Direct Air Capture: Assessing Impacts to Enable Responsible Scaling


To avoid the worst impacts of climate change, global average temperature rise needs to be limited to 1.5°C, as outlined in the Paris Agreement. This means reaching net-zero carbon dioxide (CO2) emissions globally by midcentury (IPCC 2018). The United States has committed to go further: net-zero across all greenhouse gas (GHG) emissions by 2050 (U.S. Department of State 2021). While emissions reduction is top priority, it will not be enough to meet global temperature goals; the IPCC (2018) indicates that all pathways that limit global warming to 1.5°C require carbon dioxide removal (CDR) to some extent. Large-scale carbon removal would reduce atmospheric CO2 concentrations and neutralize emissions in sectors that are difficult-to-decarbonize by midcentury, like aviation and shipping (IPCC 2018; NASEM 2019).

Carbon removal includes both natural and technological approaches that pull CO2 directly from the air and differs from carbon capture and storage (CCS), which captures CO2 at sources like cement plants. Nature-based approaches include tree planting and agricultural practices that increase carbon in soil and plant biomass, while technological approaches accelerate natural carbon cycles or directly remove CO2 from the air. Direct air capture (DAC) is a technological approach that removes CO2 from the atmosphere through reactions with certain chemicals, and can be paired with several sequestration methods. Nature-based carbon removal strategies can be scaled up given sufficient workforce and infrastructure (Fargione et al. 2021), but their potential will be constrained by land availability, sensitivity to a changing climate, and the need to maintain ecological integrity (Seddon et al. 2020). To minimize risks, a robust carbon removal portfolio should include a variety of natural and technological approaches, including DAC (Mulligan et al. 2020).

Compared to other carbon removal approaches, DAC has several benefits: it can be configured to use relatively small amounts of land; it can be sited relatively flexibly within practical constraints, so it does not compete with agricultural or other productive land uses; and it offers quantifiable and permanent CO2 removal when combined with geological sequestration or used in durable products (e.g., concrete). These are some of the reasons it has received attention and increasing investment in the past few years.

The amount of technological and natural carbon removal needed will depend on the speed at which countries are able to decarbonize. The International Panel on Climate Change (IPCC) and the National Academies of Sciences, Engineering, and Medicine (NASEM) both suggest the potential need for up to 10 billion tonnes (gigatons [Gt]) of carbon removal per year globally by 2050 to stay below 1.5°C warming (IPCC 2018; NASEM 2019). The United States’ Long-Term Strategy (LTS) points to around 0.5 GtCO2/year of carbon removal domestically by 2050 (roughly 8 percent of U.S. emissions in 2019) across all technological solutions (U.S. Department of State 2021; U.S. EPA 2021b). If mitigation efforts are less successful or nature-based solutions fall below potential, both the amount of technological carbon removal and its deployment rate would need to increase. Regardless of the ultimate amount of DAC, near-term investment and scaling are needed to make future large-scale DAC feasible (Hanna et al. 2021; Repmann et al. 2021).

Estimating carbon removal needs is typically based on Integrated Assessment Models (IAMs), which use economics as a basis to identify the most impactful, low-cost mitigation pathways but are not designed to account for social or local impacts or externalities and may not include new technologies. This paper seeks to complement assessments that prioritize cost by considering environmental and social impacts of DAC that could also help inform scale-up levels and approaches.

How DAC Works

In a DAC plant, air is exposed to a capture medium—a chemical that selectively binds CO2 while allowing the other components to pass through. Applying energy, typically as heat, releases the CO2 into a contained environment and regenerates the capture medium for subsequent rounds of capture. Ideally, a zero- or low-carbon energy source, such as renewable energy or natural gas with carbon capture, produces that heat. After the CO2 is released, it can be transported for permanent underground storage or used in products ranging from beverages to concrete.

