As atmospheric concentrations of carbon dioxide (CO2) have continued to increase, policymakers have confronted the need to not only reduce emissions, but also remove CO2 from the atmosphere. This report assesses methods for creating or enhancing terrestrial and coastal carbon sinks for atmospheric CO2. An anthropogenic carbon sink captures atmospheric CO2 and then stores it in a reservoir, either in its captured form or another chemical form. Storage reservoirs can be either on or under the land surface, or in the ocean. This report considers only land and near-shore coastal reservoirs.
Anthropogenic CO2 released to the atmosphere by fossil fuel consumption, land use change, and cement production is the dominant cause of current and projected future climate change. Removing CO2 from the atmosphere and storing it has the same impact on the atmosphere climate as simultaneously preventing emission of an equal amount of CO2. For this reason, methods that create or enhance carbon sinks are best considered as part of the toolkit for net CO2 emissions reductions, although they are sometimes misleadingly classified with solar radiation management as “geo-engineering” (Budyko, 1977; NRC, 2015a, b; PSAC, 1965). Combustion of 1 gallon of gasoline releases approximately 10 kg of CO2 to the atmosphere. Capturing 10 kg of CO2 from the atmosphere and permanently sequestering it therefore has the same effect on atmospheric CO2 as any mitigation method that simultaneously prevents combustion of 1 gallon of gasoline.
The committee repeatedly encountered the viewpoint that most CO2 removal and sequestration1 approaches, or negative emissions technologies (NETs), will be deployed to reduce atmospheric CO2 after fossil emissions have been reduced to near zero. However, this viewpoint does not consider the fact that decreasing fossil emissions once they reach low levels will likely be very expensive and therefore methods for reduced and negative emissions will likely be used in concert for centuries, even during a sustained period of net negative global emissions (see Box 1.1, Figure A). Thus,
1 Although the term “storage” might imply accumulation for future use, the committee uses this term interchangeably with the term “sequestration” in accordance with the literature reviewed (Fuss et al., 2018).
trations, which produces less than 2°C of warming in the majority of climate models (Jones et al., 2016). In the top left panel of Figure A, the atmospheric concentration of CO2 declines after modeled year 2050 by 82 ppm, from 450 ppm in 2050 to 368 ppm in 2300. The panels in Figure B show the anthropogenic emissions (brown) and sinks from NETs for four 50-year periods. The green and dark blue boxes show the predicted sizes of the “natural” land and ocean sinks, while the light blue boxes show the starting and ending atmospheric concentrations (the atmospheric concentration at the end of each period and the beginning of the next are mildly inconsistent because of small inconsistencies between the scenario and modeled predictions as described in the supplementary information of Jones et al., 2016).
The most unexpected aspect of Figure B is that the atmospheric concentration declines by 25 ppm or 196 Gt CO2 from 2050 to 2100 (see upper right panel of), despite net anthropogenic
the question remains “Which costs2 more—an emission reduction or an equivalent amount of negative emission?”
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.
For example, few alternatives to chemical fuels are likely to exist for commercial aviation. One option for zero net emissions would be to use a NET to capture and store the 2.5 kg of CO2 for each liter of aviation fuel consumed. If the price of the NET could be reduced to $100/t CO2, then the cost of fuel would increase by ~$0.25 per liter. The total CO2 emissions of the fossil/NET bundle may decrease in comparison to cellulosic biofuels, without the carbon emissions associated with biofuels production and the negative externalities associated with devoting close to 100 million hectares of cropland to produce the required feedstock (Gunnarsson et al., 2018; Owen et al., 2010).
2 The committee refers to a comparison of direct costs of attaining an emissions reduction or negative emissions. All negative emissions and reduction technologies have a full set of indirect costs that may not be reflected in direct cost estimates.
As shown in the following chapters, some NETs are already cost-competitive with other mitigation options. Additional research would further reduce costs and facilitate scale-up. However, options with sufficient capacity to create negative emissions of at least 10 Gt/y CO2 have large negative side-effects (i.e., the impact of large-scale reforestation and afforestation on food production and biodiversity), are not yet well enough understood to deploy at scale, and/or face competition with less expensive mitigation options, which impedes research and development (R&D) by the private sector.
