As discussed in Chapter 2, achieving net-negative GHG emissions globally will require large-scale development of a portfolio of CDR systems. Additional considerations alongside carbon accounting will be required for each strategy, and many of these are fundamentally spatial: What land area can support a CDR system without competing with human activities (e.g., food production, settlements) and without disturbing natural habitats? Are construction materials available, and what do they cost? What are the social and environmental risks associated with each CDR system related to their location? Can the components of the CDR system be recycled or reused across deployments?

This chapter uses a geospatial approach to highlight global opportunities for siting biological and technological CDR systems that leverage available opportunities, but avoid competing with human activities or habitat conservation. For technological CDR systems, mapping both low-carbon energy resources and storage potential helps identify opportunities for co-location and minimizes transportation distances. In areas where several CDR systems are viable, deployment will involve complex decision-making processes that must include regional stakeholders, policymakers, and local communities. We attempt to provide more details on activities that have the potential for more expansive development. This section should be considered a coarse review. Any actual project would require a more thorough study to assess feasibility and to develop deployment plans.

Each year, approximately 30 percent of human-caused carbon emissions are absorbed into terrestrial ecosystems (Friedlingstein et al., 2019), including forests, soils, and other vegetation. As reviewed in Chapter 2, several human interventions have the potential to “enhance” these biological CDR systems, by either increasing the rate of carbon dioxide removal, extending the permanence of storage, or both. Here, we examine approaches to quantifying and evaluating the technical potential of biological carbon dioxide removal strategies, and their risks and constraints, in particular as they relate to spatial aspects of land use, ecology, and climate.

Interventions related to enhancing carbon dioxide removal by forests can take several forms, including reforestation, afforestation, agroforestry, avoided conversion, and improved forest management (IFM) (See Chapter 2 for definitions.) (Griscom et al., 2017; Anderson et al., 2017). In terms of their effect on the global carbon cycle, reforestation, afforestation, and agroforestry are more clearly forms of carbon dioxide removal in so far as they primarily drive new biomass growth, whereas avoided conversion combines of carbon dioxide removal and avoided emissions. Avoided conversion and IFM prevent emissions associated with deforestation, which reduces tree and soil carbon stocks. They may additionally result in continued carbon dioxide removal, depending on the specifics of the forest’s ecological dynamics (e.g., species, age).

Estimating the potential of any of these approaches begins with considering the location: Where is the project taking place? What do we know about the local ecosystem? And what are the potential interactions with the climate system? A key parameter is above-ground forest carbon. Typically, above-ground forest carbon is estimated by first estimating above-ground biomass, and then multiplying biomass by a conversion factor (approximately 0.5) reflecting the typical carbon concentration in the woody tissues of trees and plants, which vary in their elemental composition depending on the underlying compounds (e.g., lignins [~60 – 72% C], cellulose [44% C], and hemicelluloses [28 – 30% C]) (Martin et al., 2018). Plants grow biomass over time with rates and growth forms that vary by species, climate, soil, and other location-specific parameters. There is additional carbon below ground in roots and in soil organic matter. Efforts to quantify and map these dimensions spatially have thus far relied on a combination of ground observations, remote sensing, and mechanistic modeling. Remote sensing, particularly satellite imagery, is critical for spatially comprehensive large-scale observations, especially in remote areas. It can be used to assess potential forest-based interventions relative to current land use, track changes in forests over time, and interpolate ground measurements where they are unavailable (Hansen et al., 2013; Dubayah et al., 2020; Schimel et al., 2019; Badgley et al., 2017; Homer et al., 2020; Blackard et al., 2008; Spawn et al., 2020). Remote sensing must be validated and complemented with ground information, such as the Forest Inventory Analysis provided by the United States Forest Service, the RAINFOR network in the Amazon, or the African Tropical Rainforest Observation Network in Africa (Malhi et al., 2002; Tinkham et al., 2018; Hubau et al., 2020). These kinds of data, however, are not necessarily available globally, or in interoperable formats. Mechanistic simulation models of biomass growth are useful for comparing and integrating empirical measurements and linking forest carbon dynamics to larger-scale climate models (Fisher et al., 2018). Especially important is incorporating land-atmosphere interactions, because increased vegetation in some areas changes an area’s surface roughness, albedo, and evapotranspiration in ways that contribute to warming, thus eliminating or potentially negating the climate benefit of carbon dioxide removal (Laguë et al., 2019; Bright et al., 2015; Jones et al., 2013a; Jones et al., 2013b). Integrated satellite, ground, and model-based analysis will be critical to identify and prioritize opportunities for forest-based activities that increase carbon dioxide removal.

A key limiting factor for CDR involving forests is the permanence of storage, especially in a changing climate. Carbon stored in forests is sensitive to disturbances such as wildfire, drought, and insects, all of which are sensitive to the climate, and thus depend on spatial location and are likely to change over time (Anderegg et al., 2020; Seidl et al., 2017; Pugh et al., 2019; Giglio et al., 2013; Hicke et al., 2013). Fire in forests results in approximately 6.6 GtCO₂ of emissions per year (Chuvieco et al., 2016; van der Werf et al., 2017). The historical record of wildfire occurrences (which can be assessed through satellite imagery) has been modeled as a function of climate variables to describe spatial variability, and strongly suggests that risks will increase in the future, particularly in areas that become drier or have more extreme temperatures (Williams and Abatzoglou, 2016; Barbero et al., 2015; Moritz et al., 2012). Drought decreases productivity and carbon stocks through tree and plant mortality. As a particularly striking example, the California drought in 2011 − 2015 killed more than 140 million trees, resulting in a cumulative ecosystem carbon loss of ~600 Mt CO₂, approximately 34 percent of the state's total greenhouse gas emissions over that period (Sleeter et al., 2019; Anderegg et al., 2020). Recent progress has been made characterizing the physiology of drought-induced mortality, but spatially explicit datasets and predictive models remain an area of active research. Finally, anthropogenic disturbance is also a major risk factor from a CDR perspective, whether due to changes in land ownership, conversion of land to agricultural practice, or other factors that result in stored carbon being released back into the atmosphere. However, forests intensively managed to supply long-lived timber products can potentially contribute to carbon storage.

Some studies have tried to identify the potential scale and, in some cases, the spatial distribution of reforestation and other opportunities related to forest carbon (Figure 3.1) (Griscom et al., 2017; Bastin et al., 2019). At least one study (Bastin et al., 2019; Cook-Patton et al., 2020) inflated the potential of reforestation by a factor of ~3 – 10 by failing to account for critical dimensions around climate feedbacks, ecology, land use, and human activity more generally (Veldman et al., 2019; Skidmore et al., 2019; Lewis et al., 2019; Friedlingstein et al., 2019). Significant work remains to rigorously characterize both the potential and the risks of these approaches across space and time.

Land-based carbon dioxide removal strategies for agricultural lands and grasslands have the potential to store 2 − 6.6 GtCO₂eq/yr globally in the form of soil organic carbon (SOC) (Bossio et al., 2020; Zomer et al., 2017). Interventions include changes from conventional tillage to no tillage, applying soil amendments like biochar, adding cover crops, grazing improvements (e.g., reducing intensity and using legumes), and restoration of degraded grasslands. Soil carbon dioxide capture is appealing because there are agricultural practices that can, in principle, both increase soil and crop health, and thus productivity, and increase soil organic carbon. Additionally, SOC enhancement in existing agricultural landscapes, in contrast to several other terrestrial CDR strategies like reforestation or grassland restoration, does not lead to land-use competition.

Non-intervention-specific mapping of potential opportunities for carbon sequestration in agricultural landscapes requires spatial data on the extent of croplands and pasturelands (Zomer et al., 2017) as well as the degree to which the soil organic carbon has been depleted due to historic and current agricultural land use (Sanderman et al., 2017). Combining results from a statistical model that predicts current SOC stocks using climate, topography, geology, and land information with results from a spatially-explicit database of historical land use, Sanderman et al.(2017) constructed a global map of SOC change since the dawn of agriculture. Their map shows that SOC losses are attributed to both cropping and grazing activity. The midwestern United States, large swaths of Europe, and eastern and central China have lost significant absolute amounts of SOC, primarily due to cropping (Sanderman et al., 2017). Rangelands in Argentina, southern Africa, and Australia have experienced the greatest losses in terms of percentage of historic SOC (Sanderman et al., 2017). These hotspots of loss offer opportunities for implementing SOC restoration strategies, keeping in mind, however, that only half to two-thirds of depleted SOC can be recovered (Lal, 2004) through human interventions with currently developed strategies. 

Another approach to mapping SOC sequestration potential is to model the potential increase in SOC due to interventions as a sigmoidal (“S”-shaped) function, with parameters derived from fitting models to observational data (Zomer et al., 2017). This approach assumes that if soil is properly managed to increase SOC (with the method depending on the location and soil type), SOC will increase rapidly initially, but then decrease as SOC saturates and sequestration reaches an equilibrium. Zomer et al. (2017) apply these sigmoidal SOC recovery functions with parameters representing medium and high sequestration rates (Sommer and Bossio, 2014) to a gridded global map of current SOC content within agricultural land (Figure 3.2). The resulting maps show several hotspots for SOC sequestration potential − in particular, the midwestern U.S., south-central Canada, eastern Mexico, most of Europe, Ethiopia, Sudan, most of India, and several countries in Southeast Asia. 

Yet, no such studies mapping the potential or suitability of specific cropland CDR strategies exist (e.g., cover cropping, no-till, soil amendments). Spatially, modeling the SOC sequestration potential of these strategies would require a detailed soil map, cropping system map (e.g., US Department of Agriculture’s Cropland Data Layer, or CDL), and a tool that predicts SOC potential per parcel of land given average farm performance parameters (e.g., USDA’s COMET tool) based on strategy-specific literature on sequestration efficacy (e.g., crop species performance in certain agro-ecological regions) (Poeplau and Don, 2015).

