Carbon dioxide removal technologies may involve a range of environmental, socioeconomic, and economic risks. Environmental risks, for example, range from potential damage to natural ecosystems and biodiversity loss to food insecurity and production of non-CO2 gases. Here we outline these risks for the major CDR technologies discussed in this paper.
BIOENERGY WITH CARBON CAPTURE AND STORAGE (BECCS)
BECCS is the process of growing biomass, such as switchgrass, which captures carbon dioxide through photosynthesis, burning it to produce electricity or liquid fuels, and capturing the carbon dioxide emissions associated with combustion for underground storage.
BECCS poses a series of environmental and socioeconomic risks due to its land and water requirements. Large-scale deployment can displace natural ecosystems, such as forests and grasslands, damaging wildlife and resulting in significant biodiversity loss. Similarly, large-scale deployment can decrease land availability for food production, resulting in increased prices and regional shortages. At the same time, BECCS can strain freshwater resources, contributing to water scarcity and negatively affecting freshwater ecosystems.
Whether BECCS is carbon negative, neutral, or positive is case-dependent. Factors to consider include emissions and changes to albedo from land-use change; competition with reforestation or afforestation; nitrous oxide emissions from nitrogen fertilizer use; transportation emissions if the growing and processing sites are not co-located; carbon losses during capture; and potential leakage from geological storage.
Biochar results from the process of heating biomass under low-oxygen conditions (pyrolysis) and serves as a carbon-rich and decomposition-resistant additive to soils that can improve agricultural yield. There is broad disagreement about the climate, environmental, and socioeconomic benefits and risks of biochar production and burial. Climate-related concerns include emissions from land-use change; albedo change due to the effect of darkened soil; the variability of biochar stability; the question of whether biomass sites are co-located with pyrolysis sites; and the competition between biochar and other land-based negative emissions techniques, such as BECCS, for land and water resources (whether BECCS or biochar is more climate-effective is case-dependent). Meanwhile, environmental and socioeconomic concerns include the potential for land-intensiveness (which is typically low but varies), as well as the effect of particulate matter, which biochar can release during production, transport, and distribution, on air quality. Food insecurity is likewise a concern due to the potential for land competition with food production.
OCEAN IRON FERTILIZATION
Fertilizing the ocean with iron could promote the growth of algae, which absorb carbon dioxide. It is generally recognized that ocean iron fertilization is deeply problematic, with environmental risks outweighing the potential for negative emissions. The negative and unpredictable environmental effects of ocean fertilization can include toxic algal blooms; increases in ocean acidity; decreases in oxygen; disrupted nutrient cycling; and disrupted food webs, including fish, seabirds, and ocean mammals. Fertilization can also unpredictably disrupt the yields of fisheries. Meanwhile, the overall climate benefit is limited, given that fertilization can produce additional gases, including nitrous oxide and methane, and can result in carbon storage that is largely temporary, with carbon returning to the atmosphere rather than being sequestered in the deep ocean.
ENHANCED WEATHERING (EW) AND OCEAN ALKALINIZATION
Enhanced weathering is the process of dispersing crushed silicate or carbonate minerals over land to accelerate the absorption of carbon dioxide. Meanwhile, ocean alkalinization is the process of dispersing alkaline minerals in the ocean, changing ocean chemistry and increasing carbon dioxide uptake. Enhanced weathering and ocean alkalinization have not yet been demonstrated at scale but raise several concerns. The mining and transport necessary for these techniques could have a significant environmental footprint and require a significant amount of energy. In addition, the dispersal of minerals in enhanced weathering could negatively affect terrestrial ecosystems as well as groundwater and rivers, while the dispersal of minerals in ocean alkalinization could negatively affect marine ecosystems.
DIRECT AIR CARBON CAPTURE AND STORAGE (DACCS)
DACCS captures carbon from ambient air through a chemical process and stores it underground. Although the technique has not yet been demonstrated at scale, it has elicited fewer climate, environmental, or socioeconomic concerns than other negative emissions technologies. Climate-related concerns revolve around carbon losses during transportation and storage; they also revolve around the energy intensiveness of DACCS, which could offset its climate benefit unless the energy source is clean. Environmental and socioeconomic concerns revolve around water use, which could be significant for some technologies; mining that may be required for input materials; land use, which would be relatively minimal aside from any associated clean energy infrastructure; and negative environmental effects that have not yet been anticipated or considered.