For example, at current costs for direct air capture, it would be difficult for a direct air capture/fossil bundle to compete successfully in markets for renewable fuels, because direct air capture alone would add more than $1 per liter to the cost. A related problem is that direct air capture requires considerable input of electrical and heat energy. Given that available energy is largely derived from fossil fuel today, direct air capture with net negative CO2 emissions may not become cost-competitive until low-cost zero-carbon energy is available. Finally, direct air capture must be coupled with reliable sequestration. The only existing method for sequestering large amounts of CO2 is geologic sequestration, and current rates of geologic sequestration are much lower than what would be required to impact atmospheric concentrations.
Although several companies aim to commercialize direct air capture systems (e.g., Carbon Engineering, Global Thermostat, Climeworks), Climeworks is the furthest along in the market process, selling to a comparatively small market in high-cost CO2 (i.e., CO2 used in greenhouses to enhance productivity may cost more than $1,000/t if the greenhouse is located far from a source). This market is too small to support a robust ecosystem of small innovators necessary to explore the large number of chemical recipes and physical machinery that might decrease direct air capture prices. Thus, like photovoltaics or hydraulic fracturing and horizontal drilling, the development of direct air capture will likely require long-term government investment in incentives.
BACKGROUND ON THE CARBON CYCLE AND CARBON SINKS
Isotopic evidence shows that the increase in atmospheric CO2 concentration from 280 parts per million (ppm) in 1750 to 407 ppm in 2017 was primarily caused by fossil fuel burning (IPCC, 2013; Le Quéré et al., 2016). Since 1750, 71 percent of the carbon atoms in anthropogenic CO2 emissions have originated from geologic reservoirs of coal, oil, and natural gas, 2 percent from geologic reservoirs of limestone used in cement production, and 27 percent from terrestrial ecosystems—primarily because of the clearing of forests, draining of wetlands, and the conversion of forests and grasslands to croplands and pastures (see Figure 1.1). NETs can help to reverse these transfers, by
removing CO2 from the atmosphere and transferring it back to geologic reservoirs and ecosystems.
Figure 1.1 also shows that the increase in atmospheric CO2 since the industrial revolution would have roughly doubled the observed ~125 ppm, if carbon sinks in the terrestrial biosphere and oceans had not taken up one-half of anthropogenic emissions. The “atmospheric fraction” (AF) is the annual increase in atmospheric CO2 divided by total anthropogenic emissions. Despite substantial interannual variation, much of it linked to the El Niño–Southern Oscillation (ENSO) cycle, the multiyear average atmospheric fraction has remained remarkably steady at ~45 percent since continuous measurements of atmospheric CO2 began in the late 1950s, indicating that the sum of land and ocean sinks has grown in proportion to anthropogenic emissions (see Figure 1.2). The land and ocean carbon sinks are often referred to as “natural” sinks, though a more appropriate adjective is probably “inadvertent,” because they are unintended byproducts of fossil fuel consumption and land use. The growth of the land sink is thought to
have two primary causes: CO2 fertilization of plants, which enhances photosynthesis and causes terrestrial ecosystems to gain carbon mass, and forest regrowth following agricultural abandonment in some locations (Pan et al., 2011). The ocean sink is caused both by the physical dissolution of atmospheric CO2 and by photosynthetic carbon gain by phytoplankton (Figure 1.3; Sarmiento and Gruber, 2002).
To understand the effect of NETs on future CO2 uptake by the “inadvertent” sinks, it is useful to divide the carbon sequestered by both the ocean carbon sink and the component of the land sink caused by CO2 fertilization into two separate pools. These two pools are distinguished by their characteristic time scales for carbon retention. Some carbon sinks are quick to reach equilibrium with the atmosphere, whereas others will continue to remove atmospheric CO2 over the next 10,000 years. Carbon in surface
waters of the ocean (see Figure 1.3) and in short-lived and rapidly decomposing tissues on land (such as most of the carbon in leaves and fine roots) are carbon sinks that rapidly reach equilibrium with the atmosphere. As such, carbon in this short-lived pool is closely correlated to atmospheric CO2, creating a sink whenever atmospheric CO2 increases and a source whenever it decreases. Thus, the size of the sink correlates to the time-derivative of atmospheric CO2. Carbon in the other pool has a long residence time, remaining out of equilibrium with atmospheric CO2. This characteristic results from accumulation of the carbon in this pool in the past when the atmospheric
concentration was lower, carbon in the deep ocean (residence time of ~1,000 years), and in living wood and recalcitrant dead organic matter on land (residence times of several decades to centuries). Rates of carbon addition to the long-lived pool increase with the gap between the current and past atmospheric CO2 concentrations. The associated carbon sink can thus persist through a period of declining atmospheric CO2, if the concentration remains sufficiently above the preindustrial concentration.
