Climate change is one of greatest challenges the world faces today. The rise of human societies has taken place during a stable period in the history of Earth’s climate. Over the past 8,000 years Earth’s climate maintained a relatively even balance with no large swings in the climate state (Petit et al., 1999) like those observed earlier in the paleoclimate record. Through the emissions of large amounts of greenhouse gases (GHGs) during the industrial age, humans have intervened in the planet’s carbon cycle, shifting the equilibrium that existed for the bulk of human history. The carbon cycle itself is resilient and has feedback cycles that will allow it to return to an equilibrium (Zeebe and Caldeira, 2008), but those feedbacks operate over very long timescales—on the order of thousands of years.
Climate science has revealed that there are substantial risks to society posed by the large emissions of GHGs that have been and are continuing to be emitted into the atmosphere. These risks include not just warming, but threats from sea level rise, rapid ecosystem changes, ocean acidification, and extreme weather events (IPCC, 2013a,b, 2014a; NCA, 2014). Reducing the atmospheric burden of carbon dioxide (CO2), the most prevalent and persistent GHG, is an essential component of reducing those risks. Returning the atmospheric concentration of CO2 closer to the level that Earth had during the last several millennia as humans flourished on the planet would minimize risks for human societies that have grown to depend on the stability of Earth’s climate.
Current emissions of GHGs by humans continue to push Earth further away from its historical climate state. Over the next few decades humans are likely to continue to emit large amounts of CO2 to the atmosphere. Nevertheless, avoiding some of these emissions and/or removing some of that CO2 from the atmosphere would slow this shift away from the historical state. Once anthropogenic emissions cease, it will take nature many thousands of years to remove enough of industrialized society’s CO2 emissions through natural processes such that they would no longer be of climatic concern. A more rapid return to lower CO2 concentrations would involve removing CO2 from the atmosphere. For now, while there are large sources of CO2 emission, the avoidance of emissions from fossil energy sources through the use of improved energy efficiency, deployment of carbon-free energy sources (e.g., wind and solar power), carbon capture and sequestration (CCS), and carbon dioxide removal (CDR) techniques are all components of the portfolio of possible strategies for reducing the risks from climate change. For this report, the committee examined CDR techniques in the
context of both the present, with currently available technologies, as well as the future, as technologies and other solutions may evolve.
Even if CDR technologies never scale up to the point where they could remove a substantial fraction of current carbon emissions at an economically acceptable price, and even if it took many decades to develop even a modest capability, CDR technologies still have an important role to play. As described in the recent Intergovernmental Panel on Climate Change (IPCC) report, “[m]itigation scenarios reaching about 450 ppm CO2eq in 2100 typically involve temporary overshoot of atmospheric concentrations, as do many scenarios reaching about 500 ppm to 550 ppm CO2-eq in 2100. Depending on the level of the overshoot, overshoot scenarios typically rely on the availability and widespread deployment of BECCS and afforestation in the second half of the century” (IPCC, 2014a). Furthermore, since climate stabilization requires GHG emissions to be essentially zero, it is almost inevitable that some CDR will be needed in the long term to deal with residual emissions by nonparticipatory nations, or by sectors for which fossil fuel substitutes prove difficult to implement (e.g., aviation) (NRC, 2011a). Finally, after the time emissions finally do cease, even a modest amount of CDR, on the order of 1 GtCO2/yr, can significantly shorten the time needed for CO2 to recover to preindustrial values.
As discussed throughout this report, CO2 removal from the atmosphere can be enhanced using a range of approaches from biological to chemical. To remove enough CO2 from the atmosphere to offset a substantial fraction of today’s CO2 emissions represents a major challenge given available technology and physical constraints (e.g., available land for growing bioenergy feed stocks, and disposing of sequestered CO2). To take enough CO2 out of the atmosphere to cause atmospheric concentrations to markedly decrease would be extraordinarily difficult. The challenge is to capture climatically important amounts of CO2 out of the atmosphere, to sequester it reliably and safely, and to do this in a way that is economically feasible, environmentally beneficial, and socially, legally, and politically acceptable.