There are no global mapping studies on grassland restoration opportunities. However, regional and country-level analyses provide several possible approaches for CDR mapping (Fargione et al., 2018; Winowiecki et al., 2018). Marginal farmland, abandoned farmland, and degraded land (due to overgrazing) are candidates for grassland restoration. For example, Fargione et al. (2018) mapped areas in the U.S. where cropland was abandoned to grassland between 2008 and 2012, estimated using remote sensing products such as the USDA CDL and the National Land Cover Database (Lark et al., 2015), and used those areas to estimate SOC restoration potential from a gridded soil map (Hengl et al., 2017). Other grassland SOC sequestration opportunities include strategic conversion of low-productivity (marginal) croplands into high-diversity, productive perennial grasses for bioenergy (Awasthi et al., 2017; Gelfand et al., 2013; Tilman et al., 2006). Combining the above approaches with maps of SOC debt produced by Sanderson et al. (2017) can help identify the most promising locations for grassland restoration or improved grazing for carbon sequestration.

Through managed grazing intensity, soil amendment (e.g., fertilizer or liming) is another strategy for storing or restoring SOC in grasslands (Eze et al., 2018; McSherry and Ritchie, 2013). While no maps of modeled SOC sequestration potential for pasture improvement strategies exist, a global meta-analysis of the response of SOC stock to various grassland management strategies shows that there are geographically-specific factors like climate that can help predict the response of grassland SOC to management interventions (Eze et al., 2018). Generally, more-intensive grazing leads to greater declines in SOC, but the negative effects of grazing on SOC are generally more significant in tropical and subtropical climates than in temperate climates. While heavy liming does partially offset grazing’s depletion of SOC, this strategy can succeed only with acidic soils (land leached by heavy and persistent rainfall) and is partially dependent on the location’s geology. However, the majority of the world’s grasslands are in climatic zones with lower mean average precipitation. Nitrogen fertilizer does increase SOC, particularly in climates with high mean average temperature and high mean average precipitation (Eze et al., 2018), but several studies have found that there are diminishing returns in carbon sequestration per unit of added nitrogen. Moreover, high-nitrogen fertilizer inputs could lead to nitrous oxide emissions, which have significantly higher greenhouse gas potency than carbon dioxide (See Supplement 4.1 in Chapter 4 and Chapter 1).

A few existing studies have estimated the economic potential or economic cost of the mapped suitable areas for agricultural and grassland carbon dioxide removal strategies, and they offer a more realistic estimate of realizable potential (Smith et al., 2008; Fargione et al., 2018; Griscom et al., 2017). However, social and economic conditions are likely to be the most significant determinants of realizable potential. 

Recent studies show that global peatland distribution by ecosystem and area is 83.3 percent boreal, 4 percent temperate, and 12.7 percent tropical (Leifeld and Menichetti, 2018). Using maps of global peatland distribution and maps of land use and climate, Leifeld and Menichetti (2018) estimated the potential GHG emissions if peatlands were to be fully degraded. For estimating current degree of degradation of peatlands, the study overlaid cropland areas with peatland areas and assumed that all peatlands used for cultivation are fully degraded or drained, since peatlands need to be drained before they can be cultivated (Leifeld and Menichetti, 2018). These estimates were validated using the area of degraded peatlands reported by each country (Joosten, 2010). Leifeld and Menichetti (2018) estimate the global carbon sequestration potential from peatland restoration ranges from 0.08 to 0.92 GtCO₂eq/yr (Leifeld and Menichetti, 2018), while Griscom et al. (2017) estimate a maximum global potential of 0.8 GtCO₂eq/yr with uncertainty in the upper range of about 2.4 GtCO₂eq/yr, based on expert elicitation. Opportunities for peatland restoration are globally distributed with hotspots in Southeast Asia (Indonesia), parts of Southern Africa, Northern Europe (United Kingdom, Sweden, Finland, Germany), South America (Brazilian Amazon, Venezuela, Bolivia), and the state of Alaska. However, several studies highlight the high uncertainty of peatland extent estimates globally and the need for improved mapping to guide restoration and mitigation efforts (Griscom et al., 2017; Leifeld and Menichetti, 2018; Page et al., 2011). 

Seagrass meadows are responsible for storing about 18 percent of the carbon taken up by ocean sediments globally, or about 176 − 410 MtCO₂ per year (Kennedy et al., 2010). These marine habitats have been lost at an annual rate of about 110 km² since 1980, and more than 29 percent of known historical seagrass beds have disappeared (Waycott et al., 2009). The median estimate of the carbon emitted by this habitat loss is about 150 MtCO₂eq per year (with a range of 50 − 330 MtCO₂eq) (Pendleton et al., 2012). Restoration of lost or degraded seagrass meadows is thus a significant carbon sequestration opportunity. While there is an understanding of broad distributional trends of historic and current seagrass extent, less than a quarter of seagrass beds have been mapped globally, and locations are typically point observations without meadow extent estimates (Short et al., 2016). These mapped locations along with inferred and modeled habitat studies suggest that seagrass is very widely distributed and found along coasts of every continent except Antarctica (Short et al., 2016). Seagrasses are particularly abundant off the coasts of Australia, North America (including the Caribbean Islands), and Southeast Asia. Restoration potential mapping, however, relies on a spatial understanding of where seagrass meadows have been lost. Waycott et al. (2009) performed a meta-analysis of seagrass studies to qualitatively identify potential hotspots for seagrass meadow loss. Known areas of loss include the southwestern coast of Australia, eastern Canada, the Gulf of Mexico, the Mediterranean Sea, and the Baltic Sea (Waycott et al., 2009). Seagrass extent in other regions of the world is not well mapped.

Given the vast area requirements for terrestrial CDR, many of the terrestrial strategies described in this chapter could exacerbate the growing degree of land use competition among food production, climate change mitigation strategies like solar and wind development, urbanization, and conservation. Given finite land resources, policymakers, land managers, and communities must weigh the benefits and challenges of land-intensive CDR strategies. For example, whether to reforest or manage existing forested lands for biomass energy (with or without carbon capture and storage) or for carbon sequestration remains controversial (Favero et al., 2020). Other studies have explored expanding or repurposing existing cropland for perennial herbaceous bioenergy crops like switchgrass and miscanthus. These decisions consider more than the technical suitability of a CDR strategy (e.g., technical assessments like Griscom et al., 2017) by accounting for the economics and policies to incentivize particular CDR strategies. Socioeconomic studies on the spatial planning or land use allocation of CDR strategies often use economic models driven by biophysical data that typically fall within the partial and general equilibrium modeling category of climate and land use modeling (Favero et al., 2020; Herrero et al., 2013; Michetti, 2012). 

These economic models are typically embedded within the structure of integrated assessment models (IAMs) for simulating land use and land use change due to changes in the agriculture and forestry sectors through the supply (constrained by biophysical potential) and demand for key commodities in each sector in response to commodity prices. These models attempt to simulate the land use and climate effects of certain policies, such as a bioenergy demand policy at the national level (Favero et al., 2020; Herrero et al., 2013; Schmitz et al., 2014). For example, Favero et al. (2020) used the global timber model (GTM) to characterize bioenergy policy impacts on carbon emissions, carbon sequestration, and natural forest ecosystem services. The authors found that some inefficient bioenergy policy designs, such as taxing carbon dioxide emissions from biofuels, can lead to the loss of natural forests and forest carbon stocks, whereas efficient policies can protect natural forests while increasing total forest carbon. 

Maximizing the CDR potential of engineered technologies requires co-locating the steps of the process as much as possible and using low-carbon energy. DAC plants can, in theory, be located anywhere, provided they have a source of energy (heat and/or electricity) and sufficient land. The DAC plants should thus be installed next to the energy production site if the source of heat is fixed. On account of the lower heat quality requirements associated with solid sorbent-based DAC approaches today, these plants can in some cases be sited at some industrial sites with sufficiently high-temperature waste heat that provides the necessary thermal energy - although these opportunities are limited. At other sites, waste heat at lower temperature may be available, which could be upgraded to high-enough temperatures using industrial heat pumps powered by electricity. This would substantially increase the options for DAC locations, at the cost of increased capital expense for these heat pumps. Ideally, the CO₂ sequestration site should also be co-located with the CO₂ capture site to minimize transportation costs and potential leakage through pipelines, thus maximizing the CDR potential. However, sequestering CO₂ near densely-populated areas might be risky and encounter strong public resistance (Wong-Parodi and Ray, 2009). Alternatively, captured CO₂ can be transported from its source or point of capture to CO₂ sequestration sites. For example, transportation is proposed for a maximum of 100 km for sequestration in sedimentary reservoirs. Offshore sequestration is also a possibility. 

Alternatively, in locations where large volumes of CO₂ could be captured but appropriate sequestration sites are distant, efforts can be combined to create transportation networks (Fry et al., 2017; Morbee et al., 2011). Increasing the pipeline length would require the addition of booster pumps but would not significantly increase the cost of transportation. For instance, increasing the length of a pipeline from 100 km to 200 km would add $2/tCO₂ to pipeline costs (Grant and Morgan., 2018). The pipeline transportation network might need to be more developed in some areas like Europe, where carbon hubs could be sited in various locations throughout the region and the CO₂ could be transported for sequestration in sedimentary formations below the North Sea.

This section also aims to highlight locations that minimally compete with human activities and avoid natural habitat loss. Maps show potential locations for low-carbon sources of energy such as photovoltaic (PV), concentrated solar (CSP) (see Box 3.1), and wind energy, which require large land areas on “converted lands” (Baruch-Mordo et al., 2019). Converted lands are lands already disturbed by human activities (e.g., human settlements, agricultural land, roads, and dams).

