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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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. 2018. 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.

1 Introduction As atmospheric concentrations of carbon dioxide (CO2) have continued to increase, policy makers 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, may transform it into another chemical form, and then stores it in a reservoir. 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 and climate as simultaneously preventing an equal amount of CO2 from being emitted. For this reason, methods that create or enhance carbon sinks are best thought of 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, 2015b, a; PASC, 1965). Combustion of a 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 a gallon of gasoline combustion. OVERVIEW The committee repeatedly encountered the view that CO2 removal and sequestration1 approaches, or negative emissions technologies (NETs) as they are often referred to, would primarily be deployed to reduce atmospheric CO2 after fossil emissions had been reduced to near zero. In contrast, because it is likely to be particularly expensive to decrease fossil emissions once they reach low levels, methods for negative emissions and emissions reductions will likely be used in concert for centuries, even during a sustained period of net negative global emissions, as in Figure 1.4b. Thus, the question will remain: 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 direct air capture and sequestration to capture and store the 2.5 kg of CO2 for each liter of aviation fuel consumed. If the price of direct air capture could be reduced to $100/tCO2, then this would add ~25 cents a liter to the cost of fuel. It also would likely decrease the total CO2 emissions of the fossil/direct air capture bundle in comparison to cellulosic biofuels, and do so without the carbon emissions associated with biofuels production and the negative externalities associated 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). 2 The committee is referring to a comparison of direct costs of attaining an emissions reduction or negative emissions. It recognizes that all emissions reduction and negative emissions technologies have a full set of indirect costs that may not be reflected in direct cost estimates. PREPUBLICATION COPY 15

16 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda with devoting on the order of one hundred million hectares of cropland to produce the required feedstock (Owen et al. 2010; Gundundsson and Anger 2012). As shown in subsequent chapters of this report, some NETs are already cost-competitive with other mitigation options, moreover, additional research would further reduce costs and facilitate scale-up. However, options with the capacity large enough to create negative emissions of 10 GtCO2/y or more either 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 for deployment at scale, and/or face competition with less expensive mitigation options, which impedes investment in 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 over a dollar 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 needs to be coupled with reliable sequestration. Currently the only method for sequestering large amount of CO2 is geologic sequestration, and current rates of geologic sequestration are much lower than would be required to impact atmospheric concentrations. Though there are several companies aiming to commercialize direct air capture systems (e.g. Carbon Engineering, Global Thermostat, Climeworks), Climeworks is the farthest in the market process, selling to a comparatively small market in high-cost CO2 (i.e. CO2 used in greenhouses to enhance productivity). This 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. FIGURE 1.1 The Global Carbon Budget. The cumulative contributions to the global carbon budget from 1870. Contributions are shown in parts per million (ppm). SOURCE: Le Quéré et al., 2016. PREPUBLICATION COPY

Introduction 17 BACKGROUND ON THE CARBON CYCLE AND CARBON SINKS Isotopic evidence shows that the increase in atmospheric CO2 concentration from 280 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% of the carbon atoms in anthropogenic CO2 emissions have originated from geologic reservoirs of coal, oil, and natural gas, 2% from geologic reservoirs of limestone used in cement production, and 27% from terrestrial ecosystems; primarily due to the clearing of forests, draining of wetlands, and the conversion of forests and grasslands to croplands and pastures (see Figure 1.1). NETs can be thought of as a way 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 double the observed ~125 ppm, if carbon sinks in the terrestrial biosphere and oceans had not taken up 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 multi-year average atmospheric fraction has remained remarkably steady at ~45% 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 FIGURE 1.2 Global carbon dioxide budget. The global carbon budget refers to the mean, variations, and trends in the anthropogenic perturbation 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, climate change and climate variability, and other anthropogenic and natural changes. SOURCE: Le Quéré et al., 2018. PREPUBLICATION COPY

18 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 1.3 Schematic of the global carbon cycle. Note that 1 PgC equals one billion metric tons of carbon (1 GtC), which is the same amount of carbon as in 3.67 GtCO2. Further, 1ppm CO2 equals about 7.82 GtCO2. Black numbers show the cycle in 1750; red numbers show the average of 2000-2009. SOURCE: IPCC, 2013. “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 is a result of the carbon in this pool being accumulated in the past when the atmospheric concentration was lower; carbon in the deep ocean (residence time of ~1000 years) and in living wood and recalcitrant dead organic matter on land PREPUBLICATION COPY

