Numerous CDR techniques have been proposed (Fig. 1) and the surrounding literature indicates that some CDR techniques could contribute significantly to achieving net zero or net negative CO 2 emissions15,16,26,27,28,29. While it is possible that CDR, together with mitigation, could eventually return atmospheric CO 2 to previous levels, this would only partially return the climate and other Earth system parameters, such as ocean pH, to the corresponding previous state, due to hysteresis and other effects30,31. Here we examine the potential contributions of CDR towards achieving the Paris Agreement goals, and the challenges that would be faced, complementing previous analyses which have focused on issues like the assumed role of CDR in low-carbon scenarios18,19, or the ability to compensate sectors that are particularly difficult to mitigate (e.g., air travel, agriculture and certain industries).

Several CDR techniques have been developed as prototypes, and afforestation is already in widespread use, as are some of the components involved in other techniques, e.g., bioenergy (in BECCS). However, all of these are far from the scale of CDR ref . Attempting to scale up any CDR technique would require addressing many technical and social issues, several of which are common across most or all of the techniques. One of the most important common technical issues is the total CO 2 storage capacity (see Box 2). Further issues include limits of required chemical and biological resources, how the techniques would compete with each other and other sectors for resources, the time scales involved, and the economic costs and societal impacts (see Box 1).

Box 2: Carbon storage capacity and achievability CO 2 removal methods require adequate storage reservoirs, either directly for CO 2 or for other forms of carbon (e.g., biomass, minerals and consumer products). A variety of reservoirs are possible, either quasi-permanent, confidently isolating CO 2 from the atmosphere over long timescales (e.g., >10,000 years178), or temporary, where a non-negligible amount of the removed CO 2 might return to the atmosphere within decades to centuries179. The achievability for nearly all reservoirs is qualitatively estimated (see Figure below) to be relatively high for small amounts (e.g., <1 Gt(CO 2 )), but challenging for larger amounts (e.g., >1000 Gt(CO 2 )), with considerable research needed, e.g., into ecological and economic implications, and development of adequate infrastructures for extensive deployment. For storage in the deep oceans, however, even relatively small amounts are likely to be challenging, given a lack of applicable practical experience. Deep-ocean storage, mainly via injection of liquefied CO 2 into deep-ocean waters and seabed sediments180, is mostly considered temporary, since ocean circulation will return some of the CO 2 to the atmosphere181. However, this occurs on timescales that are much longer than relevant for initial achievement of the Paris Agreement, with model simulations showing that for a CO 2 discharge depth of 3000 m, slightly less than half of the CO 2 would return to the atmosphere within 500 years182. The capacity for deep-ocean storage depicted in the Figure is based on a recent analysis179 and far exceeds CDR ref . Geological and geochemical storage capacity is considered large and quasi-permanent178,179. The main approach to geological storage is injection of CO 2 (usually compressed as a supercritical fluid), via boreholes, into deep porous rock formations like oil and gas reservoirs and deep saline formations overlain by sealing layers37. Challenges include the lack of adequate geological data in some regions, as well as the trade-offs between efforts to co-locate CO 2 capture sites and injection sites versus the development of CO 2 pipelines and ship transport networks183. Enhanced weathering techniques apply geochemical storage, reacting CO 2 with alkaline minerals, on either the land or ocean surface, and subsequently storing the weathering products55. Storage would likely be limited by logistical requirements (mining and transport) and ecological impacts rather than by mineral rock resource availability. Efforts are being made to combine geochemical storage with geological storage via in situ mineralization of liquid CO 2 injected into boreholes with geochemical conditions conducive to rapid mineralization reactions184, but considerable work is needed to determine how well this could be scaled up to tens or hundreds of Gt(CO 2 ). Biosphere-based carbon stores in trees and soils are limited in total capacity179, though both likely exceed CDR ref , with the storage capacity of soils estimated to be a few times larger than that of forests. Afforestation and soil carbon enrichment (e.g., terra preta) are well-established processes, and would be technically easier to implement in the near-term than geological and geochemical storage; however, these would compete against global trends of deforestation and top-soil degradation and loss. Challenges would likely grow rapidly at larger scales, with issues like land use competition, irrigation and fertilizer supply limits becoming increasingly significant32. In both cases, the biomass storage is temporary on timescales relevant to the Paris Agreement, and sustained ecosystem maintenance would be needed to prevent carbon from being returned to the atmosphere through changes in the local environment (e.g., disease), climate (e.g., drought, fire) or society (e.g., changing land use). Carbon capture and utilization (CCU) could also be considered a form of storage reservoir. While products such as liquid fuels or polyurethane foams would return CO 2 to the atmosphere via combustion or decay within years to decades, some products like construction materials could sequester CO 2 for centuries. However, even with extensive policy and market support actions, the removal potential is likely less than 10 Gt(CO 2 ) by 2100185. Finally, it is important to bear in mind that even small amounts of carbon storage in some reservoirs may be very difficult or even unachievable if societal and political support is lacking.