Our analysis also aims to avoid inhibiting the transition from high-carbon emissions to low-carbon electricity generation technologies. For instance, if a proposed PV plant can replace a coal-fired power plant, this use of solar electricity should be the priority, as opposed to powering a DAC plant with solar electricity next to a coal-fired power plant. This is the most efficient use of low-carbon electricity generation for reducing carbon dioxide emissions. Coal-fired plants are shown in this study to raise awareness for local priorities in the energy transition to low-carbon sources of energy and CDR.

DAC systems based on solid sorbents require an energy distribution of roughly 80 percent thermal and 20 percent electric (National Academies of Sciences, Engineering, and Medicine, 2019). Geothermal, CSP, nuclear power (Figures 3.3 and 3.4), and biomass all can provide both forms of energy. Other systems, such as electro-swing adsorption systems (Voskian and Hatton, 2019), need only electricity to run, which can be provided by solar PV and wind (Figure 3.3). To increase public acceptance, these technologies should not compete for space with urban areas, high-value regions for food production, biofuel feedstocks for difficult-to-decarbonize end uses, or natural ecosystems. PV, CSP, and onshore wind on converted lands that would have a low impact on the existing biosphere are shown in Figure 3.3 (Baruch-Mordo et al., 2019). Locations were selected based on renewable resource quality and technical suitability (i.e., slope, aspect, soil type, land cover type). Offshore wind potential maps exclude marine protected areas (Badger et al., 2015; Protected Planet, 2020). 

The energy generated by some renewable technologies such as solar and wind is temporally variable. Best-in-class solar power plants have a capacity factor as high as 35.2 percent, whereas best-in-class wind power plants have a capacity factor as high as 52 percent (Vimmerstedt et al., 2019). These types of variable renewable energy systems require alterations to ensure adequate compatibility within a technological CDR system. One option is to provide enough capacity at the plant for excess energy to be generated during operating hours and stored to operate the capture system continuously. This would be necessary for capture systems where repeated startup and shutdown can cause problems (i.e., solvent-based DAC, where the temperature of the calciner must reach 900ºC and the energy requirements are 80 percent thermal energy [National Academies of Sciences, Engineering, and Medicine, 2019]). This is also important for increasing cost-efficiency, as it ensures maximum utilization of capital-intensive capture systems. A second option is to overbuild the DAC component so it can capture the same amount of CO₂ but operate only during energy generating hours. This option is viable only for systems where repeated startup and shutdown is possible (i.e., some sorbent-based DAC and electrochemical DAC technologies) and where the DAC component features lower capital costs.

Opportunities for onshore wind, PV, and CSP are shown in 109 countries on converted lands, (already-transformed lands where the installation of these technologies at a large scale would have a low impact on natural habitats and human settlements) (Baruch-Mordo et al., 2019). Offshore wind locations were retrieved from the Global Wind Atlas repository (Badger et al., 2015), and protected marine areas were removed from the dataset (Protected Planet, 2020). Dark brown, dark red, and purple regions show the best opportunities for CSP, PV, and wind, respectively. Note that all CSP locations are also suitable for PV.

Figure 3.4 represents geothermal opportunities, with the aggregate geothermal potential at the country level shown for key countries (Geothermal Energy Association, 2015), along with the geographically-specific geothermal heat flow on land (Davies, 2013). Geothermal energy requires less land than other renewable energy technologies and can operate at high capacity factors, which means that it can operate continuously without the intermittency of wind and solar energy. Typical capacity factors for geothermal power plants range between 70 and 80 percent. The surface heat flow of the Earth is much higher along mid-oceanic ridges but is far less accessible when areas are 2,500 – 3,000 meters below sea level, with the exception of Iceland. Hence, only onshore surface heat flow is represented.

Using geothermal as a low-carbon energy source depends greatly on location and power plant technology, as some geothermal power plants can release amounts of CO₂ comparable to natural gas or even coal-fired power plants (Aksoy, 2014). The world average emissions factor for geothermal is estimated to be around 122 gCO₂/kWh, with the highest values in the Menderes and Gediz grabens (depressed areas of the Earth’s crust bordered by parallel faults) in Turkey reaching up to 1,300 gCO₂/kWh (Aksoy, 2014; Fridriksson et al., 2016). Geothermal power plants also release other more potent GHGs: hydrogen sulfide (about 3.0 wt%), methane (about 0.15 wt%), and ammonia (about 0.29 wt%) (Fridriksson et al., 2016). The release of GHGs by geothermal power plants depends on the type of plant. In a binary plant configuration, the fluid is never in contact with the atmosphere. It is pumped from the subsurface, the heat is recovered with a heat exchanger, and the fluid is injected back into the geothermal reservoir. Binary plants represent 15 percent of worldwide geothermal power plants (Geothermal Energy Association, 2015) and do not emit GHGs from geothermal fluids. However, most geothermal power plants are condensate plants (dry steam or flash plants), representing 84 percent of the worldwide installed capacity (Geothermal Energy Association, 2015). These plants are open systems that release GHGs from the fluid or steam, which originate from the host rock of the geothermal reservoir or the mantle bodies that warm up the reservoir. GHG emissions vary depending upon the location and management of the reservoir, and in some cases, reinjection of fluids depleted in GHGs can decrease GHG emissions, with time, at condensate plants (Fridriksson et al., 2016).

Different configurations exist for geothermal plants with various requirements for the temperature of the geothermal brine. Flash and dry steam power plants use geothermal fluid with temperatures higher than 180^oC, and binary power plants use temperatures in excess of 100^oC (U.S. Department of Energy [DOE], 2019). Flash plants use cooling towers before sending geothermal fluid back into the ground. Existing plants are unlikely to be retrofitted, but new geothermal power plants could include solid sorbent-based DAC modules in place of the cooling towers (DiPippo, 2005, 2004; Snyder et al., 2017), taking advantage of the remaining heat for CO₂ capture. The geothermal fluid is sent back underground to ensure the sustainability of the geothermal reservoir. Associating DAC with existing geothermal installations is the cheapest energy option, as a large share of the capital cost of geothermal energy is drilling the wells. Using DAC modules as a cooling step before using geothermal energy is another possibility for pairing DAC with existing geothermal installations, as electricity generation requires lower temperatures than that of the geothermal brine. Otherwise, geothermal wells could be drilled solely for the purpose of providing heat to a DAC plant, but the cost of capture would be significantly higher. The co-development of geothermal power plants and DAC for transitioning from fossil fuel-based energy production to renewable energy and capturing current and past CO₂ emissions is an attractive proposition (Baker et al., 2019; McQueen et al., 2020).

Another low-carbon source of energy is nuclear power. Nuclear reactors represented in Figure 3.4 are operational reactors whose current licenses expire after 2025 (other reactors that are not on the present map could see their license renewed beyond 2025), and reactors under construction (Global Energy Observatory, 2018; IAEA Power Reactor Information System, 2019). Given the dangers and controversies surrounding nuclear energy production, the goal here is not necessarily to encourage nuclear energy development, but rather to take advantage of the energy produced by existing facilities while they are still in operation (McQueen et al., 2020). Hence, they would act as a transitional energy source.

Hydropower is also a renewable source of energy, and proposed or under-construction dams are represented in Figure 3.4 (Zarfl et al., 2015). However, hydropower development is highly subject to social and environmental siting controversies, resulting in high cost overruns and long lead times, due to their well-documented impacts on terrestrial and freshwater ecosystems as well as human livelihoods.

As shown on Figures 3.3 and 3.4, energy sources for CDR are heterogeneously distributed. Generally speaking, the quality of solar energy and, to a lesser extent, wind energy is latitude-dependent. The highest latitudes for solar energy generation are 45^o for CSP and 60^o for PV, and their resource quality is highest close to the equator and decreases toward highest-latitude limits. In order of energy opportunity, optimal locations are in Africa, the Middle East, Australia, and South America. 

Conversely, the quality of wind energy is highest offshore and at higher latitudes due to higher wind speeds. In order of energy opportunity, promising locations include southern South America, New Zealand, the South Coast of Australia, and Northern Europe. More limited opportunities are found onshore at lower latitudes, the most promising being the Great Plains in the U.S.

High-quality geothermal resources are related to plate tectonics. Oceanic ridges and subduction zones disturb the surface heat flow and can trigger higher surface heat flow and higher temperatures in the subsurface over hundreds of kilometers from the tectonic plate edges depending on the local geologic context. Most of the world's opportunities lie within the vicinity of the Ring of Fire: the western U.S., Kamchatka (Russia), Japan, Taiwan, the Philippines, Papua New Guinea, and New Zealand. In order of energy opportunity, optimal locations are in Iceland, the Rift Valley in East Africa, Indonesia, Turkey, Western Europe, and Northern Africa.

Both nuclear and hydropower energy depend on large water supplies. Nuclear power is mostly generated in densely populated areas of developed countries (Europe, North America, and the east coast of Asia) on the coast, or on major rivers. Hydropower is typically developed in mountainous areas because it requires water reservoirs or large elevation gradients between the source of water and the hydroelectric turbine. Most dams under construction are in the Himalayas and South America, regions with high terrestrial and aquatic biodiversity.

Very few countries could develop all six low-carbon energy generation options in this chapter (solar CSP, solar PV, wind, geothermal, nuclear, and hydropower), but most countries have the potential to develop at least one of them. Locations with the least potential are northern Canada and northern Asia, but they are also places with low population density.