The results from Box 1.1 dispel two related scientific misunderstandings about NETs that the committee frequently encountered. The first is that NETs are qualitatively unlike other methods of climate mitigation because they offer society the only way to deliberately reduce the atmospheric concentration of CO2. Instead, atmospheric CO2 will decline once net anthropogenic emissions (emissions minus sinks from NETs) become smaller than the annual uptake by the natural sinks. At the same time, it would be extremely difficult to reduce net anthropogenic emissions enough to achieve declining atmospheric CO2 without the use of NETs because some fossil and land-use sources would be extremely disruptive or expensive to mitigate, such as some agricultural methane or CO2 from air travel. The same can be said of the omission of any major mitigation option, such as photovoltaics, wind electricity, or carbon capture and sequestration at fossil power plants. In addition, unlike other forms of mitigation, NETs provide the only means to achieve deep (i.e., >100 ppm) emissions reductions, beyond the capacity of the natural sinks. The second misconception is that the natural sinks would reverse and become sources during a period of declining atmospheric CO2. Instead, the sinks are expected to persist for more than a century of declining CO2 because of the continued disequilibrium uptake by the long-lived carbon pools in the ocean and terrestrial biosphere. For example, to reduce atmospheric CO2 from 450 to 400 ppm, it would not be necessary to create net negative anthropogenic emissions equal to the net positive historical emissions that caused the concentration to increase from 400 to 450 ppm. The persistent disequilibrium uptake by the land and ocean carbon sinks would allow for achievement of this reduction even with net positive anthropogenic emissions during the 50 ppm decline.
Nonetheless, the strengths of the land and ocean sinks decline through time in Figure 1.4 because of the concerted effects of the rapid decline in the time-derivative of atmospheric CO2 during the middle part of the century (for the rapidly equilibrating pools) and the declines in the absolute concentration after 2050 (for the disequilibrium pools). The mechanisms behind the sinks ensure that actions that decrease atmospheric CO2 will also tend to decrease sink strength. Thus, based on the dynamics of the natural terrestrial and ocean sinks, the deployment of NETs will progressively reduce their effectiveness. The perturbation airborne fraction (PAF) plotted in Figure 1.4 represents the decrease in atmospheric CO2 caused by the removal of one small unit
of CO2 by a NET. PAFs are less than 1.0 as the land and ocean sinks in the models are decreased by CO2 removal, and the monotonic decrease through time from nearly 1.0 to almost 0.5 shows that effectiveness decreases progressively. At a PAF of 0.5, two units of negative emissions are required to achieve one unit of reduced atmospheric CO2. However, this property is shared by technologies that reduce emissions; their use also weakens the sizes of the ocean and land sinks for the same reason.
This report focuses on the vital and productive role that NETs could play to reduce climate change immediately and throughout this century. The critical quantity here is net anthropogenic emissions, that is, the sum of positive emissions from fossil fuels and land use and negative emissions from NETs. The committee’s goal is to propose research that will reduce the costs and disruption of NETs, and so allow deeper reductions in net anthropogenic emissions, or the same reductions at lower cost and with greater allowable emissions from fossil fuels and land use. In addition, research on NETs will provide humanity with the long-term option of very large reductions in atmospheric CO2, like a return to preindustrial concentrations, although this is most likely a problem for the 22nd century (Hansen et al., 2017; Tokarska and Zickfeld, 2015).