The committee has examined a number of CDR techniques through this lens throughout this report. There are land management activities, in particular preserving and restoring forests, that society can sensibly do at present that will help reduce CO2 emissions, but not at the scale of current global CO2 emissions. Bioenergy with carbon capture and sequestration (BECCS) exists today, but large-scale implementation will only become cost competitive in the coming decades and only differs in net atmospheric effect from the separate use of bioenergy and CCS when fossil fuel use is minimal, which is decades off at best. Accelerated mineral weathering on land or in the ocean may be technically feasible, but at substantial cost if done on the scale required
for achieving significant impact. Direct air capture and sequestration (DACS) has the theoretical potential to effectively sequester substantial quantities of CO2 from the atmosphere provided nonfossil sources are used to power the separation of CO2 from air, but it is unclear that this approach will be cost effective in the near term. Last, the environmental and sociopolitical risks of deploying ocean iron fertilization at a large scale would likely outweigh the potential benefits. Overall, there is value in pursing multiple parts of a portfolio of these strategies, both for what can be done in the short term and what can be done in the long term.
The scale of a system that removes a CO2 molecule from the atmosphere and sequesters it reliably might be similar to the scale of the system that first put that CO2 molecule into the atmosphere. Over the past decade, humanity has been emitting about 34,000,000,000 tons of CO2 (34 GtCO2) into the atmosphere each year (Table 2.1). Because there are more than 7,000,000,000 people (7 billion) in the world, this works out to about 5 tons of CO2 per person per year, or about 30 pounds of CO2 per person per day.1 In 2010, the United States emitted about 20 tons of CO2 per person per year2—about 110 pounds per American per day. For comparison, in 2012, Americans generated >4 pounds per person per day of municipal solid waste (i.e., trash or garbage).3 CO2 is the waste we produce most prodigiously.
If CDR were to be used to avoid all climate change from U.S. CO2 emissions, the United States would need to remove 110 pounds of CO2 per day for each American. CO2 is a dilute gas in the atmosphere, making up only about 0.04 percent of the atmosphere by volume (and about 0.06 percent by mass). This means that if we were able to remove 100 percent of the CO2 molecules from a volume of air, we would need to process about 51,000 m3 (about 67,000 cubic yards) of air per American per day.4 This corresponds to a volume approximately 30 feet high (nearly 10 m) and the area of an American football field5 to be processed for each American each day. Nobody is suggesting that CDR will
1 The committee is not suggesting that everyone on the planet is responsible for equal amounts of CO2 emissions; this estimate is simply to help visualize the size of the challenge.
4 The molecular weight of dry air is 28.97 g/mol and that of CO2 is 44.01 g/mol. Therefore, if CO2 is 400 ppm by volume (see Chapter 1), it is 400 ppm × 44.01 g/mol / 28.97 g/mol = 608 ppm by mass. At sea level and 15°C, dry air is 1.275 kg/m3. Thus, 1 m3 of air contains 1.275 kg × 608 ppm = 1.275 kg × 0.000608 kg CO2/kg air = 0.000775 kg CO2/m3. 50 kg of CO2 per American per day = 50 kg CO2/(0.000775 kg CO2/kg air)/(1.275 kg/m3) = 51,000 m3.
be the only tool used to reduce CO2 emissions, but to make a substantial contribution reducing our net CO2 emissions, CDR would need to be deployed at a substantive level.
