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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CHAPTER ONE Introduction 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 ­ errestrial and coastal carbon sinks for atmospheric CO2. An anthropogenic carbon t 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-engineer- ing” (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. OVERVIEW The committee repeatedly encountered the viewpoint that most CO2 removal and se- questration1 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 dur- ing a sustained period of net negative global emissions (see Box 1.1, Figure A). Thus, 1  Althoughthe 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). 23

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N BOX 1.1 OCEAN AND TERRESTRIAL SINKS The dynamics of the ocean and terrestrial sinks are inherently more complicated than these simplified images and remain challenging to predict with models (Friedlingstein et al., 2014). Several global models predict that the terrestrial sink will eventually switch to a source under business-as-usual emissions due to warming-accelerated decomposition of organic matter in high-latitude soils, nutrient limitation of CO2 fertilization, and forest die-back from heat, drought, and insect pests (Joos et al., 2001; Prentice et al., 2001). Similarly, the future size of the oceanic sink depends on changes in ocean circulation resulting from a host of factors, including the strong wind-speed dependence of CO2 exchange between the atmosphere and surface ocean and the effects of warming and glacial melt water on the overturning circulation (DeVries et al., 2017; Sarmiento et al., 1998; Swart et al., 2014). Figure A depicts the predicted carbon exchanges from six global models under the RCP (Rep- resentative Concentration Pathway) 2.6 scenario for future atmospheric greenhouse gas concen- FIGURE A  RCP2.6 scenario and simulated carbon fluxes. SOURCE: Jones et al., 2016. 24

Introduction BOX 1.1  Continued 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 FIGURE B  The four stages of succession of the differing balance between flux components. SOURCE: Jones et al., 2016. continued 25

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N BOX 1.1  Continued emissions of 164 Gt CO2 during the same period (fossil and land use minus NET), because the land and ocean carbon sinks take up 196+164 Gt CO2. The sinks persist for over a century of declining ­ atmospheric CO2 in Figure B, because the disequilibrium uptake of CO2 by the long-lived carbon pools in the land and ocean models dominates out-gassing by the short-lived carbon pools. The 2050-2100 results imply that an identical 25 ppm reduction in atmospheric CO2 could be achieved solely by reducing the fossil and land use emissions to 164 Gt CO2, without any deploy- ment of NETs. Similarly, over the 250-year period from 2050 to 2300 in Figure B, the large net ocean and land sink causes the atmospheric concentration to decline by 75 ppm, though net anthropogenic emissions are slightly positive. At the same time, reducing the average 2050-2100 fossil fuel and land-use emissions to the ~3.3 Gt/y CO2 in Figure B (less than 10 percent of today’s emissions) would likely be very difficult without the use of NETs to offset emissions that would be prohibitively expensive or disruptive to eliminate, such as some agricultural methane and nitrous oxide (N2O) emissions, or CO2 emissions from air travel. In this sense then, NETs would be essential to achieving reduc- tions in atmospheric CO2 such as those in Figure B, because they would be essential to reducing net anthropogenic emissions enough so that remaining land and ocean sinks could reduce the atmospheric concentration. Moreover, it may become necessary to reduce atmospheric CO2 beyond the capacity of natural sinks. NETs offer the only way to achieve such deep reductions. 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 compo- nent 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 avia- tion. 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 crop- land 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. 26

Introduction As shown in the following chapters, some NETs are already cost-competitive with other mitigation options. Additional research would further reduce costs and facili- tate 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 prob- lem 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 reli- able 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 27

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 1.1  Cumulative contributions to the global carbon budget since 1870. NOTE: Contributions are shown in parts per million (ppm). SOURCE: Le Quéré et al., 2016. 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 revolu- tion would have roughly doubled the observed ~125 ppm, if carbon sinks in the terres- trial 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 frac- tion 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 appro­ priate adjective is probably “inadvertent,” because they are unintended ­ yproducts b of fossil fuel consumption and land use. The growth of the land sink is thought to ­ 28

Introduction FIGURE 1.2  Global carbon dioxide budget. NOTE: The global carbon budget refers to the mean, variations, and trends in the anthropogenic pertur- bation of CO2 in the atmosphere since the beginning of the industrial era. It quantifies the input of CO2 to the atmosphere by emissions from human activities, the growth of CO2 in the atmosphere, and the resulting changes in land and ocean carbon fluxes in response to increasing atmospheric CO2 levels, cli- mate change and climate variability, and other anthropogenic and natural changes. SOURCE: Le Quéré et al., 2018. 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 photo­ ynthetic carbon s 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 com- ponent 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 29