There are currently two leading DAC technologies, determined by capture media: solvent and sorbent plants (Figure 1). Solvent plants use a liquid solvent (e.g., dilute potassium hydroxide [KOH]) to absorb CO2 as ambient air passes through. The dissolved CO2 then binds with calcium to form a stable solid mineral. CO2 is released through application of high-temperature heat (900°C/ 1,652°F), which is achieved today with natural gas, but could be achieved through other options like clean hydrogen in the future. Solvent-type plants are built at larger scale today because of cost advantages specific to the plants’ components. Sorbent plants use a solid sorbent as the capture medium, which functions like a filter, selectively binding CO2 and allowing other components to pass through. Sorbent plants use low-temperature heat to release the CO2 from the capture medium—80–120°C (176–248°F), which allows use of renewable energy and waste heat—and are built in a more modular fashion with many smaller repeating units.

Figure 1 | Illustration of Two Leading Types of DAC Systems in Development Today

Notes: CO2/H2O = Carbon dioxide/water; GJ/tCO2 = Gigajoules/tonnes of carbon dioxide; O2 = Oxygen; CaO = Calcium oxide; CaCO3 = Calcium carbonate; Ca(OH)2 = Calcium hydroxide; CO23-= Carbonate. These schematics show the contactor unit for the sorbent-based DAC (top) and solvent-based DAC (bottom). In step 1, the contactor ensures “contact” between CO2 in the air and the capture media. The key difference is that in the sorbent-based approach, ambient air passes through a porous support where it contacts the sorbent, whereas the solvent-based approach uses a counterflow configuration: ambient air passes horizontally through a packed bed while solvent is pumped through vertically. In step 2, the capture media is regenerated for subsequent capture.

Source: Based on NASEM (2019).

DAC is a nascent industry: the largest commercial plant in operation is Climeworks’ Orca plant in Iceland, which came online in September 2021 and uses geothermal energy to remove 4,000 tonnes of carbon dioxide per year (tCO2/year) (Figure 2). Additionally, the first million tonne/megaton (Mt) scale plant is in development in the United States, with another planned for Scotland, and there are over a dozen smaller-scale DAC plants operating globally already (Beuttler et al. 2019; Carbon Engineering 2021).

Figure 2 | Part of Climeworks’ 4,000 tCO2/year DAC Plant in Iceland

Note: tCO2/year = Tonnes of carbon dioxide per year.

Source: Climeworks.

American public officials have stated DAC’s importance in achieving global climate goals by providing $3.5 billion for DAC hubs—locations where DAC will be able to store or use at least 1 MtCO2/year—in the 2021 Infrastructure Investment and Jobs Act (IIJA, also known as the Bipartisan Infrastructure Law) (U.S. Senate 2021). Due to DAC’s commercial immaturity, operational data are limited or in some cases nonexistent, and technologies and processes are expected to change as more plants are deployed. New approaches are emerging that can shift or minimize resource intensity, like all-electric DAC configurations or sorbents with longer life times. For these reasons, this analysis is preliminary and conclusions may change as technologies develop and more data become available.

Scope and Methods

This assessment considers a range of impacts, beyond DAC’s intended goal of carbon removal, from building and operating DAC plants—both on-site and in the supply chain—including pollutants, use of land, energy, and water, and benefits like job creation. These impacts are associated with a range of activities in the construction and operation of DAC plants (Figure 3).

Impacts were assessed based on a literature review of papers from peer-reviewed, government, and industry sources. Policy and procedural recommendations were assembled from a broad review of environmental justice; community engagement; and just transition literature from academic sources, and from community organizations, nongovernmental organizations (NGOs), and government sources. Peer-reviewed papers specifically examining the environmental impacts of DAC are few (e.g., Deutz and Bardow 2021; Terlouw et al. 2021; Madhu et al. 2021), given the nascency of the technology.

We discuss the broad categories of impacts expected at the plant level (capturing 1 MtCO2/year) and at a larger potential scale by 2050 (0.5 GtCO2/year, as outlined in the U.S. LTS, or 500 1-Mt plants), using the previous sources as well as those specific to the impacts of each activity. We assess impacts in relation to today’s leading DAC technologies.

While impacts of end uses and transportation of captured CO2, such as CO2 pipelines and geological sequestration, concern stakeholders and communities (Livermore Lab Foundation 2021; Batres et al. 2021) and must be considered in any build-out of DAC, they fit into a larger landscape of needs related to carbon management. As such, we do not consider these impacts in-depth and limit this analysis to elements specific to DAC or required for the construction and operation of DAC plants.

Figure 3 | Scope of DAC Activities Considered

Notes: PV = Photovoltaic; CO2= Carbon dioxide.