These numbers indicate that, to make a substantive difference to the global climate, CDR would need to occur at a truly massive scale. Because CDR must operate on each CO2 molecule, there are no easy wins at the scale of the climate problem. Although atmospheric CDR approaches might be able to cost-effectively address some portion of our CO2 emissions, it cannot be assumed that these approaches will be able to feasibly be scaled up to address a major fraction of current CO2 emissions. As discussed in Chapter 5 of the companion volume (Climate Intervention: Reflecting Sunlight to Cool Earth), the committee recommends that efforts to address climate change should continue to focus most heavily on mitigating greenhouse gas emissions in combination with adapting to the impacts of climate change because these approaches do not present poorly defined and quantified risk and are at a greater state of technological readiness.
Some CDR approaches, such as afforestation and reforestation, are already recognized as valuable both for the CDR and sequestration, but also for other co-benefits, including ecosystem services such as protection of watersheds from erosion, nutrient retention, good water quality, wildlife habitat and diversity, recreational opportunities, and other social benefits (Millennium Ecosystem Asessment, 2010; Plantinga and Wu, 2003). Accelerated mineral-weathering approaches aim to accelerate the natural processes that neutralize CO2 acidity (Kheshgi, 1995) and thus could potentially provide substantial environmental benefit to neutralizing some of the acidification of the ocean caused by excess anthropogenic CO2. There may be other CDR approaches that may be unable to scale up to match current or future CO2 emissions, but they may nevertheless be cost effective at modest scale and/or provide valuable co-benefits.
Costs for various CO2 capture approaches currently range from $50 to more than $1,000 per ton CO2 (tCO2), and costs for various sequestration approaches range from $6/tCO2 to hundreds of dollars per ton of CO2 (see Table 2.2). As such, some CDR approaches might not be cost competitive with least-cost mitigation options today but could potentially become cost competitive at some future date if and when costs of deployment decline and a price has been placed on carbon emissions that reflects the social costs of those emissions. The most recent estimate for the social cost of a ton of carbon emissions to society is $12 to $120 (Interagency Working Group on Social Cost of Carbon, 2013; see also http://www.epa.gov/climatechange/EPAactivities/economics/scc.html).
Developing the ability to capture climatically important amounts of CO2 from the atmosphere and sequester it reliably and safely on scales of significance to climate change requires research into how to make the more promising options more effective, more environmentally friendly, and less costly. At this early stage successful development also requires soliciting and encouraging new synergies and approaches to CDR. Such research investments would accelerate this development and could help avoid some of the greatest climate risks that the lack of timely emissions reduction may make inevitable. The committee recognizes that a research program in CDR faces difficult challenges to create viable, scalable, and affordable techniques, but the committee argues that the situation with human-induced climate change is critical enough (see Chapter 1) that these CDR techniques need to be explored to assess their potential viability, and potential breakthrough technologies need to be nurtured as they arise.
Prioritizing a research portfolio will be challenging, as will the temptation to narrow the portfolio to those technologies closer to economic feasibility. Ongoing relevant research (e.g., bioenergy, CCS) also has the potential of advancing atmospheric CDR technologies and approaches. The scope of existing relevant programs could be broadened to include a wider portfolio. No major new bureaucracies are needed to facilitate enhanced research in this area.
It is possible that future research and development efforts could provide low-cost ways to reduce net anthropogenic CO2 emissions through CO2 capture from the atmosphere. However, the sheer mass of CO2 under consideration, and its diffuse presence in the atmosphere, present challenges to any effort to remove a substantial fraction of it and dispose of it safely in a reliable reservoir.
Overall, the committee concludes that there would be great value in the United States pursuing
- An expanded program of research and field studies to assess and improve strategies for performing and monitoring geologic sequestration;
- The exploration of strategies such as accelerated mineral weathering that enhance ocean uptake of carbon dioxide and/or increase the ocean’s ability to store carbon without causing adverse effects (ocean iron fertilization does not appear to be a promising strategy in this regard);
- Continued research on combining biomass energy with carbon dioxide capture and sequestration, including exploration of approaches that do not form and sequester concentrated CO2; and
- A program of fundamental research in science and technology to solicit, foster, and develop approaches for scrubbing carbon dioxide from the atmosphere that hold the potential to bring costs and energetics into a potentially feasible range.