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 1.3  Schematic of the global carbon cycle. NOTES: One PgC equals 1 billion metric tons of carbon (1 Gt C), which is the same amount of carbon as in 3.67 Gt CO2. Further, 1 ppm CO2 equals about 7.82 Gt CO2. Black numbers show the cycle in 1750; red numbers show the average of 2000-2009. SOURCE: IPCC, 2013. waters of the ocean (see Figure 1.3) and in short-lived and rapidly decomposing tis- sues 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 resi- dence 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 30

Introduction 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 as- sociated 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) be- come 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 agri- cultural 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 emis- sions 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 posi- tive 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 equilibrat- ing pools) and the declines in the absolute concentration after 2050 (for the disequi- librium 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 re- duce 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 31

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 1.4  Perturbation airborne fraction in Jones et al., 2016. 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 re- ductions 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). 32

Introduction 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 tech- nologies 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 accel- erate as greenhouse gases continue to accumulate and (2) under business as usual e ­ missions, 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 govern- ments that mean global warming should not exceed 2°C above the preindustrial value, and led to the Cancun agreement under the UNFCCC that committed gov- ernments 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 an- nounced an intent to withdraw) to limit total warming below 2°C, and with an aspira- tional 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 proba- bly 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. 33

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N 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 break- throughs. 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 reduc- tions 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 emis- sions 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 tem- porarily 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 tempera- tures 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 readi- ness, required land, impacts on food production and biodiversity, required water, ni- trogen 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 recom- mendation 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 Founda- tion, Incite Labs, and the Linden Trust for Conservation, with support from the Na- tional Academy of Sciences’ Arthur L. Day Fund. 3  See also DOE, 2016, Socolow et al., 2011, and Tavoni and Socolow (2013). 34

Introduction FIGURE 1.5  Panel a depicts CO2 emission pathways until 2100, and panel b depicts the extent of net negative emissions and BECCS using RCP 2.6 in 2100. SOURCE: Fuss et al., 2014. The committee was not tasked with performing a systematic review of all the litera- ture related to NETs. Fortunately, because of the widespread interest in NETs, the com- mittee had access to several recent and comprehensive reviews including Fuss et al., 2018, Minx et al., 2018, and Nemet et al., 2018. 35

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N BOX 1.2 ASSESSING THE NEED FOR NETS To complete its task, the committee assessed the urgency of developing new and/or im- proved NETs. Apart from two NETs with large co-benefits, NETs will only be deployed to reduce atmospheric CO2. Thus, the coming economic and social demand for NETs can only be assessed in reference to a plan for climate mitigation. The committee assessed the need for NETs using the target from the Paris Agreement that warming be limited to substantially less than 2°C and ideally 1.5°C. And recently, the IPCC, 2018 concluded that limiting global warming to 1.5°C with limited or no overshoot will require the use of NETs by the middle of this century. Any statement in this report about the need for an emissions reduction of a particular size should not be interpreted as a normative statement (a value judgment on what should be), but rather as a statement about action required given a decision to meet the Paris agreement or to provide NETs to the international market by most nations, many corporations, and several U.S. states and local governments. Nonetheless, the committee is acutely aware that the U.S. government has announced an intention to withdraw from the Paris Agreement. It is useful to ask how different the report’s conclusions would be without the constraint of the Paris agreement. The committee believes that its conclusions and recommendations are generally robust, simply because the economic rewards for success would be so large. This is due to the U.S. government’s ongoing commitment to reduce climate change, as evidenced by the recently adopted 45Q rulea that provides a tax credit of $50/t CO2 for capture and storage, and the ongoing commitments to the Paris agreement by most nations, many corporations, and several U.S. states and governments. a https://www.law.cornell.edu/uscode/text/26/45Q. 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 man- groves, 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 man- agement practices within forests or agricultural lands that increase the total inventory of carbon in the terrestrial biosphere. These include the following: º Management methods on croplands or pastures, such as reduced tillage 36