Source: Authors’ analysis.

DAC Siting and Configuration

DAC can be sited flexibly within certain practical constraints. Siting decisions will likely be made by first identifying regions with land and energy availability and proximity to storage (Figure 4) and/or CO2transportation infrastructure. First-generation plants may prioritize on-site use or storage of CO2 or connecting to existing CO2 pipelines to avoid the need for new transport infrastructure. After land, energy, and storage criteria are met, locations would be further narrowed by other environmental, community, regulatory, and economic considerations, including community willingness to host DAC plants and local zoning restrictions. Considerations like water scarcity, proximity to solvent/ sorbent producers, existing roads, and/or availability of disturbed or brownfield land, among others, could also influence siting location.

Figure 4 | Map of Potential for Deployment of Renewable Energy (Solar, Wind, Geothermal) and Geological Storage for DAC

Notes: This map shows where potential for solar, wind, or geothermal power is located, CO2 could be stored in geological formations, and potential for both energy and storage are colocated. Due to data limitations, wind and solar availability are mapped only on converted lands, while geothermal potential is mapped across any land. Utilization of CO2 in products would expand the above range to include areas where renewable energy, but not geological storage, is available. Transport infrastructure for CO2 and/or consideration of locations where DAC could be powered by fossil fuel with carbon capture and storage (CCS) would also expand the above range.

Sources: Developed by Hélène Pilorgé using solar and wind data from Baruch-Mordo et al. 2019; geothermal data from NREL n.d.; Williams et al. 2008; and geological storage data from USGS 2013.

Importantly, DAC does not need to compete with productive or agricultural land uses and can be sited on marginal land. However, siting to maximize net CO2 removal by, for example, colocating with low-carbon thermal energy resources, may impact local communities, ecosystems, or culturally significant lands. Although on-site pollutant emissions from DAC operation are expected to be small, other environmental and social impacts such as land and water use, noise associated with construction and plant operation, using fossil fuels, and the visual impact of the facility can add to existing burdens that some communities face. Low-income communities, and particularly low-income Black, Latino, and Indigenous communities, in both rural and urban areas, bear a higher burden of pollution and corresponding poor health outcomes than do higher-income, majority-white communities (Mohai et al. 2009), so it is vital that DAC siting decisions consider existing burdens on nearby communities.

Given that DAC plants are a new type of facility, it is particularly important that early projects are carefully evaluated through social and environmental impact assessments (EIAs) and use robust community engagement processesto involve impacted communities in decision-making.3 As communities are uniquely able to assess the relative risks and benefits of siting a DAC plant in their area, developers should proactively engage with communities as early as possible in the project time line to identify suitable sites and the technical and nontechnical aspects of the DAC configuration that meet communities’ needs.

DAC plants could be sited far from communities to avoid direct impacts on people or closer to reduce commuting time for workers, distance and cost to transport materials used at the facility, and transport-related emissions. Optimal distance from a community will differ by project and would need to consider trade-offs related to commute distance, land disturbance for road construction, and costs and distance to transport capture media and other materials that are used on an ongoing basis.

There may be additional ways to reduce or change impacts through DAC configuration and choice of energy resource, capture media, and other materials needs: research is ongoing to optimize processes and materials use. For example, Climeworks estimates that the GHG impact of construction materials will decrease by more than half from today’s 4,000 tonne carbon dioxide/year (tCO2/year) plant to a future 100,000 tCO2/year version, on a per tonne removed basis (Terlouw et al. 2021). Other improvements like extending sorbent lifetimes will also help reduce environmental impact. Low-carbon concrete and/or steel could also be used to reduce embodied emissions. All these choices carry trade-offs, depend on many factors (Figure 5), and will need to be determined on a project-by-project basis to reflect local circumstances and community needs.

Beyond the siting and energy impacts, DAC scale-up will increase materials demand, impacting communities where materials like steel, cement, and chemicals are produced. While project developers can prioritize materials with lower carbon intensity and environmental impact, reducing the climate and environmental impacts of materials and industrial production will require broader action from governments and regulators.

Figure 5 | Interconnected Factors Involved in Determining Impacts of DAC Plants

Note: NG = Natural gas.

Source: Authors’ analysis.

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