CDR approaches that have value on a smaller scale can have other co-benefits but are unlikely to individually scale to contribute significantly to the problem at hand. The committee concludes there would be value in pursuing
- Research on land use management techniques that promote carbon sequestration and
- Research on accelerated weathering as a CO2 removal or sequestration approach that would allow conversion to stable, storable, or useful carbonates and bicarbonates.
Note that these research topics are not prioritized and, although they are listed together, these research topics do not necessarily require equal levels of investment.
The development of a research program on CDR may involve modeling, field research, satellite measurements, and laboratory studies. As such, this research will likely involve the efforts of multiple agencies, laboratories, and universities. It would be useful to have some coordination of the research efforts involved in these multiple organizations to avoid duplication and ensure that the most important questions are addressed. Although other organizations could perhaps fill this coordinating role, the U.S. Global Change Research Program (USGCRP) is the most obvious possibility and is a logical choice given the overlap of many research topics with the climate change research agenda. USGCRP coordinates and integrates federal research on changes in the global environment and their implications for society (http://www.globalchange.gov/about/overview). Thirteen departments and agencies participate in USGCRP, and USGCRP agencies interact with a wide variety of groups around the world including international, national, state, tribal, and local governments, businesses, professional and other nonprofit organizations, the scientific community, and the public.
Recommendation 2:6The committee recommends research and development investment to improve methods of carbon dioxide removal and disposal at scales that
6 Note that Recommendations 1, 3, 4, and 5 involve both CDR and albedo modification or albedo modification only, and are found in the Summary of this report and discussed in more detail in Chapter 5 of the companion report, Climate Intervention: Reflecting Sunlight to Cool Earth.
matter, in particular to minimize energy and materials consumption, identify and quantify risks, lower costs, and develop reliable sequestration and monitoring.
- It is increasingly likely that, as a society, we will need to deploy some forms of CDR to avoid the worst impacts of climate change, but without research investment now such attempts at climate mitigation are likely to fall well short of needed targets.
- Many of the strategies discussed for carbon dioxide removal provide viable and reasonably low-risk approaches to reducing atmospheric concentrations of carbon dioxide. Because the natural rate of carbon dioxide removal is currently being overwhelmed by anthropogenic emissions, additional CDR would need to be sustained at large scales over very long periods of time to have a significant effect on carbon dioxide concentrations and the associated risks of climate change.
- Absent some unforeseen technological innovation, large-scale carbon dioxide removal techniques have costs comparable to or exceeding those of avoiding carbon dioxide emissions by replacing fossil fuels with low-carbon-emission energy sources. Widespread CDR will likely occur only in a policy environment in which there are limits or a price is imposed on emissions of carbon dioxide, and in that case CDR will compete directly with mitigation on a cost basis (i.e., cost per ton of CO2 removed versus cost per ton of CO2 emission avoided).
- Decisions regarding deployment of CDR will be largely based on cost and scalability. Carbon dioxide removal strategies might entail some local or even regional environmental risk, but in some cases, CDR strategies may have also substantial co-benefits.
- Several federal agencies should have a role in defining and supporting CDR research and development. The committee recommends a coordinated approach that draws upon the historical strength of the various agencies involved and uses existing coordination mechanisms, such as the U.S. Global Change Research Program, to the extent possible.
For decades, the UN Framework Convention on Climate Change has recognized the important role of forests in CO2 removal from the atmosphere with reliable sequestration, although there has been controversy over how best to measure and assign credit for captured CO2. Far more controversial has been the suggestion that CO2 could be removed from the atmosphere by fertilizing the ocean with iron, for which there is a near consensus that at climatically relevant levels of deployment potential risks
outweigh potential benefits. Indeed, few observers today think that iron fertilization of the ocean is an attractive and effective way to markedly reduce atmospheric CO2 concentrations.