Introduction 19 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 1.4A depicts the predicted carbon exchanges from six global models under the RCP 2.6 scenario for future atmospheric GHG concentrations, which produces less than 2°C of warming in the majority of climate models (Jones et al., 2016). In the top left panel of 1.4A, 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 1.4B show the anthropogenic emissions (brown) and sinks from NETs during 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 1.4B is that the atmospheric concentration declines by 25 ppm or 196 GtCO2 from 2050 to 2100 (see upper right panel of Figure 1.4B), despite net anthropogenic emissions of 164 GtCO2 during the same period (fossil and land use minus NET), because the land and ocean carbon sinks take up 196+164 GtCO2. The sinks persist for over a century of declining atmospheric CO2 in Figure 1.4B, 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 GtCO2, without any deployment of NETs. Similarly, over the 250-year period from 2050 to 2300 in Figure 1.4B, 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 GtCO2/y in Fig. 1.4—less than 10% 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 N2O emissions, or CO2 emissions from air travel. In this sense then, NETs would be essential to achieve reductions in atmospheric CO2 like those in Fig. 1.4B, because they would be essential to reduce 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. (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, providing that the concentration stays sufficiently above the pre-industrial 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 PREPUBLICATION COPY

20 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 1.4A RCP2.6 scenario and simulated carbon fluxes. SOURCE: Jones et al., 2016. same time, it would be extremely difficult to reduce net anthropogenic emissions enough to achieve declining atmospheric CO2 without the use of NETs because of fossil and land use sources that would be extremely disruptive or expensive to mitigate, such as some agricultural methane or CO2 from air travel. Please note that 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. Also, 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 this reduction to be achieved 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 which 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 atmospheric fraction (PAF) plotted in Figure 1.4C is the decrease in atmospheric CO2 caused by the removal of one small unit of CO2 by an NET. PAFs are less than one as the land and ocean sinks in the models are decreased by CO2 removal and the monotonic decrease through time from nearly 1 to PREPUBLICATION COPY

Introduction 21 FIGURE 1.4B The four stages of succession of the differing balance between flux components. SOURCE: Jones et al., 2016. FIGURE 1.4C Perturbation Airborne Fraction in Jones et al., 2016. almost 0.5 shows that effectiveness decreases progressively. At a PAF of 0.5, 2 units of negative emissions are required to achieve one unit of reduced atmospheric CO2. However, it is important to understand that 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—the sum of positive emissions from fossil fuels and land use and negative emissions from NETs. The 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 PREPUBLICATION COPY

22 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda 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 pre-industrial 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 multi-meter 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, 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 US 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, and time lags in the carbon cycle and climate system likely mean that only about 2/3 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 be kept beneath 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 GtCO2/ppm). Article 4 of the Paris Agreement states that increases in atmospheric CO2 should cease “in the second half of the century”, although it is important to understand that 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 reduction in net anthropogenic emissions required to meet the 2°C target, let alone 1.5°C, would now be quite difficult and expensive, even with technological breakthroughs. For example, projected costs of limiting atmospheric CO2 beneath even 500 ppm average over $1000 per ton of CO2 by 2100 in the IAM studies reviewed in the latest IPCC report (IPCC, 2014a). 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% of global cropland—be devoted to NETs (IPCC, 2014a). 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 over-shoot 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). Given that 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.). PREPUBLICATION COPY

Introduction 23 FIGURE 1.5 Panel a depicts carbon dioxide 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. BOX 1.2 Assessing the Need for NETs To complete its task, the committee assessed the urgency of developing new and/or improved 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 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. Whenever this report states the need for an emissions reduction of a particular size, this should not be interpreted as a normative statement (a value judgement on what should be), but rather as a statement about action required given a decision to meet the Paris target or to provide NET’s to the international market created by such a decision by most nations, many corporations, and several US states and local governments. Nonetheless, the committee is acutely aware that the US 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 target. 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 US government’s ongoing commitment to reduce climate change, as evidenced by the recently adopted 45Q rulea that provides a tax credit $50/tCO2 for capture and storage, and the ongoing commitments to the Paris target by states, local governments, corporations and other countries. ____________________ a https://www.law.cornell.edu/uscode/text/26/45Q Improved understanding of future climate-related risks, coalescence around a 2°C target, and the prominence of NETs in the conclusions of IPCC, 2014a has 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 concerns about them, including: cost, technological readiness, required land, impacts on food production and biodiversity, required water, nitrogen and energy, permanence of PREPUBLICATION COPY

24 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda stored carbon, far-field emissions of CO2, emissions of non-CO2 GHG’s, and biophysical impacts on climate (NRC, 2015b). Other similar syntheses include Socolow et al., 2011, Tavoni and Socolow, 2013, and DOE, 2016. The 2015 National Academies report also recommended 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. This recommendation ultimately led to this current study. The specific charge to the committee is in Box 1.3. The sponsors of the study are the US Department of Energy, National Oceanographic and Atmospheric Administration, US Environmental Protection Agency, US 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. A related study on carbon use that also came out of the 2015 National Academies report on climate intervention is briefly described in Box 1.4. It should be noted that the committee was not asked to do a systematic review of all the literature related to NETs. National Academies studies depend on the expertise of its committee members to do its assessment and develop its conclusions and recommendations. Fortunately, because of the widespread interest in NETs, the committee had access to several recent and comprehensive reviews including: Minx et al., 2018, Fuss et al., 2018, and Nemet et al., 2018. NEGATIVE EMISSIONS TECHNOLOGIES In response item E of the task statement, 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: o 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”). o 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”). 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 sequestration3 (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 breakdown lignin (up to 25% of all biomass). 3 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. PREPUBLICATION COPY