The biosphere, soils, and the hydrosphere currently store large amounts of CO₂. Section 3.2 detailed the potential opportunities and risks of storage in the biosphere. The lithosphere has the largest storage potential with the lowest chance of CO₂ re-release to the atmosphere in sedimentary basins or mafic and ultramafic rocks. The assignment of a technological readiness level (TRL) to a given technology is a method of assigning its maturity. This is described in greater detail in Figure 4.4 of Chapter 4. The sequestration of CO₂ in deep sedimentary formations containing saline aquifers or depleted oil and gas reservoirs possesses the highest TRL and has the capacity to sequester past, present, and future emissions permanently (Szulczewski et al., 2012). Even with a low TRL, mafic and ultramafic rocks are promising avenues for CO₂ sequestration, as carbon mineralization occurs much more quickly in these rocks.

This subsection focuses on potential resources for CO₂ sequestration. The geological formations proposed are thus potentially usable for the sequestration of CO₂, without making any statement on their technical, economic, legal, or socioeconomic feasibility. For any of these resources, local studies will be required to more precisely assess their capacity for CO₂ sequestration. This information is not exhaustive; valuable data is scattered across the literature and other opportunities may exist in addition to those presented here.

No global database of sedimentary formations suitable for CO₂ sequestration exists today. There are local studies with variable degrees of certainty and granularity. In Europe, the CO2StoP study was completed in 2014 for the European Commission (Poulsen et al., 2014). In the United States, the U.S. Geological Survey (USGS) issued a national assessment of geologic carbon dioxide storage in 2013 (USGS, 2013), and the Gulf Coast Carbon Center (GCCC) features a database that highlights sequestration of greenhouse gases in brine (Hovorka et al., 2012).

At the global level, Bradshaw and Dance (2005) issued a map of a high-level estimate of the prospects of sedimentary basins for CO₂ sequestration, later used by the IPCC (Metz et al., 2005). The authors classified the sedimentary basins into three types: 1) world-class petroleum basins: basins flagged as “high priority” or “frontier” basins by the USGS World Petroleum Assessment, 2) prospective basins: small petroleum basins and non-highly-deformed sedimentary basins (basins that have endured tectonic activity and have broken-up or irregular rock layers), and 3) non-prospective basins: highly deformed sedimentary basins and other geological provinces, including fold belts, metamorphic, and igneous rocks.

Global sedimentary basin maps are available and provide other characteristics such as type of basin, how well they are explored, and the maximum thickness of basin sediments (CGG Robertson, 2019). More granular data about the thickness of the sediments were gathered in one global dataset by Laske et al. (2013) (Exxon Production Research Company, 1985; Laske et al., 2013). To maintain a supercritical state, which reduces the risks of leakage, CO₂ needs to be sequestered at pressures greater than 73.8 bars, corresponding to geostatic pressures occurring deeper than 800 meters. In order to ensure safe injection and trapping of CO₂, the threshold of 1,000 meters is preferred. Combining these two datasets might help select basins with sediment thickness greater than 1,000 meters and help identify areas within these basins that are deeper than 1,000 meters. This combined information provides a rough guide to areas that can be explored for future CO₂ sequestration projects.

Additional information helps narrow the choices for suitable sedimentary formations. It is important to ensure that large amounts of CO₂ can be injected in formations and stay trapped in the subsurface pore space. This requires pairing permeable geological formations with a regional seal, which is a caprock of the formation, or barrier, that is impermeable to CO₂. One rock that has the appropriate seal properties is shale, which has low permeability to both CO₂ and hydrocarbons. The presence of a seal is critical in identifying an appropriate geologic sequestration site. Additionally, the presence of major oil and gas fields is a good indicator of a large reservoir and a reliable caprock since the same mechanisms that have reliably stored oil and gas for millions of years will be storing CO₂ in a dedicated sequestration project. In addition, regulatory frameworks and accessibility might define the feasibility of a CO₂ sequestration project. The Global CCS Institute (Consoli, 2016; Consoli and Wildgust, 2017) used these parameters to investigate prospective and potential basins in major countries across the world. The Institute also studied the capacity of prospective basins, indicators for storage readiness, and the legal and regulatory framework of each country. This study does not consider some parameters, including injectivity, well placement, regulations, and commercial drivers for CO₂ sequestration. Therefore, the authors define a prospective sedimentary basin as one suitable to sequester human-caused CO₂ based on current knowledge and data. In most cases, a prospective basin has published CO₂ sequestration assessments. In those nations that have not completed a CO₂ sequestration assessment, a basin could also be defined as prospective if it hosts major gas fields or is known to have suitable geology based on existing data.

Regional studies can provide more detailed information in specific areas, and multi-country assessments are of great interest as they aim to provide highly detailed, homogeneous data across large geographic areas. Here we present one multi-country study in Europe (CO2StoP) and discuss two datasets for the U.S. (Hovorka et al., 2012; Poulsen et al., 2014; U.S. Geological Survey (USGS), 2013). Other local studies exist, including the GEODISC program in Australia (Bradshaw and Rigg, 2001).

CO2StoP is a regional study that was conducted mainly in the European Union and some other European countries as a collaboration of volunteer countries (Poulsen et al., 2014). In this study, a reservoir formation is defined as a “mappable body of rock that is continuous in the subsurface and which is both porous and permeable” and a storage unit as “a part of a reservoir formation that is at depths greater than 800 meters and which is covered by an effective cap rock.” Each storage unit may contain one or more daughter units, which are defined as “structural of stratigraphic traps which have the potential to immobilize CO₂ within them” (e.g., domes in deep saline aquifers sealed by caprocks or proven oil and gas fields). For each of the assessment units, the study provides general information (unit type [e.g., whether the reservoir is a saline aquifer or a hydrocarbon field], lithology, geographic area, country, geological basin) and specific information for pore-volume estimation (area, average thickness, average area of net sand, average ratio of net-versus-gross in the vertical direction, and average porosity). This project in particular has developed its own tool to calculate the capacity of the storage and daughter units and uses a geographical information system (GIS) for their localization. The geology of Europe is diverse, and there are sedimentary basins of various sizes in every country. Like the Global CCS Institute study (Consoli, 2016; Consoli and Wildgust, 2017) at the world level, this study shows that countries investigating their territory for sedimentary basins suitable for CO₂ sequestration find several locations worth investigating further for potential CO₂ injections in subsurface pore space.

The 2013 National Assessment of Geologic Carbon Dioxide Storage Resources provides geologic storage information down to the storage assessment unit (SAU) level, which is defined as a “mappable volume of rock that consists of a porous reservoir.” For each SAU, the list of parameters includes: 

• Area, thickness, depth from surface of the SAU, and density of CO₂  at depth;

• Thickness, porosity, and permeability of the net porous interval, which is the part of the SAU that contains an appropriate lithology with sufficient porosity to store CO₂; and

• Trapping pore volume and efficiencies of trapping mechanisms.

Each parameter has a minimum, maximum, and likely value. These parameters can be used to identify the best SAUs in a given area and calculate other indicators, like the injectivity of CO₂ in a given SAU.

In the U.S., the Gulf Coast Carbon Center (GCCC) brine database focuses on deep saline aquifers (Hovorka et al., 2012). It contains most of the following parameters for each basin, in some cases associated with a map showing their geographical variability:

• Depth of the formation

• Permeability or hydraulic conductivity 

• Thickness (as well as the net sand thickness and the thickness of the seal) 

• The percentage of impermeable shale in the aquifer

• Rock/water reaction

• Hydrocarbon production

• Low direction

• CO₂ solubility in its brine (temperature, pressure, and water salinity at depth)

• Porosity 

• Water chemistry

• Rock mineralogy

Both U.S. datasets, from the USGS and the GCCC, offer high-level information on the potential physical reaction of the basin to CO₂ injections (capacity, pressure build-up) and, to some extent, the efficiency of trapping mechanisms likely to occur in the reservoir. The GCCC also provides information (rock/water reaction, water chemistry, and rock mineralogy) that can help predict the response of the reservoir rock to CO₂ input. CO₂ can indeed trigger dissolution and precipitation in the reservoir and alter the porosity and the permeability of the formation.

The GCCC also created the MATLAB module Enhanced Analytical Simulation Tool for CO₂ storage capacity estimation and uncertainty quantification (EASiTool), which estimates the total CO₂ that can be injected into a sedimentary formation over a given period of time through a given number of wells, accounting for the formation’s parameters. This information is vital to evaluate a CO₂ sequestration project’s potential (Ganjdanesh and Hosseini, 2018, 2017; Hosseini et al., 2006).

All of the studies presented above have been used to create a global dataset of prospective basins for CO₂ sequestration (Figure 3.5). Mainly based on the study from Consoli (2016), this map shows potential opportunities in countries that are large emitters of GHGs and offers a high-level understanding of how much information we have from each dataset and the suitability of each formation.

The GCCC and USGS datasets provide great detail. This map uses basin shapes from the USGS dataset, and basins that have injectivities lower than 0.25 MtCO₂/yr (Baik et al., 2018) were discarded because they are not suitable for CO₂ injection and sequestration. The CO2StoP study in Europe has a high level of knowledge and provides a GIS file that was used for this map. Studies from Consoli (2016) and Consoli & Wildgust (2017) were used to select basins in the Robertson Basins & Plays GIS shapefile, and other sources were used when necessary (Geoscience Australia, 2017; Pitman et al., 2012; Tartarello et al., 2017; Campbell et al., 2015). Consoli (2016) ranked the countries, using four categories: full, moderate, limited, and very limited knowledge (Consoli, 2016). This information is reflected in different colors on the map.  