Biomass-based techniques

Numerous biomass-based CDR techniques have been proposed, all removing CO 2 from the atmosphere by photosynthesis. Some then use the biomass for primary carbon storage (e.g., in trees, humus, peat, etc.), while others involve combustion and subsequent storage of the products (e.g., compressed CO 2 and biochar).

Afforestation (here also including reforestation) involves increasing forest cover and/or density in previously non-forested or deforested areas. Principally the carbon storage potential is large compared to CDR ref , given that historic deforestation was 2400±1000 Gt(CO 2 )16. However, since much of this deforestation was to make space for current agriculture and livestock, extensive land-use competition could be expected for such a degree of afforestation32. More realistic estimates therefore range from about 0.5–3.5 Gt(CO 2 )/yr by 2050, increasing to 4–12 Gt(CO 2 )/yr by 210027,28,33, implying a total removal potential of about 120–450 Gt(CO 2 ) from 2015 to 2100 (assuming linear increases in the CO 2 uptake rate, starting at zero in 2015).

Combining biomass energy with carbon capture and storage (BECCS), which can be used for either electricity generation or the production of hydrogen or liquid fuels34, is widely assumed in integrated assessment model scenarios to be able to provide sufficient CDR to keep \(\Delta \bar T_{\mathrm{s}}\) below 2 °C18,19. The range of estimates of the maximum removal potential of BECCS is large, again partly based on assumptions about land-use competition with agriculture, economic incentives for extensive development and deployment, and other factors, such as nature conservation. High-end estimates for BECCS in the literature involve underlying assumptions such as the use of forestry and agriculture residues35, the transition to lower meat diets, and the diversion of over half the current nitrogen and phosphate fertilizer inputs to BECCS, resulting in an uptake of ~10 Gt(CO 2 )/yr by 205032,33, with estimates for 2100 being similar or possibly even higher27,36. This would also depend on the development of both bioenergy and carbon capture and storage (CCS) technologies, infrastructures, and governance mechanisms to allow a capacity several orders of magnitude greater than current prototypes37,38,39. Assuming a linear development to 10 Gt(CO 2 )/yr until 2050 and constant thereafter would imply a cumulative removal potential by 2100 of ~700 Gt(CO 2 ), i.e., exceeding CDR ref . Various factors may reduce this, but it could also increase under the high-end assumptions mentioned above.

Biochar, a stable form of carbon produced by medium temperature pyrolysis (>350 °C) or high temperature gasification (~900 °C) of biomass in a low oxygen environment, can be buried or ploughed into agricultural soils, enriching their carbon content. Various gases or oils can also be produced by the pyrolysis process. While biochar production could principally be applied to a similar amount of biomass as assumed for BECCS (i.e., ~700 Gt(CO 2 ) removal by 2100), many additional factors come into play40,41, including feedstock type and source, labile carbon fraction, char yield, required energy input, the mean soil residence time of the biochar carbon, sink saturation, and priming effects (i.e., accelerated organic matter decomposition). This results in a much lower estimated maximum removal potential for biochar, ~2–2.5 Gt(CO 2 )/yr28,41, or up to ~200 Gt(CO 2 ) by 2100, although, as with BECCS, this could possibly be enhanced by additional use of residue biomass from agriculture and forestry41.