ORIGIN AND PURPOSE OF THE STUDY
The United Nations Framework Convention on Climate Change (UNFCCC) pledged in 1992 to “prevent dangerous anthropogenic interference with the climate system” and initiated an international effort to reduce CO2 emissions. Negative emissions technologies were brought into the framework of the UNFCCC by the Kyoto Protocol of 1997, which included reforestation and afforestation as part of its Clean Development Mechanism (UNFCCC, 2013). In the two decades since the Kyoto Protocol, scientific research has improved understanding of greenhouse gas concentrations and the amount of warming that would cause “dangerous anthropogenic interference with the climate system.” Recent work (IPCC, 2012, 2013; NASEM, 2016) concludes that (1) damages from anthropogenic climate change are already occurring and will accelerate as greenhouse gases continue to accumulate and (2) under business as usual emissions, the climate system is in danger of crossing one or more thresholds for rapid and catastrophic change, such as multimeter sea-level rise from the loss of a major continental ice sheet.
The improved understanding of risks and damages created a consensus among many in the scientific community, nongovernmental organizations (NGOs), and governments that mean global warming should not exceed 2°C above the preindustrial value, and led to the Cancun agreement under the UNFCCC that committed governments to “hold the increase in average global temperature below two degrees” (UNFCCC, 2011). This in turn led to the adoption of Article 2 of the UNFCCC Paris agreement in 2016 by many nations of the world (although the United States has announced an intent to withdraw) to limit total warming below 2°C, and with an aspirational target of 1.5°C.
The 2°C target is exceedingly challenging—the global mean temperature has already risen about 1°C over the 20th century, and time lags in the carbon cycle and climate system likely mean that only about two-thirds of the warming that will eventually occur at current concentrations of atmospheric greenhouse gases has been reached (Hansen et al., 2011). The CO2 concentration, currently 407 ppm (2017), would probably need to remain below 450 ppm to prevent more than 2°C of warming (IPCC, 2013). It is currently increasing at about 2 ppm per year (Figure 1.2, 7.82 Gt CO2/ppm). Article 4 of the Paris agreement states that increases in atmospheric CO2 should cease “in the second half of the century,” although preventing the increase of atmospheric CO2 does not require that anthropogenic emissions cease, only that they be less than or equal in strength to carbon sinks.
Studies using integrated assessment models (IAMs) conclude that the reduction in net anthropogenic emissions required to meet the 2°C target, let alone the 1.5°C target, would now be quite difficult and expensive to achieve, even with technological breakthroughs. For example, the projected costs of limiting atmospheric CO2 below even 500 ppm average more than $1,000/t CO2 by 2100 in the IAM studies reviewed in the latest IPCC report (IPCC, 2014b). Moreover, the lowest cost trajectories for achieving the 2°C target chosen by the economic optimizations in IAMs often include massive deployment of NETs that would avoid the steeper costs of relying on emissions reductions alone. Some scenarios require that 600 million hectares of land (equal to nearly 40 percent of global cropland) be devoted to NETs (IPCC, 2014b). Net negative emissions in the second half of the century and beyond, achieved by the combined action of NETs, emissions reductions, and natural sinks, would allow atmospheric CO2 to temporarily overshoot levels consistent with 1.5°C or 2°C of warming at equilibrium, as in the time-series of atmospheric concentrations in RCP 2.6 (Figure 1.5; Fuss et al., 2014). Because the time required to reach equilibrium temperature is extensive (centuries), subsequent reductions in atmospheric CO2 could in principle keep global temperatures from exceeding the 1.5°C or 2°C target (Box 1.2.).
Improved understanding of future climate-related risks, coalescence around a 2°C target, and the prominence of NETs in the conclusions of IPCC (2014b) have led to substantial interest in negative emissions and to the recognition that far less is known about some NETs than about most traditional forms of carbon mitigation. In 2015, the National Academies published Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration (NRC, 2015b), which described and assessed relevant NETs and sequestration approaches and related concerns, including cost, technological readiness, required land, impacts on food production and biodiversity, required water, nitrogen and energy, permanence of stored carbon, far-field emissions of CO2, emissions of non-CO2 greenhouse gases, and biophysical impacts on climate (NRC, 2015b).3 That report’s recommendation for an R&D investment to minimize energy consumption and materials required by NETs, identify and quantify risks, reduce costs, and develop reliable sequestration and monitoring led to this current study. In addition, a recommendation from that report led to a related study on carbon use (see Box 1.3).
