One category of risk with respect to CDR is associated with its effectiveness, generally, and with specific reference to the category of agriculture, forestry, and other land uses CDR. As noted above, some have stated that CDR is essential, in combination with large-scale decarbonization, in order to achieve net-zero GHG emissions by 2050 and to limit the risk of harm from adverse climate impacts.52 Other researchers have stated that there are risks to relying on CDR for this purpose. The IPCC states, for example, that “CDR deployed at scale is unproven, and reliance on such technology is a major risk in the ability to limit warming to 1.5°C.”53

Some have stated the concern that such reliance might lead to delaying or avoiding current emissions reductions on the assumption that the effect of the CO2 emitted will be counteracted through future CDR.54 These researchers have stated the concern that it may be difficult to deploy a sufficient amount of CDR to balance the emissions under these circumstances.55 If CDR does not function as anticipated, the risk of climate impacts could increase, and the range of mitigation options to avoid such impacts might be limited.56 Some have questioned the future technical and economic viability of CDR and state that such a delay may create risks for communities that are more vulnerable to the effects of climate change.57

Some researchers have stated concerns regarding effectiveness that are specific to AFOLU CDR. One concern is the durability of carbon removal, a concern for AFLOU CDR, as forests and soils can act as either sources or sinks for atmospheric CO2.58 A forest fire or a change in agricultural tillage practices could return carbon to the atmosphere, and the CDR benefit from the soils and forests would then be lost. Some have stated an additional concern that efforts to enhance forest carbon stocks in one location will displace deforestation to a different location in a process known as leakage. Such leakage could reduce the climate mitigation effectiveness of the AFOLU CDR.59

Some have expressed a range of concerns about the feasibility of CDR. These have included concerns about the costs, scalability, and economic viability of some CDR methods. In particular, there is concern that such economic or biophysical constraints may cause CDR methods to fail to

52 DOE, Office of Fossil Energy and Carbon Management, “Is CDR Necessary to Achieve Net-Zero by 2050?” in Carbon Dioxide Removal Frequently Asked Questions, https://www.energy.gov/sites/default/files/2021-11/Carbon- Dioxide-Removal-FAQs.pdf#page=5.

53 IPCC, “Chapter 2: Mitigation Pathways Compatible with 1.5°C in the Context of Sustainable Development,” in Global Warming of 1.5°C: An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty, 2018, p. 96.

54 N. Grant et al., “The Policy Implications of an Uncertain Carbon Dioxide Removal Potential,” Joule, vol. 5, no. 10 (2021), p. 2593.

55 N. Grant et al., “Confronting Mitigation Deterrence in Low-Carbon Scenarios,” Environmental Research Letters, vol. 16, no. 6 (2021), p. 064099 (hereinafter Grant 2021).

56 Grant 2021.

57 Kevin Anderson and Glen Peters, “The Trouble with Negative Emissions,” Science, vol. 354, no. 6309 (2016), p. 182.

58 T. Bhattacharyya et al., “Soil as Source and Sink for Atmospheric CO2,” in Carbon Utilization: Applications for the Energy Industry, ed. Malti Goel and M. Sudhakar (Singapore: Springer, 2017), p. 61. See also CRS Report R46312, Forest Carbon Primer, by Katie Hoover and Anne A. Riddle.

59 Charlotte Streck, “REDD+ and Leakage: Debunking Myths and Promoting Integrated Solutions,” Climate Policy, vol. 21, no. 6 (2021), p. 843.

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achieve the CO2 removal capacity necessary to stabilize the climate.60 For some CDR methods, such as BECCS and ocean alkalinity enhancement, some have stated concerns that their implementation may be constrained by requirements for land, fertilizer resources, water, or mined minerals. For other CDR methods, such as biochar and EW, there are concerns regarding limited capacity to produce the carbon residue and the crushed rock necessary for these methods.

Some researchers have questioned the feasibility of stabilizing global temperatures using BECCS, as it has not been implemented at a commercial scale and there are uncertainties regarding scale- up and cost.61 Some studies have scrutinized the implementation and the timeline for using BECCS.62 Other studies indicate that the resources required to support BECCS could rival the requirements of current global food production.63 Using BECCS to stabilize global temperature increases at or below 2oC is estimated to require the removal, by BECCS, of 3.3 gigatons of carbon per year by 2100.64 The IPCC Special Report on Global Warming of 1.5oC found that this level of BECCS could require 25%-46% of the world’s cropping area by 2100.65 A study of this level of BECCS found that it is comparable to current U.S. and global agricultural resource requirements:

[R]emoving 3.3 Gt of carbon per year from the atmosphere using BECCS would require the annual mobilisation of … 60 to 371 Mt of nutrients (N and P2O5), 1250 to 2490M ha of marginal land in the US, and 7800 to 15700B m3 of water. As a means of comparison, 17 Mt of N and P2O5 nutrients are used annually in the US, 721M ha of land are harvested for cereal production in the world … and 7980B m3 of water is withdrawn—including green water—for the world agriculture.66

