments in NETs, intellectual property and economic benefits will likely accrue to the nations that develop the best technology.

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³https://www.law.cornell.edu/uscode/text/26/45Q.

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