In addition to mixing biochar into soils, recent studies have focused on replenishing or enhancing organic carbon in cultivated soils through various agricultural practices42, such as limiting tilling, and composting (rather than burning) crop residues. While these ideas are generating considerable interest, including the COP21 4 per mille initiative43,44, their ability to be scaled up to being relevant for the Paris Agreement is poorly known, due to saturation and other effects. Earlier studies45 suggested a very limited possible role for soil enrichment; however, more recent analyses suggest a physical removal potential of ~200 Gt(CO 2 ) by 210041, i.e., a significant fraction of CDR ref , and this could possibly be increased up to 500 Gt(CO 2 ) by practices such as soil carbon enrichment at greater depths43,44. Soil carbon enrichment may be more closely associated with co-benefits for agriculture than with trade-offs like competition for biomass, so that it might be seen as particularly attractive to pursue in the near term, while trade-offs and similar issues with other techniques are being resolved.

Ocean iron fertilization (OIF) is the proposal to fertilize iron-poor regions of the ocean to spur phytoplankton growth and increase the detritus carbon flux to the deep ocean46. The general conclusion emerging from modelling work, perturbative field studies, and analyses of natural iron enrichments downstream of islands, is that some oceanic carbon uptake could likely be achieved, particularly in the iron-limited Southern Ocean46. However, while early studies indicated that CO 2 removal by OIF might be capable of far exceeding CDR ref , later studies showed that this neglected many limiting factors, so that the removal capacity is likely less than 400 Gt(CO 2 ) by 210047. Furthermore, this would likely result in significant side effects in the oceans, like disruption of regional nutrient cycling, and on the atmosphere, including production of climate-relevant gases like N 2 O15. Although there are reasons to encourage further research48, the limited removal potential and significant side effects, along with international legal developments that restrict large-scale deployment (see Box 1), make it unlikely that OIF will be employed to contribute significantly to the Paris Agreement goals. It seems similarly unlikely that related ocean carbon cycle techniques, such as using wave-driven pumps to enhance oceanic upwelling and thus increase the rate of mixing of fresh CO 2 into deep-ocean waters, will contribute significantly49.

Many further biomass-based CDR techniques have been proposed, such as accelerating the formation of peatlands, or burying timber biomass in anoxic wetlands. A recent assessment15 has concluded that the expected CO 2 removal capacity for each of these would likely be less than 100 Gt(CO 2 ) by 2100, and several would have significant environmental side effects. Further research may reveal greater CO 2 removal potentials, but current literature indicates that none would be capable of significantly contributing to achieving the Paris Agreement goals.

The biomass-based techniques share a wide range of research needs (Fig. 3), which are relevant to their possible roles in the Paris Agreement context, and can be grouped under three broad categories: (1) the technical carbon removal potential and how this can be increased; (2) social and environmental impacts and how trade-offs can be minimized while capitalizing on co-benefits and synergies; and (3) development and operational costs. Given the current state of research and development, it is not yet possible to generally prioritize any of these categories above the others, although this may be possible in dedicated studies of individual techniques. Several technique-specific aspects of the first two categories were discussed above.