The specific charge to the committee is provided in Box 1.4. The study sponsors are the Department of Energy, National Oceanographic and Atmospheric Administration, Environmental Protection Agency, U.S. Geologic Survey, V. Kann Rasmussen Foundation, Incite Labs, and the Linden Trust for Conservation, with support from the National Academy of Sciences’ Arthur L. Day Fund.
3 See also DOE, 2016, Socolow et al., 2011, and Tavoni and Socolow (2013).
The committee was not tasked with performing a systematic review of all the literature related to NETs. Fortunately, because of the widespread interest in NETs, the committee had access to several recent and comprehensive reviews including Fuss et al., 2018, Minx et al., 2018, and Nemet et al., 2018.
NEGATIVE EMISSIONS TECHNOLOGIES
In response to item E of the Statement of Task, the committee focused on five major approaches (see Figure 1.6):
- Coastal blue carbon (Chapter 2)—Land-use and management practices that cause an increase in 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 within forests or agricultural lands that increase the total inventory of carbon in the terrestrial biosphere. These include the following:
- Management methods on croplands or pastures, such as reduced tillage
- or the planting of cover crops that increase the total amount of undecomposed organic carbon in the soils (“agricultural soils”).
- Planting forest on lands that used to be forest, but were converted to another use (“reforestation”), or planting forest on lands that were originally grasslands or shrublands (“afforestation”).
- Management practices that increase the amount of carbon per unit land area on existing forest, such as accelerating regeneration after disturbance or lengthening harvest rotations (“forest management”).
- Bioenergy with carbon capture and sequestration4 (BECCS; Chapter 4)—Photosynthesis captures atmospheric CO2 and energy from sunlight and stores both in plant tissues. BECCS combines the production of energy from plant biomass to produce electricity, liquid fuels and/or heat with capture and sequestration of any CO2 produced when using the bioenergy and any remaining biomass carbon that is not contained in the liquid fuels. This report focuses on biomass combustion for power and thermochemical conversion to fuel because they have the highest carbon negative potential, as opposed to biological biomass conversion that is fundamentally limited due to the inability to break down lignin (up to 25 percent of all biomass).
- Direct air capture and sequestration (Chapter 5)—Chemical processes capture and concentrate CO2 from ambient air so that it can be injected into a storage reservoir. In some incarnations, the captured CO2 may be reused in products. Capture and reuse in short-lived products, such as chemical fuels, is not included in this report as a NET, because the carbon in the products would be returned quickly to the atmosphere. However, capture in long-lived products, such as many structural materials, is included, because the product itself is then the storage reservoir. Carbon capture and re-use is the subject of a separate National Academies study that is discussed in Box 1.3.
- 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). Carbon mineralization includes 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 captured through either BECCS or direct air capture are injected into ultramafic and basaltic rocks where it mineralizes in the pores.
- Geologic sequestration (Chapter 7)—Supercritical CO2 is injected into a
4 BECCS includes both combustion-based methods that utilize biomass to generate electricity, with CO2 being sequestered from the flue gas, and pyrolysis-based methods that use biomass to produce liquid biofuels and biochar, with the biochar represented the negative carbon potential.
- geologic formation where it remains in the pore space of the rock for a long period of time. This is not a NET, but rather an option for the sequestration component of BECCS or direct air capture. It is treated by a separate chapter, to avoid repetition in the BECCS and direct air capture chapters.
The committee used the above list of NETs as an organizing framework for both information gathering and report development. Many of the elements on the list are well documented in an extensive literature and are relatively straightforward to describe to policymakers and the public. The collection includes NETs at very different stages of technological readiness, and so the recommended research program called for in item A of the Statement of Task spans the full range from basic scientific research to final pre-deployment studies. Given the relative nascency of coastal blue carbon, direct air capture, and carbon mineralization approaches in the literature, these approaches are described in much more technical and comprehensive detail in their respective chapters, with supplemental technical details for direct air capture and carbon mineralization included in Appendices D and E.