FIGURE 1.6  Negative emissions technologies considered in this report. 37

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N BOX 1.3 NATIONAL ACADEMIES STUDIES ON CARBON REMOVAL AND USE: DEVELOPING A RESEARCH AGENDA FOR CARBON DIOXIDE REMOVAL AND RELIABLE SEQUESTRATION & DEVELOPING A RESEARCH AGENDA FOR UTILIZATION OF GASEOUS CARBON WASTE STREAMSa Recognizing the important role of CO2 removal and sequestration technologies in meeting greenhouse gas reduction goals, the National Academies convened an ad hoc committee to eval- uate the state of science and viability of technologies for removing CO2 from the atmosphere. The resulting report, Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration (NRC, 2015b), recommended investments in research and development (R&D) for technologies that remove CO2 from the atmosphere. Building on this report, the National Academies convened two committees, one to identify research needs for CO2 removal from the atmosphere and sequestra- tion, and one to study utilization of concentrated carbon waste gas streams. The Committee on Developing a Research Agenda for Carbon Dioxide Removal and Reliable Sequestration assessed the benefits, risks, and “sustainable scale potential” for atmospheric CO2 removal and sequestra- tion approaches and defined the essential components of an R&D program, including estimates of the program’s cost and potential impact. The Committee on Developing a Research Agenda for Utilization of Gaseous Carbon Waste Streams developed a research agenda for conversion of concentrated waste gas streams of CO2, methane, and biogas as a feedstock into commer- cially valuable products. That committee surveyed the current landscape of carbon utilization technologies and identified key factors and criteria associated with making these technologies commercially viable. Together, the two committees’ reports provide an assessment of research needs and opportunities to remove and utilize gaseous carbon. a See http://dels.nas.edu/Study-In-Progress/Developing-Research-Agenda-Utilization/DELS-BCST-16-05. BOX 1.4 STATEMENT OF TASK A. Identify the most urgent unanswered scientific and technical questions needed to: i.  assess the benefits, risks, and sustainable scale potential for carbon dioxide removal and sequestration approaches in the terrestrial and nearshore/coastal environment; and ii. increase the commercial viability of carbon dioxide removal and sequestration; B. Define the essential components of a research and development program and specific tasks required to answer these questions; C. Estimate the costs and potential impacts of such a research and development program to the extent possible in the timeframe of the study. D. Recommend ways to implement such a research and development program. E. The list of carbon dioxide removal approaches to be examined would include land man- agement, accelerated weathering, bioenergy with capture, direct air capture, geologic sequestra­ ion, nearshore/coastal approaches, and other approaches deemed by the study t committee to be of similar viability. 38

Introduction or the planting of cover crops that increase the total amount of undecom- posed organic carbon in the soils (“agricultural soils”). º Planting forest on lands that used to be forest, but were converted to an- other use (“reforestation”), or planting forest on lands that were originally grasslands or shrublands (“afforestation”). º 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 i ­nability 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 prod- ucts. 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 sepa- rate 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 (particu- larly mantle peridotite, basaltic lava, and other reactive rocks). Carbon mineral- ization includes both at the surface (ex situ) where CO2 in ambient air is miner- alized 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. 39

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N 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 infor- mation 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 chap- ters, with supplemental technical details for direct air capture and carbon mineraliza- tion included in Appendices D and E. The committee’s focus on sequestration in terrestrial and nearshore/coastal environ- ments 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 tech- nologies exist for oceanic sequestration, some with potentially limited environmen- tal 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 decarbon- ized 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, 40

Introduction 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 reduc- tions 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, po- tential 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 recom- mendations 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 seques- tration, cost per ton of CO2, barriers to cost reductions, secondary impacts (includ- ing 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 commit- tee 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 achiev- able 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 uncer- tain. Each chapter attempts to explain clearly how these estimates were obtained. In 41

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N 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 in- clude considerations of all the processes that might cause CO2 emissions, from carbon capture to ultimate sequestration, and possible leakage of CO2 back to the atmo- sphere 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 off- sets that stipulate storage for the next 20-100 years (Hamrick and Gallant, 2017). For- tunately, 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 +P ! T e– r t dt = Tr . If Tr>1, then the expense required to recapture and re-sequester additional present cost of recapturing and re-sequestering escaped CO2 forever is P , 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, con- tinued 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 op- tions) and once a method or technology is implemented at scale. The committee also 42

Introduction 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: (1) 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; (2) 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 (3) 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) demon- stration (engineering and economics), and (4) deployment (scale-up barriers, econom- ics, 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 graphi- cally 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 43

N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 1.7  Illustration of coordination between research phases, technology readiness level (TRL), prototype scale, stage-gates, and institutions. 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). Synthesis 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, Chap- ter 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 environ- mental and societal impacts. 44

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To achieve goals for climate and economic growth, “negative emissions technologies” (NETs) that remove and sequester carbon dioxide from the air will need to play a significant role in mitigating climate change. Unlike carbon capture and storage technologies that remove carbon dioxide emissions directly from large point sources such as coal power plants, NETs remove carbon dioxide directly from the atmosphere or enhance natural carbon sinks. Storing the carbon dioxide from NETs has the same impact on the atmosphere and climate as simultaneously preventing an equal amount of carbon dioxide from being emitted. 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, Negative Emissions Technologies and Reliable Sequestration: A Research Agenda assesses the benefits, risks, and “sustainable scale potential” for NETs and sequestration. This report also defines the essential components of a research and development program, including its estimated costs and potential impact.

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