Ocean alkalinization and/or ocean iron fertilization would need to be applied over vast regions to have a chance at making a climatically detectable difference, and thus both ideas potentially involve intervening in Earth system processes, for better or worse, at a massive scale. The idea of interfering in Earth system properties at large scale is also common to albedo modification proposals, such as putting particles in the stratosphere. Furthermore, both involve activities that have effects across international borders and/or on an international commons such as the ocean. These properties have caused some (e.g., The Royal Society, 2009) to lump CDR and albedo modification (“solar geoengineering” or “solar radiation management”) together under a single umbrella term (“geoengineering”).
In some contexts, it might be useful to treat various CDR proposals and albedo modification proposals jointly. This is especially true of those CDR approaches that raise novel risks and governance issues (e.g., ocean fertilization, ocean alkalinization [or “ocean alkalinity addition”]). However, many proposed CDR approaches do not pose novel risks or governance issues (e.g., land management, BECCS).
For the next decades and perhaps the remainder of the century, atmospheric CO2 emissions are likely to be much greater than the amount of atmospheric CO2 removed. Thus, from a practical standpoint, it is often useful to consider these proposals in the context of other proposed means of reducing net CO2 emissions (e.g., near-zero-emission energy sources and increased energy efficiency).
Addressing the challenge of climate change will require a portfolio of solutions, and as the anthropogenic contributions to climate change persist, the effectiveness of that portfolio becomes increasingly critical. Both CDR strategies and other technologies and approaches that lead to lower CO2 concentrations in the atmosphere (e.g., CCS, solar energy, wind energy, and energy efficiency improvements) offer the potential to slow the growing concentrations of CO2 and other GHGs in the atmosphere. Although CDR techniques hold promise, they are not sufficiently advanced to the point of being deployable at scales and costs necessary to substantively address the challenges climate change represents, nor are they likely to ever be sufficient to singularly address these challenges. To determine if and when these techniques can be a major component of a mitigation portfolio requires research targeted at assessing and improving
the efficacy of these techniques for reducing atmospheric carbon content as well as fostering new methods and approaches. Key areas of focus are provided in the previous section, “Research.”
It is clear, however, that atmospheric CO2 removal is and can be valuable, especially given the current likelihood that total carbon emissions will exceed the threshold experts believe will produce irreversible environmental effects. For example, land management and reforestation can remove CO2 from the atmosphere and, when done well, can have substantial co-benefits. BECCS could represent an important mechanism for reducing atmospheric CO2 concentrations in the future once fossil fuel emissions are significantly reduced. Other approaches have been proposed (e.g., DACS and accelerated chemical weathering) that would benefit from additional research and analysis. Some of these approaches may never be cost effective, creating challenges to the development of a research portfolio that does not negatively affect research into mitigation opportunities that may be less expensive. Overall, there is much to be gained in pursing multiple parts of a portfolio of climate change strategies including research on various CDR techniques.
To be effective, carbon dioxide removal must be pursued collectively by a number of international participants. In contrast, albedo modification could be undertaken unilaterally. The environmental and climate system consequences of albedo modification are as yet poorly characterized, and the governance issues are complex as well. Some forms of carbon dioxide removal also involve environmental risk, for example from changes in ocean ecology or induced seismicity from underground injection of CO2 or from the use of inappropriate reservoirs. The barriers to deployment of CDR approaches are largely related to high costs, slow implementation, limited capacity, and policy considerations. If carbon removal technologies are to be viable, it is critical now to embark on a research program to lower the technical barriers to efficacy and affordability while remaining open to new ideas, approaches, and synergies. As is true for mitigation and adaptation, society must take advantage as soon as possible of CDR strategies that can help avoid the worst effects of warming. We will lose the opportunity if society delays in research and development to lower the technical barriers to efficacy and affordability of CDR for deployment.