The committee’s focus on sequestration in terrestrial and nearshore/coastal environments is not intended to undervalue the potential of technologies or practices for oceanic sequestration, but instead is a response to the Statement of Task. The oceans already contain 36,000 Gt C, mostly in the form of bicarbonate (equivalent to 132,000 Gt CO2). Once the fossil fuel age is over, almost all of the anthropogenic CO2 in the atmosphere will ultimately make its way into the oceans (centuries to millennia) and finally into carbonate minerals on the sea floor (tens of millennia). Promising technologies exist for oceanic sequestration, some with potentially limited environmental impact, and the capacity of the oceanic carbon reservoir is obviously enormous. Explored approaches include increased biomass production and ocean alkalization. This committee was not convened to cover the physical, chemical, and biological dimensions of oceanic options, or the complex international rules and negotiations that would permit oceanic disposal. Consideration of oceanic options would require a separate study. Near-shore/coastal is included with the terrestrial options, rather than with oceanic options, because the near-shore/coastal ecosystems assessed in this study store carbon in living plant tissues and undecomposed organic matter in soils/sediments, like terrestrial ecosystems, and unlike most oceanic options.
The exclusive focus of this report on NETs is also a reflection of the Statement of Task. Although the report addresses the relationship between NETs and decarbonized power (i.e., a direct air capture system powered by fossil fuel has a much higher cost/net CO2 than a system powered by decarbonized power), it does not address critical mitigation options such as enhanced energy efficiency, renewable electricity,
or reduced deforestation because they are not NETs. Their exclusion is in no way a statement about priorities. The committee is acutely aware that the possibility of large negative emissions in the future might result in a moral hazard, by reducing the will to cut emissions in the near term (Anderson and Peters, 2016). Emissions reductions are vital to address the climate problem. However, policymakers benefit from consideration of the broadest possible portfolio of technologies to find the most inexpensive and least disruptive solution, including those with positive, near-zero, and negative emissions. In addition, a broad portfolio of technologies (including multiple NETs) offers increased resiliency to managing the risks of surprises arising from nature and mitigation actions. Furthermore, the possibility of irreversible consequences of temporary warming is another reason to quickly develop NETs so that they can more quickly reduce net anthropogenic emissions.
Framework for Assessing Individual NETs
The Statement of Task specifies two main purposes of this report: (1) to assess each NET and identify the most critical unanswered questions about benefits, costs, potential scale, and risks, and the most important barriers to commercial viability and (2) to propose an R&D program, with estimated costs and implementation (including monitoring and verification, institutional structures, and research management). NETs span a range of technological readiness, and therefore the assessments and recommendations for the different options are highly heterogeneous. Nonetheless, Chapters 2-7 share a few elements. Each chapter focuses on two scales, the United States and the globe, and on research designed to funded by the United States. Each defines the approach and describes the technology, impact potential for CO2 removal and sequestration, cost per ton of CO2, barriers to cost reductions, secondary impacts (including co-benefits), and requirements and costs of the proposed research agenda. The committee developed these estimates, research agenda, and costs based on its expert judgment after reviewing the relevant literature and hearing from experts at committee workshops and webinars.
Impact Potential. Upper-bound estimates of the potential rate and capacity for carbon capture and sequestration are constrained primarily by hard barriers such as available pore space in geologic reservoirs or available land area. Practically achievable rates and capacities reflect the committee’s judgment about levels of deployment that could be achieved given economic, environmental, societal, and other barriers to scale-up. Thus, the practically achievable estimates required the uncertain integration of a large number of option-specific factors, many of which are themselves uncertain. Each chapter attempts to explain clearly how these estimates were obtained. In
general, the committee restricted its investigation to methods that have a practically achievable potential of at least 1 Gt/y CO2e globally. In addition, these estimates include considerations of all the processes that might cause CO2 emissions, from carbon capture to ultimate sequestration, and possible leakage of CO2 back to the atmosphere after sequestration. For example, the carbon implications of the input energy requirement for a technology such as direct air capture is needed.
A particularly challenging problem associated with carbon flux is permanence, that is, that leakage from a CO2 storage reservoir may occur a century or more after initial sequestration. How long does sequestered carbon need to remain below ground or locked in the organic matter in an ecosystem? Because CO2 has an average residence time in the atmosphere of more than a century, and because dissolved CO2 acidifies the oceans and persists for millennia before being deposited in carbonate sediments, a typical answer is that carbon needs to be stored, on average, for millennia.