Offshore basins offer attractive opportunities for CO₂ sequestration (Offshore Storage Technologies Task Force, 2015; Ringrose and Meckel, 2019) due to their depth and remoteness. Offshore basins identified as highly prospective by Bradshaw and Dance (2005) were selected in the Robertson Basins & Plays GIS shapefile (Bradshaw and Dance, 2005; CGG Robertson, 2019) and displayed in this map. All datasets (detailed above) were combined with the map of sediment thickness by Laske et al. (2013) and resampled into a 3 km x 3 km raster. This gives a high-level estimate of the areas that might be too shallow for CO₂ injections (< 1,000 m). 

This map (Figure 3.5) shows that in most countries that are large GHG emitters, numerous opportunities exist for future onshore or offshore CO₂ sequestration. Operating, future, and completed projects for CO₂ sequestration are shown on the map, along with current CO₂-EOR locations in the U.S (Global CCS Institute, 2019). Most current and future activity is in Australia, East Asia, Europe, and North America.

Attempts have been made to estimate global and regional capacity potential for CO₂ sequestration. Lower estimates show that the global resource available is roughly 7,000 GtCO₂, over three times as much capacity as the total GHG emitted since the beginning of the Industrial Revolution (~ 2,035 +/- 205 GtCO₂ [Le Quéré et al., 2015]). The amount of CO₂ that has to be sequestered by 2050 to reach the 2^o C climate goal is estimated to be 10 GtCO₂/yr (National Academies of Sciences, Engineering, and Medicine [NASEM], 2019; United Nations Environment Programme [UNEP], 2017), requiring between 10,000 and 14,000 injection wells (Ringrose and Meckel, 2019). Beyond the CO₂ sequestration capacity of each country or region, Pozo et al. investigated the question of equity between countries for CO₂ sequestration (Pozo et al., 2020). Here we present two approaches for capacity estimates: a top-down approach using global datasets to provide a uniform dataset and estimate local resources (Kearns et al., 2017) and a bottom-up approach aggregating numerous local studies to identify local resources and ultimately provide a global dataset (Consoli and Wildgust, 2017). The areas studied to create the capacity estimates are presented in Table 3.1.

Kearns et al. used a holistic approach to build a homogeneous worldwide dataset (Kearns et al., 2017). Their methodology consists of a modified version of a method developed by the International Energy Agency Greenhouse Gas R&D Programme (IEAGHG). The IEAGHG assumes that about half of global sedimentary basins’ areas are covered by an adequate seal. The low bound assumes a closed system, where pressure is unable to dissipate, whereas the high bound assumes an open system, where the pressure build-up is negligible. This leads to the sequestration of 0.037 GtCO₂ per 1,000 km³ of sedimentary basin for the lower bound and 0.26 GtCO₂ per 1,000 km³ of rock for the higher bound. Contrary to the IEAGHG method, which assumes a fixed thickness of sedimentary layers, Kearns et al. used the sediment thickness from Laske et al. (2013) to calculate the CO₂ sequestration potential (Laske et al., 2013). The authors distinguished between onshore, offshore technical, and offshore practical storage as some offshore basins might be difficult to access. Offshore practical resources are located within 200 miles of land masses larger than 10,000 km² and exclude waters deeper than 300 meters and latitudes higher than the polar circles. These criteria significantly reduce the offshore capacity of coasts with narrow and steep continental shelves (e.g., Africa and Japan), and the capacity of northern countries (e.g., Russia and Canada). Total capacity of onshore and practical offshore sedimentary formations ranges from 8,000 to 55,000 GtCO₂. While this approach primarily estimates global and regional capacity ranges, it also identifies specific sedimentary basins for carbon dioxide storage.

Consoli & Wildgust took a different approach to identify prospective basins. They studied assessments from countries that are major GHG emitters (Consoli and Wildgust, 2017). As stated above, to be considered prospective, sedimentary basins should contain permeable saline formations that are paired with a regional seal or have existing oil and gas fields. They also need to be reachable by trucks and equipment (or in the case of marine basins, ships). And finally, they need to be available for use in accordance with the laws and regulations of the country in which they are located. Regional assessments tend to differ in methodology, quality, and physical and chemical parameters provided, which makes direct comparisons difficult. In that context, multi-national surveys are key to harmonizing the methodology and data available. The authors also indicated that their methodology is not appropriate to estimate total resources for CO₂ sequestration, as a more detailed, site-scale study of prospective basins is needed to estimate actual resources before starting any CO₂ sequestration project. 

Despite all these challenges and the variability of geology from country to country, substantial CO₂ sequestration resources are available in most regions of the world. Most of the countries that published regional assessments have identified sufficient resources to support multiple sequestration projects. This approach helps identify prospective basins worth further investigation.

In summary, sedimentary formations potentially suitable for CO₂ sequestration are distributed around the world. Detailed local field studies and careful subsurface modeling are necessary to select appropriate injection sites. Together with technical or economical feasibility, factors such as population density, public acceptance, and local regulations are essential to ensure the success of injection projects. Also, estimated capacities (Table 3.1) suggest that the CO₂ sequestration resource should not be a limiting factor. In order of the most collective pollution since the start of the Industrial Revolution, the top emitters of CO₂ are North America, Europe, China, and Australia. Figure 3.5 shows these regions all have potential CO₂ sequestration opportunities that might require long-distance CO₂ transportation, depending on the co-location of CO₂ sources and potential sinks. This could be partly overcome by using other rock types for CO₂ sinks.

Mafic (basalts) and ultramafic (peridotites and serpentinites) rocks also have the potential to sequester CO₂, as discussed in Section 2.1. The technology has a lower technology readiness level (TRL) than CO₂ injection in sedimentary formations, but these rocks are much more reactive with CO₂ and would present interesting alternatives to injection in locations where sedimentary basins are nonexistent, too shallow, or unable to permanently store CO₂ at depth. These rocks are found around the globe but are not homogeneously distributed. Basalts are associated with current or past volcanism, while ultramafic rocks are located in present or past mountain ranges. In addition, basalts, peridotites, and serpentinites are part of the oceanic floor, where they are widely abundant, but they are not considered accessible for CDR due to their significant depths. (The ocean floor is an average of 5,000 meters below sea level.) The map (Figure 3.6) presents ultramafic rocks on land, basalts on land, and basalts offshore, excluding marine protected areas and exclusive economic zones (200 nautical miles, or about 370 kilometers, from shore) (Coleman and Irwin, 1974; Flanders Marine Institute, 2019; Hartmann and Moosdorf, 2012; Johansson et al., 2018; Kelemen, 1998; McGrail et al, 2017; Protected Planet, 2020; Sigfússon et al, 2015; Whittaker et al., 2015).

A variety of alkaline waste materials from the mining and construction and demolition industries possess significant potential to sequester large volumes of CO₂ (Renforth, 2019). Recent research has identified at least 12 types of waste material that can capture CO₂ through mineralization or enhanced weathering 1 (Renforth, 2019). These alkaline materials can be broken down into their constituent components based upon their chemical composition, as indicated in Table 3.2. The functional unit of each alkalinity source is indicated in the first column, followed by the current estimated lifecycle emissions in kgCO₂. The method for calculating mineral carbonation and enhanced weathering potential (defined in chapter 2) is outlined in Renforth (2019).

It is possible to capture CO₂ from ash residue as a byproduct of gasification within the BECCS negative-emission system, with a potential of 186.2 ± 126.1 kgCO₂/t_ash (Vassilev et al., 2013). This enhances the potential impact of BECCS beyond standalone estimates of emissions reduction. The biomass composition from the four samples in Table 3.2 was based on a combination of agricultural residue; herbaceous, woody, and animal-based biomass; and algae (Renforth, 2019). The average ash composition of the biomass is approximately 6.9 ± 1.1 wt% (Vassilev et al., 2013; Mckendry et al., 2002; de jong et al., 2014; Demirbas, 2014; Zanzi, 2001). In terms of energy content, the ash has an average higher heating value (includes the latent heat of vaporization of water) of 19.1 ± 0.3 GJ/t_ash. 

Cement also possesses the latent ability to sequester CO₂, with a high carbonation and enhanced weathering potential of 510 and 773 kgCO₂/t_cement, respectively (Vanoss and Padovani, 2003). There is potentially even higher activity during building demolition, particularly when cement has been mixed with concrete, as the particle size has been reduced and the material is exposed to the elements (Washbourne et al., 2015). This enhanced sequestration rate is equal to approximately 85 tCO₂/ha/yr. Some cement formulations may include a mixture of clinker and gypsum at a ratio of 9:1. Clinker in turn is created by heating limestone in a kiln to ~1500 °C, which also produces cement kiln dust (CKD) as a byproduct. CKD has the ability to sequester CO₂ up to 330 ± 11.6 kgCO₂/t_CKD. Within the U.S., a small number of cement plants produce CKD at a CKD-to-clinker ratio of 1:10, with an average CO₂ sequestration potential of 115 ± 17 kgCO₂/t_clinker. 

More than 300 million tonnes of lime is produced globally every year (USGS, 2018), with an estimated carbon mineralization potential of 776.9 ± 12.9 kgCO₂/t_lime and enhanced weathering ratio of 1165 ± 19.4 kgCO₂/t_lime. Future annual carbon dioxide removal ratios for lime are predicted to be between 60 and 143 MtCO₂/yr by 2100 (Renforth, 2019). Lime is a versatile material and, between 1975 and 2003 within the U.S. and E.U., it was used in a variety of industries (Farell, 2009; EuLA, 2014). More specifically, of the 300 million tonnes of annual lime production, roughly 41 ± 1% was used in steel production, 27 ± 0.6%  was used in chemical production, and 22 ± 0.9% was used in environmental applications such as acid neutralization. As lime is highly reactive, it may recarbonate over time. Renforth (2019) suggested that 20 percent of lime use is intrinsically linked to reaction with CO₂. For example, 8.5 percent of lime production reacts with the CO₂ from biomass in the recovery of sodium hydroxide from the Kraft process in the paper manufacturing industry (Renforth, 2019). Lime is also used to dilute extremely low-pH acids, such as sulfuric acid in the drainage of mine wastes. 