Fig. 3 Schematic of research needs for proposed biomass-based CDR techniques. A broad range of issues would need to be clarified to better understand the removal potentials, costs, trade-offs and risks prior to a possible implementation of any biomass-based CDR technique, as detailed in two recent assessments15,16, including: (1) the most effective biomass types to use for various techniques; (2) the applied technologies, especially for carbon capture and biomass pyrolysis; (3) the scalability, noting that modest deployment levels of biomass-based techniques could largely be constrained to local environmental and socio-economic impacts, while extensive deployment (e.g., at levels comparable to CDR ref ) could result in significant limitations due to land and biomass availability, biomass growth rates, and competition, e.g., for water and nutrient resources, with natural ecosystems, agriculture, and other biomass-based CDR techniques; (4) impacts of choices of biomass types and the extent of implementation on regional biodiversity, wildlife, and overall ecosystem resilience; (5) impacts of differences in the albedo of the respective biomass type (e.g., trees and energy crops) versus the albedo prior to the biomass growth; (6) the carbon payback, i.e., the temporary reduction in effectiveness of a terrestrial biomass CDR technique resulting from CO 2 released due to disturbances to the ecosystem during biomass planting; (7) implications of the production of numerous non-CO 2 gases with impacts on climate and air quality, such as volatile organic compounds (VOCs) like isoprene, and the long-lived greenhouse gas N 2 O; (8) the ability to co-locate biomass processing sites (BECCS plants and biochar pyrolysis facilities) with biomass growth locations and product storage and/or burial sites, as well as the necessary transport infrastructure if these are not co-located; (9) economic implications – not only the operational costs, but also the economic impacts, e.g., due to competition with agriculture Full size image

For the third category, estimating development and operational costs has been particularly challenging, despite their importance in determining whether any technique could viably contribute to climate policy around the Paris Agreement. Published values for all of the techniques discussed above can presently only be taken as broadly indicative, and are typically of the order of $100/t(CO 2 ), with the range of values given in the literature for each technique often being a factor of three or more27,28. This uncertainty is due to numerous factors, including extremely limited commercial experience with full-scale operations (e.g., for CCS or biochar), storage site properties and the details of CO 2 transport or co-location of infrastructure for BECCS, land-use and resource competition with agriculture, and the compensating revenue from electricity or fuels produced by BECCS and biochar plants36. Complicating things further, land and resource competition might result in operational costs for biomass-based CDR techniques actually increasing as implementation scales grow, in contrast to the typical falling costs for most technologies as they grow in scale.

Mineralization-based and other abiotic techniques

Abiotic CDR techniques for removing CO 2 from the atmosphere can be roughly distinguished into two main approaches: spreading weathering materials over large open spaces (enhanced weathering and ocean alkalinisation/liming); and capturing CO 2 in some form of enclosure or on constructed machinery (direct air carbon capture and storage, abbreviated DACCS).

A review of proposals for terrestrial enhanced weathering50 divides these into (1) ex situ techniques, which involve dispersing mined, crushed and ground silicate rocks (e.g., olivine51,52) in order to increase the exposed surface area and thus allow a more rapid uptake of CO 2 , particularly in warm, humid regions where CO 2 removal would be most rapid52, and (2) in situ techniques, which are forms of underground geological/geochemical sequestration (see Box 2). Similarly, ocean alkalinization has been proposed via distribution of crushed rock into coastal surface waters53, as slowly sinking micrometre-sized silicate particles deposited onto the open-ocean sea surface54,55, or via dispersion of limestone powder into upwelling regions56. Ocean alkalinization would contribute to counteracting ocean acidification, in turn allowing more uptake of CO 2 from the atmosphere into the ocean surface waters. Terrestrial enhanced weathering could also enhance ocean alkalinity, via either riverine run-off, or mechanized transport and mixing of the alkaline weathering products into the oceans, though both may vary strongly regionally. Further proposals include combining enhanced weathering and ocean alkalinisation using silicates to neutralize hydrochloric acid produced from seawater57, or heating limestone to produce lime (combined with capture and storage of the by-product CO 2 ), which has been a long-standing proposal for dispersal in the oceans to increase ocean alkalinity58, in turn allowing additional CO 2 uptake from the atmosphere by the ocean.