Ocean-based CDR has not been implemented at a commercial scale, and there are uncertainties regarding scale-up and cost.67 Some researchers have questioned the feasibility of ocean alkalinity enhancement at scale, due to the magnitude of mining the required minerals. A National Academies of Sciences, Engineering, and Medicine (NASEM) report estimated that the required mining effort would be equivalent to that of the global cement industry.68

60 P. Smith et al., “Biophysical and Economic Limits to Negative CO2 Emissions,” Nature Climate Change, vol. 6, no. 1 (2016), p. 42. See also S. Fuss et al., “Betting on Negative Emissions,” Nature Climate Change, vol. 4, no. 10 (2014), p. 850 (hereinafter Fuss 2014).

61 Fuss 2014.

62 G. F. Nemet et al., “Negative Emissions—Part 3: Innovation and Upscaling,” Environmental Research Letters, vol. 13, no. 6 (2018).

63 IPCC, Global Warming of 1.5°C: An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre- Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty, 2018. On page 343 this report states the following:

The average amount of BECCS in these pathways requires 25–46% of arable and permanent crop area in 2100. Land area estimates differ in scale and are not necessarily a good indicator of competition with, for example, food production, because requiring a smaller land area for the same potential could indicate that high-productivity agricultural land is used.

64 P. Smith et al., “Biophysical and Economic Limits to Negative CO2 Emissions,” Nature Climate Change, vol. 6, no. 1 (2016), p. 42.

65 IPCC SR1.5 2018, p. 343.

66 Mathilde Fajardy and Niall MacDowell, “Can BECCS Deliver Sustainable and Resource Efficient Negative Emissions?” Energy & Environmental Science, vol. 10, no. 6 (2017), p. 1389.

67 Committee on a Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration, Ocean Studies Board, Division on Earth and Life Studies, and NASEM, A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration, 2022, p. 17 (hereinafter NASEM OBCDR 2022).

68 NASEM OBCDR 2022, p. 195.

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Some researchers have raised concerns about the economic feasibility of direct air capture due to the wide range of estimates of potential costs associated with this developing technology.69 Cost estimates vary based on the type of DAC technology used and whether the energy comes from a fossil fuel or renewable energy source.70 Reliance on DAC would require a large scale-up, and there is uncertainty about the potential rate of implementation.71

Some researchers have also stated concerns about the feasibility of biochar and enhanced weathering. Although the methods of biochar pyrolysis and crushing stone for EW are well understood, these technologies have not been deployed on a large scale for CDR. There are potential limitations on production capacity for both methods and uncertainties regarding cost.72

Some have raised concerns that different types of CDR may have unintended potential side effects. These side effects may be biophysical, such as changes to landscapes and ecosystems, or economic, such as changes to the cost of food.

Some researchers have expressed concerns about potential side effects of ocean-based methods of CDR. For ocean alkalinity enhancement, some researchers have raised concerns about the environmental impacts of the mining associated with this method of CDR. In addition, some researchers have stated concerns regarding potential unintended ecological effects to the ocean ecosystem of both ocean fertilization and the chemical mechanisms of ocean alkalinity enhancement.73

Both biochar and EW CDR methods involve the addition of materials to soils, which some researchers believe could lead to the potential addition of toxic substances. Some researchers have stated that the EW CDR method could result in the addition of heavy metals such as nickel and chromium to soils.74 Nickel and chromium can have toxic effects on plants.75 Some researchers have stated concerns that biochar could be a source of organic contaminants when added to soils, including polycyclic aromatic hydrocarbons (PAHs), and dioxins, as well as heavy metals.76

69 K. Sievert et al., “Considering Technology Characteristics to Project Future Costs of Direct Air Capture,” Joule, vol. 8, no. 4 (2024), p. 979.

70 The two main types of DAC technology are solid sorbent and liquid solvent approaches with energy sources that include natural gas and renewable hydrogen. See NASEM NET 2019, p. 223; see also S. Shayegh et al., “Future Prospects of Direct Air Capture Technologies: Insights From an Expert Elicitation Survey,” Frontiers in Climate, vol. 3 (2021) (hereinafter Shayegh 2021).

71 Shayegh 2021. See also NASEM, Accelerating Decarbonization in the United States: Technology, Policy, and Societal Dimensions (Washington, DC: The National Academies Press, 2024) (hereinafter NASEM 2024), which states on p. 424 that “technological options like BECCS and DAC are unlikely to be deployed at levels that would materially affect 2030 emissions, carbon sinks in the United States through 2030.”

72 J.C. Minx et al., “Negative Emissions—Part 1: Research Landscape and Synthesis,” Environmental Research Letters, vol. 13, no. 6 (2018), p. 063001.

73 NASEM OBCDR 2022, p. 195. See also p. 8.

74 Fatima Haque, Yi Wai Chiang, and Rafael M. Santos, “Risk Assessment of Ni, Cr, and Si Release from Alkaline Minerals during Enhanced Weathering,” Open Agriculture, vol. 5, no. 1 (2020), p. 166.