Waste from metal mining offers great potential to sequester CO₂, mainly due to the large quantities of ultrabasic rocks within the host material. Significant quantities of such waste are produced in the metal mining process (Renforth, 2019). Most carbon mineralization research has targeted the host rocks of rare materials and metals, such as ultramafic rocks hosting platinum group elements (PGEs), 2, before kimberlite pipes hosting diamonds (Mervine et al., 2018), and more common materials such as nickel. Both PGE and diamonds offer potential carbon dioxide removal of 100-200 MtCO₂/yr. Nickel is split into two categories of tailings, Ni-laterite and Ni-sulphide. Laterite has a carbon sequestration potential of 251 ± 26.7 and 377.2 ± 40.1 kgCO₂/t_laterite for mineral carbonation and enhanced weathering, respectively, while sulphide offers potential savings of 367.5 ± 7.6 and 555.3 ± 11.7 kgCO₂/t_sulphide. Carbonating mine tailings can have other benefits, such as providing physical stability to tailings (Vanderzee et al., 2018), dust reduction, immobilization of toxic metals (Hamilton et al., 2016), and neutralizing hazardous materials like asbestos (Assima et al., 2013). Asbestos and brucite have high carbonation rates, as carbonation has been detected in their drainage waters. However, it is unclear if the entire formation of rock can actually weather at a meaningful reaction rate (Harrison et al, 2013).

The aluminum industry produces on average approximately 3.45 ± 0.04 tonnes of red mud (also known as bauxite residues) for every tonne of aluminum produced (Bertram et al., 2017). According to Power et al. (2011), around 120 Mt of red mud is produced globally, with a current stock of 3 Gt. The substance has a high carbon dioxide sequestration capacity of 44  – 66 kgCO₂/t_{red mud}. The carbonation and enhanced weathering potential were based on chemical composition alone and equate to 46.8 ± 8.1 kgCO₂/t_{red mud} and 128.3 ± 18.1 kgCO₂/t_{red mud}, respectively.

Roughly 500 Mt of slag is manufactured every year (USGS, 2016). For every tonne of steel produced, an additional 185 ± 5 and 117 ± 6 kg/t_steel of blast furnace and steel slag are formed as a byproduct of the crude steel production process, respectively (USGS, 2018). Pig iron production in blast furnaces may be reduced due to movement toward a circular or regenerative economy over the next decades (Neelis and Patel, 2006), although the production of steel from scrap has declined slightly since the 1970s (from 45 to 35 percent). It is difficult to predict how current stocks (25 Gt [Krausmann et al., 2017]) will be used over the next century. It is estimated that the total carbonation potential of slag will equate to 320 – 870 MtCO₂/yr, with an enhanced weathering potential of 480 – 1,300 MtCO₂/yr (Renforth, 2019). 

Figure 3.7 illustrates the location of the most notable waste alkalinity feedstocks. In particular, the U.S. has high quantities of fly ash, cement kiln dust, and steel slag as a legacy of its high industrial output.

Combining low-carbon energy resources (described in Section 3.3.2) with CO₂ capture (described in Section 2) and sequestration sites (described in Section 3.3.3) assists in identifying potentially feasible locations for technological CDR systems. These proposed systems are defined as “potential” (see Section 1.3 for definitions of the terminology), as the characteristics of each location are unique, and ultimately a life-cycle analysis (LCA) would be necessary to assess the CDR potential of a specific system in a given location (Section 4). Before launching any project, it is essential to assess the feasibility of building a low-carbon energy plant (or pairing the DAC plant with an existing source of low-carbon energy), and the potential for geologic sequestration in that location. Below, we propose some examples of technological CDR systems and the potentially relevant locations for each. We focus on geologic sequestration in sedimentary basins as this technology has a higher TRL. This exercise can be considered a first step toward more thorough local analysis.

Technological CDR approaches available today operate with heat and electricity. All of the low-carbon sources of thermal energy identified above (geothermal, CSP, nuclear, and biomass) currently convert most of their thermal energy into electricity. For energy plants dedicated to supplying a DAC plant, the share of thermal energy converted into electricity could be reduced to match the needs of this DAC plant, i.e., 80 percent thermal, 20 percent electric. (Actual proportions will depend on the conversion efficiency from heat to electricity). Since most of the heat provided is too low for solvent-based DAC but matches sorbent-based DAC requirements (~100^oC), the section below focuses on this latter technology. If only electricity is available, the heat for sorbent-based DAC could be provided by industrial heat pumps. In the future, DAC plants that need only electric energy, such as the electro-swing adsorption systems developed by Voskian and Hatton (2019), might enter the market. Opportunities for electric-powered DAC are also shown below.

As highlighted in Section 3.3.2, potential geothermal energy, existing nuclear power plants, and potential hydropower facilities are unevenly distributed around the globe, according to the specificities of each of these resources. Figure 3.8 identifies opportunities for pairing geothermal, nuclear, and hydropower facilities with DAC within 100 kilometers of sedimentary basins. Pairing sorbent-based DAC with geothermal, nuclear, or hydropower would require using the geothermal heat or the waste heat from the nuclear plant and part of the electricity generated at the facility, particularly in the case of hydropower, where the dam would have to provide the electricity necessary for heat generation via industrial heat pumps. The plant’s carbon dioxide-capturing potential will depend on the pairing configuration: the type of plant for geothermal facilities, and the share of energy allocated to DAC for all above-cited sources of energy. A case study in the U.S. details calculations of carbon dioxide capture potential when a sorbent-based DAC plant is paired with geothermal or nuclear energy (McQueen et al., 2020).

Most of the nuclear power plants operating or under construction in East Asia, western Europe, and the U.S. are near sedimentary basins, and thus show potential for a “nuclear energy–sorbent-based DAC–sedimentary reservoirs” CDR system. Co-location of sedimentary basins and hydropower facilities currently under construction is variable. Regions with the best opportunities are Europe, Turkey, the Sichuan province in China, Southeast Asia, and along the coast of Brazil, but most hydropower facilities are not co-located with sedimentary basins. In the Himalayas, South America, and East Africa, hydropower might be better co-located with basalt or ultramafic rock formations.

Some geothermal plants are located over sedimentary basins, which might look ideal for the proposed potential CDR system. However, high-temperature geothermal resources tend to be incompatible with high-quality sedimentary reservoirs because the heat reduces the porosity and permeability of sedimentary rocks and thus their ability to sequester CO₂. This geothermal energy–sorbent-based DAC–CO₂ sequestration potential CDR system would therefore likely require transporting CO₂ long distances from the capture location to injection locations in sedimentary basins. Alternatively, basalt and ultramafic rocks have high CO₂ capture and mineralization potential and are better co-located with high-quality geothermal resources.

A California case study detailed opportunities for pairing DAC plants with geothermal energy onsite and transporting CO₂ to injection points in geologic formations. Geothermal energy is widely available in California, with required temperatures of over 100^o C necessary to regenerate the solid sorbents of a DAC plant. Suitable areas are located near the Salton Sea, the Geysers, Mammoth Lake, and in the northeast region of the state. Geothermal opportunities (geothermal wells with available data for warm-water flow rate and temperatures) have been shown in California along with sequestration options by Baker et al. (2019). While geological sequestration has a 50+ year safety record, CO₂ is unlikely to be injected under the city of Los Angeles due to a lack of public acceptance in such a large population center. So the Sacramento and San Joaquin basins in the Central Valley are more likely to be used for CO₂ sequestration. The geothermal area of the Geysers is thus ideally placed for setting up a geothermal–sorbent-based DAC–sedimentary reservoir system. Another geothermal hot spot is in Imperial County, south of the Salton Sea. The closest sedimentary basins are in Arizona, thus requiring collaboration between states.

Most global geothermal opportunities are actually closer to basalts or ultramafic rocks, but in California none of these formations has been assessed for CO₂ sequestration. The CarbFix pilot project in Iceland has demonstrated success with this option, where CO₂ is injected in basalt and permanently mineralized in less than a year (Aradóttir et al., 2011; Gislason et al., 2010; Gunnarsson et al., 2018; Matter et al., 2016; Snæbjörnsdóttir et al., 2017). The project developed a method for injecting CO₂ and H₂S from the geothermal plant (10,000 tCO₂/yr and 6,000 tH₂S/yr) and is now successfully injecting CO₂ captured from air as well (about 50 tCO₂/yr). This project shows that geothermal power plants have the potential to be carbon-neutral or even negative. This might help expand use of geothermal energy, which is renewable but has been criticized for its GHG emissions (as described in Section 3.3.2).

Since most geothermal plants are located over or close to basalts, the CarbFix project sets the precedent for the development of a geothermal energy–sorbent-based DAC–CO₂ sequestration in basalts CDR system. The method requires large amounts of water and has been successfully tested on fresh water at pilot scale (Aradottir et al., 2011; Gislason., 2010; Matter et al., 2011, 2016; Snæbjörnsdóttir et al., 2017; Gunnarson et al., 2018) and tested in the laboratory (Wolff-Boenisch et al., 2011). Salt water has a different pH than freshwater, which might affect the carbon mineralization of CO₂, but in areas where fresh water is not readily available, seawater could be an adequate alternative (Snæbjörnsdóttir et al., 2020). If seawater is proven usable in that system, it could, for instance, be implemented at a small scale on volcanic islands in the Pacific Ocean (Hawaii already has a geothermal power plant) and would have the combined advantage of reducing the plants’ dependence on energy imports and implementing CCS. When CO₂ sequestration in basalts reaches the commercial scale, large-scale projects could take place in Iceland and Washington state (U.S.), where pilot projects are already operating. Local opportunities for CO₂ sequestration in basalts coupled with geothermal energy also exist in most countries around the Pacific Ocean.