Due to the abundance of the required raw materials, the physical CO 2 removal potential of enhanced weathering is principally much larger than CDR ref . However, since the current rate of anthropogenic CO 2 emission is ~200 times the rate of CO 2 removal by natural weathering59, the surface area available for reactions would need to be increased substantially via grinding and distribution of the weathering materials. This would imply large investments, including energy input, for the associated mining, grinding and distribution operations. Given that removing a certain mass of CO 2 requires a similar mass of weathering material, the operations would need to be comparable to other current mining and mined-materials-processing industries, which could have significant impacts on sensitive ecosystems, as could the large amounts of alkaline weathering products that would be produced, especially in the runoff regions, about which very little is presently known.

DACCS could possibly be designed so that it requires a substantially reduced dedicated land or marine surface area compared to other CDR techniques, and might also allow the environmental impacts to be more limited and quantifiable. However, scaling up from small-scale applications of direct air capture technologies, such as controlling CO 2 levels in submarines and spaceships60,61, to removing and storing hundreds of Gt(CO 2 ) would involve substantial costs, especially due to the high energy requirements of three main technology components: (1) sustaining sufficient airflow through the systems to continually expose fresh air for CO 2 separation; (2) overcoming the thermodynamic barrier required to capture CO 2 at a dilute ambient mixing ratio of 0.04%; and (3) supplying additional energy for the compression of CO 2 for underground storage.

While components (1) and (3) can be quantified using basic principles, and several studies61,62 indicate that combined they would probably require 300–500 MJ/t(CO 2 ) (or ~80–140 kWh/t(CO 2 )), the energy and material requirements of the separation technology (2) are much more difficult to estimate. The theoretical thermodynamic minimum for separation of CO 2 at current ambient mixing ratios is just under 500 MJ/t(CO 2 )62. However, thermodynamic minimum values are rarely achievable. Current estimates for the efficiency of DACCS are technology-dependent, ranging from at best 3 to likely 20 or more times the theoretical minimum61, or ~1500–10,000 MJ/t(CO 2 ), implying that removing an amount equivalent to CDR ref by 2100 would require a continuous power supply of approximately 400–2600 GW. Combined with the energy requirements for (1) and (3) (equivalent to about 100 GW), this represents about 20–100% of the current global electricity generation of ~2700 GW.

A wide range of chemical, thermal, and also some biological (algae and enzymes) techniques have been proposed for the separation technology, but the focus of research has been on two main approaches60,62,63,64,65: adsorption onto solids, e.g., amine-based resins that adsorb CO 2 when ambient air moves across them, followed by release of concentrated CO 2 by hydration of the resins in an otherwise evacuated enclosure; and absorption into high-alkalinity solutions with subsequent heating-induced release of the absorbed CO 2 . While the environmental and societal impacts of these technologies could likely be much better constrained in comparison to the other CDR techniques, they are still important to consider, and include environmental impacts due to placement of the capture devices and CO 2 storage sites, mining and preparation of materials like resins that would be used in the systems, and the possible release of amines and other substances used in the separation process66.

The physical CO 2 removal potential of DACCS far exceeds CDR ref , provided the high energy requirements could be met; there are no significant principal limitations in terms of the material availability or CO 2 storage capacity (see Box 2), and even the manufacture of millions of extraction devices annually would not be unfeasible (compared to, e.g., the annual global manufacturing of over 70 million automobiles). Large investments in DACCS might, however, be unlikely as long as large point sources (e.g., power or industrial plants) continue to be built and operated, since the same effective reduction of atmospheric CO 2 levels via CCS applied to higher-concentration sources will generally be much less energy intensive and thus less expensive than CO 2 capture from ambient air61. In general, for any possible longer-term application of CDR in climate policy, a major lynchpin will likely be development of CCS, both in terms of the carbon capture technologies and the storage infrastructure, since CCS is fundamental to both BECCS and DACCS, and since it is likely to be most economically favourable to first apply CCS to remaining large point sources.