Introduction 25 BOX 1.3 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 CDR approaches to be examined would include land management, accelerated weathering, bioenergy with capture, direct air capture, geologic sequestration, nearshore/coastal approaches, and other approaches deemed by the study committee to be of similar viability. BOX 1.4 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 Streams4 In recognition of the role that carbon dioxide removal and sequestration technologies can play in meeting greenhouse gas reduction goals, the National Academies convened an ad hoc committee to evaluate the state of science and viability of technologies for removing carbon dioxide from the atmosphere. The resulting report, Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration (NRC, 2015), recommended investments in research and development for technologies that remove carbon dioxide from the atmosphere. Building on this report, the National Academies convened two committees, one to identify research needs for carbon dioxide removal from the atmosphere and sequestration, 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 carbon dioxide removal and sequestration approaches, and defined the essential components of a research and development 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 carbon dioxide, methane and biogas as a feedstock into commercially valuable products. The 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. • Direct air capture (Chapter 5) - Chemical processes capture CO2 from ambient air and concentrate it, so that it can be injected into a storage reservoir. In some incarnations, the captured carbon dioxide may be reused in products. Capture and reuse in short-lived products, like 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, like 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 1.4. 4 See http://dels.nas.edu/Study-In-Progress/Developing-Research-Agenda-Utilization/DELS-BCST-16-05. PREPUBLICATION COPY

26 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 1.6 Negative Emissions Technologies considered in this report • Carbon mineralization (Chapter 6) - Accelerated “weathering”, in which carbon dioxide 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 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 relatively straightforward to describe to policy makers 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 task statement 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 C and D. 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 GtC, mostly in the form of bicarbonate (equivalent to 132,000 GtCO2). Once the fossil fuel age is over, almost all 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). There are promising technologies for oceanic sequestration, some with potentially limited environmental impact, and the capacity of the oceanic carbon reservoir is obviously enormous. Approaches that have been explored include increased biomass production and ocean alkalization. This committee was not formulated 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. The PREPUBLICATION COPY

Introduction 27 rationale for including nearshore/coastal with the terrestrial options, rather than with oceanic options, is that the nearshore/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 relationship between NETs and decarbonized power is addressed i (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), the report does not include critical mitigation options such as enhanced energy efficiency, renewable electricity, or reduced deforestation as 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 represent 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, policy makers 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. Additionally, a broad portfolio of technologies—including multiple NETs—offers increased resiliency to help manage risks of surprises from nature and mitigation actions. Furthermore, the possibility of irreversible consequences of temporary warming is another reason to develop NETs quickly so that they can more quickly bring down net anthropogenic emissions. Framework for Assessing Individual NETs Items A-D of the task statement specify the main components 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 a research and development program, with estimated costs and implementation (including monitoring and verification, institutional structures, and research management). NETs span a range of technological readiness; as such, the assessments and recommendations for the different options are highly heterogeneous. Nonetheless, Chapters 2-7 have a few elements in common. Each chapter focuses on two scales, the US and the globe, and on research designed to be US-funded. Each includes approach definitions and descriptions of the technology, impact potential for carbon dioxide 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 judgement 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 judgement 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 the practically achievable estimates were obtained. In general, the committee restricted its investigation to methods that have a global practically achievable potential of at least 1 GtCO2/year 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 leakage of carbon stored in terrestrial and coastal ecosystems or in saline aquifers, for example, may occur a century or more after initial sequestration. How long does sequestered carbon need to remain below ground or PREPUBLICATION COPY

28 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda locked up 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 1000-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. But in the absence of high-capacity NETs, economic calculations still need to 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 $/tCO2, 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: = . If Tr>>1, 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% and T is 100 years, then the present cost of effective permanence adds 20% 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 are capable of creating 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 carbon dioxide 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) 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 PREPUBLICATION COPY

Introduction 29 FIGURE 1.7. Illustration of coordination between research phases, technology readiness level (TRL), prototype scale, stage-gates, and institutions. 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 no program funding is needed for pilot and demonstration-scale prototypes, as there may be no eligible projects—i.e. no projects able to achieve the program metrics and thus not move on to the next stage of development. Synthesis The research agendas proposed in Chapters 2-7 are brought together in Chapter 8, into an integrated research proposal and a single list of research priorities. In addition, Chapter 8 examines national and global scale carbon dioxide removal and sequestration holistically, rather than one option at a time, to better understand 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. PREPUBLICATION COPY

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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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