Concentrated solar power (CSP) technology focuses heat from the sun’s rays in order to create a focused beam of energy, driving a generator producing steam, which in turn generates electricity. The thermal energy produced by CSP during the day can be stored for four to six hours in molten salts. This provides a shorter night gap compared to PV technologies, and CSP-powered plants can even run continuously (Ortega et al., 2008). As 80 percent of the total energy requirements of today’s leading DAC technologies are thermal (Beuttler et al., 2019), this technology could be a valuable source of energy for DAC. Some of the heat could be used directly for regenerating the solid sorbent of a DAC plant, and the remaining heat for electricity production. Here we examine the overlap of CSP with renewable electricity options such as PV and wind for the additional electricity generation required. Figures 3.9 to 3.12 show the co-location of wind and PV with CSP on converted lands and geological reservoirs for CO₂ sequestration (within 100 kilometers for sedimentary reservoirs, as explained in the introduction of this section).

CSP siting is restricted by operating conditions that require it to be within 45^o of the equator. The combination that could be largely deployed on converted lands is a CSP + PV–sorbent-based DAC–sedimentary reservoirs CDR system. Regions around the world with the highest potential for such siting on converted lands are southern Africa, the Arabian Peninsula, the western U.S., Australia, Morocco, and Algeria (Figure 3.9). In Mexico and East Africa’s Rift Valley basalt is predominant for CO₂ sequestration, and it could also be an alternative in countries where sedimentary reservoirs appear to be the best option. However, no assessment of available capacity has been made for basalts. In the Arabian Peninsula, Oman hosts the largest ophiolite (serpentine rock formation) in the world, which could be another potential option for CO₂ sequestration in ultramafic rocks (Figure 3.11).

The potential for CSP is generally higher at lower latitudes, whereas the potential for onshore wind generation increases at higher latitudes. These two technologies are thus rather incompatible, except in the Great Plains region of the United States, which has one of the highest global concentrations of onshore wind opportunities on converted land. Here, opportunities are restricted to converted land to outline opportunities on lands already disturbed by human activities. Siting DAC plants in these locations would have two advantages: avoiding competition with human activities (food production or human settlements) and preserving pristine ecosystems. Other local opportunities might exist, but at a much lower scale. 

One of the most flexible CDR systems is carbon dioxide capture plants that rely solely on electricity provided by solar PV or wind turbines. This requires a DAC facility that needs only electricity, or another source of heat for a sorbent-based DAC plant. Electric DAC technology for CO₂ capture, such as the faradaic electro-swing reactive adsorption technology, is still under development at the laboratory scale (Voskian and Hatton, 2019). Through experimentation, the process has demonstrated a high faradaic efficiency and low energy consumption (40 – 90 kJ/mol) (Voskian and Hatton, 2019). Additionally, this technology primarily utilizes electric energy, making it suitable for either continuous operation or operations requiring repeated start-up and shut-down. While this CDR system is not ready to be deployed, it may be in the future.

Other possible DAC systems requiring only electrical energy would require identifying another source of thermal energy for a sorbent-based DAC. Various opportunities may exist, depending on local context. For instance, many industrial sectors produce heat necessary for their processes (e.g., cement, iron and steel, refining, chemicals, food processing). This excess heat, if not consumed in some other way at the facility, could be used to regenerate the sorbent of a DAC plant installed onsite, while the plant’s electricity needs could be met by a nearby PV or wind farm. If no waste heat is available, industrial heat pumps powered by PV or wind electricity could be used. This new type of industrial heat pump can raise the temperature of the working fluid from warm ambient temperatures (30 – 40^o C) to about 100^o C. This system would be implemented in warm climates.

Solar PV and wind opportunities on converted lands are available around the world, as are CO₂ sequestration options (Figure 3.13). All CSP opportunity areas are also viable for PV, and PV can generate electricity at latitudes as high as 60^o. This offers more opportunities than CDR systems using CSP, particularly in large GHG-emitting countries such as China, the European Union (member state emissions combined), India, Russia, and the U.S. (Figure 3.14). Sequestration options are mainly in sedimentary reservoirs, which could allow for faster implementation. In India, the Deccan Traps and the Rajmahal basalts are potential CO₂ reservoirs and could be the sequestration choice in these areas. However, there are low prospects for DAC in India as the country suffers from regular power shortages. Since these basaltic provinces are located under a highly industrialized region in India, they could be used for CO₂ sequestration from point-source capture at industrial facilities. 

A fully electric system would also favor onshore and offshore wind at higher latitudes. Here, offshore wind is displayed if it has potential generation of more than 20 GWh/yr. As previously mentioned, opportunities for onshore wind are widely available in the Great Plains of the United States. Numerous opportunities also lie in northern Europe, in particular on the coast of Norway, the UK, and the North Sea, where the wind speed is high and sedimentary reservoirs are widely available to create a wind–electric DAC–sedimentary reservoirs CDR system. For the same reasons, Cape Horn (Chile), the Cape of Good Hope (South Africa), New Zealand, and Tasmania are also good candidates (Figure 3.15).

The above examples consider siting CSP, PV, and wind farms only on converted lands, in order to minimize the impact of such installations on ecosystems and human activities. For the same reason, marine protected areas were not considered opportunities for offshore wind farms. We focused on using low-carbon energy and co-locating the elements of a potential CDR system to optimize the amount of CO₂ removed from the air. Other considerations are necessary to evaluate the impact of a potential CDR project, though the rapid reduction of the global atmospheric carbon dioxide stock is the ultimate goal. Appropriate uses of low-carbon energy, the transportation of CO₂, and other resources for CO₂ sequestration are discussed below. 

Carbon dioxide removal compliments CO₂ reductions. Removal cannot replace reductions. DAC and other CDR methods are meant to offset sectors that are hard to avoid, in addition to removing legacy emissions. It is more efficient to avoid releasing CO₂ in the atmosphere in the first place. For instance, building a PV or wind farm to replace a coal-fired power plant is more efficient than keeping the coal-fired power plant and using the PV or wind farm next to it to power a DAC plant. Figure 3.16 shows the global distribution of coal- and natural gas-fired power plants. Coal-fired power plants are concentrated in China (946), the U.S. (338), Europe (326), India (254), and Russia (96) (World Resources Institute [WRI], 2019). 

The transition from fossil fuel energy to renewable energy is underway. For instance, the Public Service Company of Colorado in the U.S. (PSCo) is replacing 600 MW of coal-fired units with 1,800 MW of renewable energy (1,131 MW wind, 707 MW solar PV) and 275 MW of battery storage. This should reduce coal-generated electricity in Colorado’s energy grid from 44 percent in 2017 to 24 percent in 2026 and increase renewable energy from 28 percent to 53 percent (Pyper, 2018). This might lower Colorado’s GHG emissions by 60 percent for CO₂ and 90 percent for nitrogen oxides and sulfur dioxide, compared to state emissions in 2005. Transitioning to low-carbon energy production is crucial for limiting carbon dioxide emissions and reduces reliance on CDR systems in the long term. However, the latest IPCC report (2018) stated that CDR systems will be necessary to reach the goal of preventing warming from increasing beyond 1.5^o C or 2^o C globally. To power these CDR systems, the world will need to produce more renewable energy than is needed to replace fossil power.

Social impacts like unemployment rates are also important considerations when transitioning from one technology to another. Coal miners generally do not migrate far when they lose their jobs (Danson, 2005; Gore and Hollywood, 2009; Hollywood, 2002), leading to local social disasters when coal mines are closed. Pai et al. (2020) analyzed the job-transition potential of shifting toward low-carbon energy and concluded that solar PV has more potential than wind in coal areas in India (Pai et al., 2020). Their study also included Australia, China, and the U.S., but they examined the total potential for each renewable technology (rather than constraining their analysis to converted areas already disturbed by human activity), which might overestimate the true renewable energy potential.

Other social parameters must also be considered for a successful CDR project. For example, public acceptance (Wong-Parodi and Ray, 2009), misunderstanding, and the perceived risks posed to communities related to CO₂ injection in the subsurface might prevent CDR system development in densely populated areas.

In addition to technical, economic, and social considerations, local field studies of potentially suitable sites for DACCS or BECCS are essential. Indeed, global or regional datasets might lack important local characteristics. For instance, peatlands (Sections 2.5 and 3.2.3) are often poorly mapped, according to Chico et al. (2020). The authors analyzed mapping data for Europe, where peat bogs are particularly abundant in Ireland and the UK and also appear in France and Spain. A map issued by Xu et al. (2017) shows that peatlands are widely distributed in these areas. Preserving these environments is crucial because, though peatlands represent less than three percent of the Earth’s land surface, they store 20 percent of soil carbon (Chico et al., 2020). Installing wind turbines in these environments would disturb them and could cause large releases of CO₂ into the atmosphere. Northern Europe has large wind resources, as shown on Figures 3.3, 3.13, 3.15B, and 3.17, and because peatlands are poorly mapped, peat bogs might still prove to be good places to build wind farms.

Western Europe is one of the most densely populated areas in the world and one of the biggest emitters of GHGs. Because of that density, offshore CO₂ sequestration options are probably better than those on land. Figure 3.17 shows the sedimentary reservoirs selected by the CO2StoP study in the North Sea, which has large available resources and is where the Sleipner CO₂ sequestration project has been operating since 1996.