75 S. Kumar et al., “Nickel Toxicity Alters Growth Patterns and Induces Oxidative Stress Response in Sweetpotato,” Frontiers in Plant Science, vol. 13 (2022), p. 1054924. See also M. Shahid et al., “Chromium Speciation, Bioavailability, Uptake, Toxicity and Detoxification in Soil-Plant System: A Review,” Chemosphere, vol. 178 (2017), p. 513.

76 M. Brtnicky et al., “A Critical Review of the Possible Adverse Effects of Biochar in the Soil Environment,” Science of The Total Environment, vol. 796 (2021), p. 148756.

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Some researchers have suggested that the land area required for AFOLU CDR could compete with food production. The cost of food could increase as agricultural land is converted to forest, although price increases could be lower when afforestation occurs only in tropical areas rather than worldwide.77

Similarly, some researchers have stated the concern that implementation of BECCS as a CDR method could have the side effect of competition for agricultural resources, including arable land and water for crop production.78 Some have proposed that in scenarios that include a range of CDR methods, possible competition for arable land and resources resulting from the implementation of BECCS might be avoided.79 This might be accomplished by limiting BECCS feedstocks to waste forestry and agricultural biomass and limiting land use to areas already producing corn ethanol.80

There are potential advantages and co-benefits associated with different types of CDR that are related to the particular implementation of each CDR mechanism. Some of these are specific to particular types of CDR and others are associated with more than one type. Some of the advantages and co-benefits of CDR are presented in this section.

AFOLU CDR may facilitate climate adaptation and biodiversity co-benefits.81 Sequestering carbon in soils in the form of soil organic matter, a type of AFOLU CDR, can increase the amount of water available to plants.82 This can be a climate change adaptation strategy, allowing crops to be less vulnerable to drought. In addition, some increases in soil organic matter are associated with crop yield increases in maize and wheat.83

BECCS has the unique co-benefit among types of CDR that it also produces energy, in addition to removing CO2 from the atmosphere.84 BECCS can be used to produce heat or electricity by combustion. It can also be used to produce biomass-derived hydrogen.85

Ocean alkalinity enhancement has the potential co-benefit of reducing ocean acidification that is driven by increased atmospheric CO2 concentrations.86 Reducing ocean acidification would have

77 IPCC, Climate Change 2014: Mitigation of Climate Change Working Group III Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2014, p. 841. See also U. Kreidenweis et al., “Afforestation to Mitigate Climate Change: Impacts on Food Prices Under Consideration of Albedo Effects,” Environmental Research Letters, vol. 11, no. 8 (2016), p. 085001; and M. Wise et al., “Implications of Limiting CO2 Concentrations for Land Use and Energy,” Science, vol. 324, no. 5931 (2009), p. 1183.

78 M. Bonsch et al., “Trade-Offs Between Land and Water Requirements for Large-Scale Bioenergy Production,” GCB Bioenergy, vol. 8, no. 1 (2016), p. 11.

79 NASEM, Negative Emissions Technologies and Reliable Sequestration: A Research Agenda (Washington, DC: The National Academies Press, 2019), p. 8.

80 NASEM 2024, p. 428.

81 P. C. Buotte et al., “Carbon Sequestration and Biodiversity Co-Benefits of Preserving Forests in the Western United States,” Ecological Applications, vol. 30, no. 2 (2020).

82 Rattan Lal, “Soil Organic Matter and Water Retention,” Agronomy Journal, vol. 112, no. 5 (2020), pp. 3265-3277.

83 E. E. Oldfield et al., “Global Meta-Analysis of the Relationship Between Soil Organic Matter and Crop Yields,” SOIL, vol. 5, no. 1 (2019).

84 IEA, “Bioenergy with Carbon Capture and Storage,” https://www.iea.org/energy-system/carbon-capture-utilisation- and-storage/bioenergy-with-carbon-capture-and-storage.

85 M. Bui et al., “Delivering Carbon Negative Electricity, Heat and Hydrogen with BECCS—Comparing the Options,” International Journal of Hydrogen Energy, vol. 46, no. 29 (2021), p. 15298.

86 NASEM OBCDR 2022, p. 196. See also CRS Report R47300, Ocean Acidification: Frequently Asked Questions, by Caitlin Keating-Bitonti and Eva Lipiec.

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a beneficial effect on some marine organisms that produce calcium carbonate to make shells and other skeletal structures.87

Both the biochar and EW CDR methods involve adding materials to soils. Some researchers have found that both methods may have co-benefits in promoting soil fertility.88 Biochar may provide a source of plant nutrients in soil and may increase soil water-holding capacity.89 EW CDR methods can increase soil pH, increasing nutrient availability for corn and soybean crops.90

DAC has the advantage of flexibility in terms of siting that could allow it to be implemented near favorable geological formations for carbon sequestration, reducing the need for pipeline transport.91