In contrast, it is difficult to imagine any set of policies that would guarantee 1,000-year carbon storage in an ecosystem, as reflected in current markets for forestry offsets that stipulate storage for the next 20-100 years (Hamrick and Gallant, 2017). Fortunately, economic calculations typically produce more achievable requirements than a purely scientific analysis, because economic discounting reduces the present cost of any re-emission that occurs far in the future. However, in the absence of high-capacity NETs, economic calculations still must include the costs of climate change that cannot be mitigated if caused by CO2 re-emitted after the end of the fossil era.
However, NETs provide the possibility of capturing and re-sequestering CO2 as it is lost from storage reservoirs. If the price of capture and sequestration is P $/t CO2, the economic discount rate is r, escape from reservoirs occurs at a constant rate per unit of carbon stored, and the average residence time of CO2 in a reservoir is T, then the additional present cost of recapturing and re-sequestering escaped CO2 forever is then the expense required to recapture and re-sequester escaped CO2 in perpetuity has small present value relative to the cost of initial capture and sequestration. For example, if the discount rate is 5 percent and T is 100 years, then the present cost of effective permanence adds 20 percent to P. Moreover, continued monitoring would reduce costs further, because re-sequestration would avoid sites with a poor record of containment.
Each chapter also estimates costs to establish NET and/or sequestration projects and for annual operation in two cases: prior to scale-up (i.e., today’s cost for almost all options) and once a method or technology is implemented at scale. The committee also
examined barriers to scale-up other than current costs, and the potential for future cost reductions. The estimated costs are specifically not reductions in the size of the economy caused by public investment in carbon removal and sequestration, like those produced by general equilibrium models in economics.
Secondary Impacts. Because NETs could make a substantial contribution to solving the climate problem only if they can create billions of tons of negative CO2 emissions, collateral benefits and costs are inevitable, and potentially substantial. The committee assessed:
- environmental impacts including emissions of non-CO2 greenhouse gases, biophysical effects of land cover change on climate and river runoff (primarily from changes in albedo and evapotranspiration), increased extinction from habitat loss, and changes in nitrogen runoff;
- potential co-benefits including electricity generation or biofuel production for BECCS, new industries, and improved agricultural productivity, soil nitrogen retention, and soil water holding capacity for cropland soil CO2 removal and sequestration; and
- societal impacts from changes in the supply of food, fiber, water, and other materials and public acceptance of scale-up.
Research Agendas. Chapters 2-7 propose and justify research programs for each NET as well as considerations for implementation of the research programs, including assessing constraints for the development and deployment of NETs imposed by the legal system, infrastructure requirements, public perception, and system-integration requirements. The recommended research agenda is presented in terms of (1) basic science questions (knowledge gaps), (2) development (technology issues), (3) demonstration (engineering and economics), and (4) deployment (scale-up barriers, economics, and governance). Each chapter contains estimated costs of the research agenda and outlines implementation of the research agenda—monitoring and verification, institutional structures, and research management.
Many of the research agenda budgets are intended to be staggered over a period of several years. Therefore, the simple addition of budget line items does not provide an accurate picture of total annual budgets for any given component or task. Moreover, as projects are scaled up from bench, to pilot, to demonstration-scale prototypes, they should pass through a comprehensive review (stage-gate) before funding is allocated for scale-up to the next prototype size. The coordination between these stage-gates, prototype scales, technology readiness levels, and research phases is shown graphically in Figure 1.7. This approach is intended to reduce technology and financial risk. As such, it is entirely possible that no program funding is needed for pilot and
demonstration-scale prototypes, because there may be no eligible projects (i.e., no projects able to achieve the program metrics and thus not advance to the next stage of development).
The research agendas proposed in Chapters 2-7 are combined in Chapter 8 into an integrated research proposal and a single list of research priorities. In addition, Chapter 8 examines national- and global-scale CO2 removal and sequestration holistically, rather than one option at a time, to understand better the interactions among NETs. Because the scale of deployment is potentially so large, interactions are inevitable, including competition for the same land, water, and materials and synergistic environmental and societal impacts.