Injecting CO₂ captured onshore into offshore sedimentary reservoirs requires pipelines. Buffers of 100, 250, and 500 kilometers around these reservoirs are displayed in Figure 3.17 for a rough estimate of the length of these pipelines. Pipelines of about 500 kilometers can reach most of the northwestern countries, but reaching most of the European opportunity areas and implementing low-carbon energy–DAC–offshore sedimentary reservoir systems in Europe would require building a dense network of CO₂ pipelines. Europe’s industrial and electricity sectors are still not carbon-neutral, and some of these facilities might choose to implement point-source capture in an effort to avoid releasing most of their CO₂ into the atmosphere. An operating network of CO₂ pipelines, such as the one proposed by Morbee et al. (2011) with booster stations and carbon hubs to collect the CO₂ from major pipelines, might stimulate projects for CO₂ capture from industrial streams or from the air and lead an effort toward true CDR systems and negative emissions. A similar project led by Bellona is looking into injecting CO₂ in sedimentary formations off the coast of Norway after it is transported by a pipeline network or ships from several locations in northern Europe (Bellona Europa, 2020).

According to the CO2StoP study, other offshore CO₂ sequestration reservoirs hold promise for avoiding very long-distance transportation of CO₂. Southern Europe has opportunities off the coast of Portugal, in the Adriatic Sea, and in northeastern Europe in the Baltic Sea (Figure 3.5).

As another example, a study in India by Garg et al. (2017) investigated pipeline trajectories for minimizing the transportation distances by linking several point sources of CO₂ to potential sequestration fields. A similar study has been carried out in the U.S. for developing pipelines in the Midwest. These pipelines would link ethanol plants that produce high-purity CO₂ as the result of the fermentation process with sequestration reservoirs, which are often not co-located with ethanol plants (Fry et al., 2017). Co-location efforts like these are beneficial for areas with great potential for CO₂ removal but few opportunities for carbon sequestration.

Another technological system that captures CO₂ and sequesters carbon dioxide is BECCS (described in Section 2.7). These systems capture CO₂ via photosynthesis when biomass grows and, when that biomass is used for bioenergy generation, a carbon-containing product such as CO₂ or biochar is sequestered. The ability to both capture CO₂ and produce energy makes BECCS attractive compared to other CDR technologies that require energy. However, BECCS requires significant land area, which can compete with the need to protect existing ecosystems and other CDR approaches, such as reforestation or afforestation (described in Sections 2.4 and 3.2). To avoid competing with food production and harming ecosystems, two approaches have been proposed: using waste biomass only (Baik et al., 2018) or managing biomass growth and production from land already disrupted by human activities (Daioglou et al. 2019). The second approach is similar to the one taken by Baruch-Mordo et al. (2019), described above in Section 3.3.2, which restricts opportunities for solar and wind to converted lands. Opinions on dedicated land use for bioenergy (whether in the form of production forests or dedicated energy crops) still differ, due to potential competition with other important land uses.

Moreover, the definition of waste biomass varies. Most definitions of waste include high-moisture biomass, agricultural wastes destined for landfills, the organic portion of municipal solid wastes, and industrial waste sludge from the food and beverage industries (Faaij, 2018). Broader classifications of waste biomass incorporate “residues,” which include woody biomass residues as a byproduct of timber or pulp and paper mills. Also, biomass growth varies throughout the year, so the composition and availability of wastes may be seasonally specific. 

BECCS deployment assessments will need to examine spatial co-location of suitable storage basins and biomass availability to minimize long-distance transport of biomass and CO₂ or other carbon products. Biomass availability varies widely and is based on local ecosystem and development contexts. While global aggregated storage capacity is generally not considered a limiting factor for BECCS or CCS deployment, the capacities of storage sites vary widely and may lead to regional storage constraints. Consideration of the storage and injection rate capacity of storage formations at a fine spatial scale is crucial in determining potential storage sites suitable for near-term BECCS deployment. As discussed in Section 3.3.3, further investigations are needed to determine locations suitable for CO₂ injection. Also, importing biomass from countries will require rigorous governance to ensure sustainable land management. Interregional cooperation (i.e., developing joint agreements for managing carbon, setting shared carbon sequestration goals, and trading negative emissions credits and biomass) is central to sustainably and affordably scaling up BECCS. This multilateralism in biomass and carbon credits trading provides important opportunities to create value for key providers of CO₂ removal (Fajardy & McDowell, 2020).

Daioglou et al. (2019) provided a global map of potential opportunities in 2050 and 2100 for biomass cultivation on abandoned agricultural lands and on grassland, shrubland, savannah, and tundra environments. This information should be correlated with reliable geological sequestration in order to show opportunities for a full BECCS system. The choice among BECCS, biological CDR techniques, and other CDR approaches relies on multiple parameters, including the co-benefits of each CDR approach (e.g., electricity production and biodiversity increase), local energy supply, local incentives and regulations, and public opinion.

No global mapping of available waste biomass alongside geologic sequestration capacity exists, but Baik et al. (2018) studied waste biomass availability in the U.S. and correlated biomass sources with sequestration in sedimentary reservoirs. In addition to showing regions with the greatest waste biomass availability, the authors estimated potential CO₂ capture using BECCS to be in the hundreds of MtCO₂/yr (Baik et al., 2018). Similar studies in other countries would help determine their available waste biomass to implement BECCS without disturbing current ecosystems and competing with reforestation and afforestation.

The above sections discussing DAC and BECCS siting focused on sequestering CO₂ in sedimentary reservoirs, as this technology is currently the best understood and the most ready to be implemented at large scale in the next few years. Opportunities for CO₂ injection in basalts and ultramafic rocks (peridotites and serpentinites) are also shown above, as all of these options might be viable for in-situ CO₂ sequestration. CO₂ sequestration options other than in-situ injection in subsurface pore space can be considered as well. As described in Section 3.3.8, mines and industrial facilities produce significant amounts of alkaline wastes that can be reacted with CO₂ and offset part of the facility’s emissions.

Figure 3.18 shows the spatial distribution of carbon mineralization opportunities in the U.S. for asbestos, gold, copper, and nickel mine tailings, as well as for multiple industrial sources of alkalinity (CKD, lime, red mud, steel slag, and fly ash), along with in-situ opportunities for CO₂ injection (Kirchofer et al., 2013; USGS, 2003 and 2010).

Sedimentary rocks react very slowly with CO₂. Therefore, Figure 3.18 explores the total capacity of each basin in terms of CO₂ sequestration. The EASiTool module was used to provide a rough estimate of the capacity that exists within the local area in addition to the thickness and porosity of each reservoir (Ganjdanesh and Hosseini, 2017; Hosseini et al., 2019). The fair category corresponds to roughly 0.1 Gt or less, the good category to 0.1 to 1 Gt, and the excellent category to 1 Gt or more. These estimates account for a maximum of 100 wells per basin, 10 km spacing between CO₂ injection wells, and an injection duration of 20 years.

Reservoir capacity is trickier to estimate for mafic and ultramafic rocks, as each rock behaves differently. Injection of CO₂ in these rocks takes place at a lower TRL than sedimentary basins, but they react more readily with CO₂. To remain in a supercritical state, CO₂ should be injected deeper than 800 meters into basalt rock. The pilot project CarbFix has shown full mineralization of CO₂ at depth in less than a year (Aradóttir et al., 2011; Gislason et al., 2010; Gunnarsson et al., 2018; Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Ultramafic rocks (peridotites and serpentinites) react even more quickly than basalts and, thus, CO₂ might not have to be sent deeper than 800 meters to avoid release back into the atmosphere and to ensure full mineralization in subsurface pore space. Their TRL remains at an initial level, and only lab-scale experiments have taken place.

Finally, ultramafic rocks, particularly mine tailings, can also be used for surficial carbon mineralization. Asbestos crystals are needle-shaped, so they have a large surface area, which increases their reactivity with CO₂. Two types of minerals take the form of asbestos: chrysotile (asbestiform serpentine) and crocidolite (asbestiform amphibole), the latter being more hazardous to human health. The ranking of asbestos mine tailings is associated with the following categories: “Good” indicates the presence of chrysotile, “fair” indicates a combination of chrysotile and crocidolite (implying a riskier access to the chrysotile resource), and “poor” indicates the presence of crocidolite only, which makes the mine tailings unsuitable for use. Using mine tailings has distinct advantages: It avoids mining additional alkalinity while sequestering CO₂, some wastes can be upcycled into products, and it can help mitigate the risks of hazardous materials like asbestos..

All CO₂ sequestration opportunities in the U.S. are displayed in Figure 3.18, showing a heterogeneous distribution and indicating that although distinct opportunities are regional, most regions have potential for storage. The same type of analysis could be done on a more granular level regionally to provide stakeholders with a comprehensive overview of CO₂ sequestration opportunities.

Through a series of existing and original spatial mapping efforts, this chapter explored global opportunities for the deployment of CDR systems. Multiple CDR approaches are available in most regions of the world and depend on the availability of resources. For example, waste alkalinity sources from mine waste and industrial aggregate are produced in different volumes, depending on the region. Approaches with high TRL levels (as described in Chapter 2) are ready to be deployed immediately, and local regions can leverage their expertise and resources, ideally through global coordination, collaboration, and transparency.

Actual CDR deployment requires detailed local studies of the energy supply network and the sources of CO₂ emissions. Some CDR approaches are energy-intensive and may therefore be limited in their efficacy. In all cases, carbon dioxide removal must be deployed in tandem with reducing emissions by switching to low-carbon energy sources or avoiding emissions by implementing point source capture. Also, biological CDR solutions have additional environmental co-benefits compared to technological approaches, but technological approaches may result in more durable storage and remove more CO₂ per land area. For any CDR approach, spatial analysis can guide deployments that maximize emission capture efficiency (discussed in Chapter 4). Ultimately, more accurate and granular data, further analysis, and a broad consideration of stakeholder and community interests and needs are all required to guide the deployment of future CDR projects. Geospatial analysis is simply a tool, not a determination.