We assess the peer-reviewed literature on the ten pathways, which comprises over 11,000 papers. For the conventional pathways, our scoping review covered over 5,000 papers, a minority (186) of which provide cost estimates. Estimates of potential scale were informed by a structured estimation process and an expert opinion survey. For the non-conventional utilization pathways, we build upon existing CO 2 removal estimates (also derived from a scoping review28 of over 6,000 papers—of which 927 provide usable estimates—and an expert judgement process) and identify preliminary published research on the relationship between CO 2 removal and CO 2 utilization to offer estimates of the scale and cost of CO 2 utilization.

Where possible, we calculate breakeven costs in 2015 US dollars per tonne of CO 2 for each pathway (hereafter, all costs stated are in US dollars). The breakeven CO 2 cost represents the incentive per tonne of CO 2 utilized that would be necessary to make the pathway economic (see Supplementary Materials, S1.2). This can be thought of as the breakeven (theoretical) subsidy per tonne of CO 2 utilization, although we are not recommending such a subsidy.

Conventional utilization pathways

Dependent on a multitude of technological, policy and economic factors that remain unresolved, each of the conventional pathways—chemicals, fuels, microalgae, building materials and CO 2 -EOR—might utilize around 0.5 Gt CO 2 yr−1 or more in 2050. We also estimate that between 0.2 and 3.2 Gt CO 2 yr−1 could be removed and stored in the lithosphere or in the biosphere for centuries or more.

Chemicals

CO 2 can be transformed efficiently into a range of chemicals, but only a few of the technologies are economically viable and scalable. Some are commercialized29, such as the production of urea30 and polycarbonate polyols31. Some are technically possible but are not widely adopted, such as the production of CO 2 -derived methanol in the absence of carbon monoxide32 (methanol is a platform chemical for a multitude of other reaction pathways, including to fuels, and is mainly manufactured via the hydrogenation of a mixture of CO and 1%–2% CO 2 ). Breakeven costs per tonne of CO 2 , calculated from the scoping review, for urea (around −$100) and for polyols (around −$2,600) reflect that these markets are currently profitable. The estimated utilization potential for CO 2 in chemicals is around 0.3 to 0.6 Gt CO 2 yr−1 in 2050, and the interquartile range of breakeven costs obtained from the scoping review is −$80 to $320 per tonne of CO 2 .

Currently, the largest-scale chemical utilization pathway is that of urea production. 140 Mt CO 2 yr−1 is utilized to produce 200 Mt yr−1 of urea 33. Urea is produced from ammonia (which is generated by the energy-intensive Haber–Bosch process; 3H 2 + N 2 → 2NH 3 ) and CO 2 according to 2NH 3 + CO 2 ⇌ CO(NH 2 ) 2 + H 2 O; coal or natural gas typically provides the necessary energy. Within days of being applied as fertilizer, the carbon in urea is released to the atmosphere. For urea to be net zero carbon, it would require its carbon to be sourced from the atmosphere—for example, using direct air capture—and the energy source would need to be renewable. All nitrogen-based fertilizers produce N 2 O, a greenhouse gas that is around 300 times more potent than CO 2 over a 100-year time horizon34. Increasing urea production may therefore have a negative impact on climate35.

For the production of polymers, the utilization potential of CO 2 is estimated to be 10 to 50 Mt yr−1 in 2050. In the current market structure, around 60% of plastics have applications in sectors other than packaging—including as durable materials for construction, household goods, electronics, and in vehicles. Such products have lifespans of decades or even centuries36.

Fuels and microalgae

Fuels derived from CO 2 are argued to be an attractive option in the decarbonization process37,38 because they can be deployed within existing transport infrastructure. Such fuels could also find a role in sectors that are harder to decarbonize, such as aviation39, since hydrocarbons have energy densities that are orders of magnitude above those of present-day batteries32. The long-term use of carbon-based energy carriers in a net zero emissions economy relies upon their production with renewable energy, and upon low-cost, scalable, clean hydrogen production—for example via the electrolysis of water or by novel alternative methods.

Here we consider products such as methanol, methane, dimethyl ether, and Fischer–Tropsch fuels as potential CO 2 energy carriers for transportation. The estimated potential for the scale of CO 2 utilization in fuels varies widely, from 1 to 4.2 Gt CO 2 yr−1, reflecting uncertainties in potential market penetration. The high end represents a future in which synfuels have sizeable market shares, due to cost reductions and policy drivers. The low end—which is itself considerable—represents very modest penetration into the methane and fuels markets, but it could also be an overestimate if CO 2 -derived products do not become cost-competitive with alternative clean energy vectors such as hydrogen or ammonia, or with direct sequestration.

A CO 2 -to-methanol plant operates in Iceland, and various power-to-gas plants operate worldwide. However, these plants represent special cases that may be difficult to replicate because they are exploiting geographic advantages, such as the availability of cheap geothermal energy. Although the production of more complex hydrocarbons is energetically and therefore economically expensive11, rapid cost-reductions could potentially occur if renewable energy—which represents a large proportion of total cost—continues to become cheaper, and if policy stimulates other cost reductions. The US Department of Energy’s target for the cost of hydrogen production—$2 per kg of H 2 —is roughly equivalent to $2 per gasoline-gallon equivalent, and would require carbon-free electricity to cost less than $0.03 kWh−1 (accounting for kinetics and other losses to the enthalpy of electrolysis-based hydrogen production, around 40 kWh per kg H 2 )40. In recent years, several wind and solar power auctions around the world have been won with prices below41 $0.03 kWh−1.

The interquartile range for breakeven costs for CO 2 fuels from our scoping review was $0 to $670 per tonne of CO 2 . Negative breakeven costs appear in studies that model particularly beneficial scenarios, such as low discount rates, free feedstocks, or free or low-cost renewable electricity.

For pathways that have high capital costs, the benefits of economies of scale and learning could be considerable42. This is particularly relevant for the algal pathways thatrequire photobioreactors43 and for the fuel synthesis pathways that require electrolysers44. Microalgae are a subject of long-standing research interest because of their high CO 2 -fixation efficiencies (up to 10%, compared with 1%–4% for other biomass45), as well as their potential to produce a range of products such as biofuels, high-value carbohydrates and proteins, and plastics43. The microalgae pathway has complex production economics and the estimated CO 2 utilization potential for microalgae in 2050 ranges from 0.2 to 0.9 Gt CO 2 yr−1, with a breakeven cost interquartile range from the scoping review of $230 to $920 per tonne of CO 2 .

Concrete building materials

CO 2 utilization pathways in concrete building materials are estimated to remove, utilize and store between 0.1 and 1.4 Gt CO 2 yr−1 over the long term—with the CO 2 sequestered well beyond the lifespan of the infrastructure itself—at interquartile breakeven costs of −$30 to $70 per tonne of CO 2. The high end might reflect a scenario (amongst other possibilities) in which CO 2 is used as a cement curing agent in the entirety of the precast concrete market and in 70% of the pourable cement markets. The estimate also includes aggregates that are produced from carbonated industrial wastes, such as cement and demolition waste, steel slag, cement kiln dust, and coal pulverized fuel ash.

Cement requires the use of lime (CaO), which is produced by the calcination of limestone in an emissions-intensive process. As such, unless calcination is paired with carbon capture and sequestration, it is difficult for building-related pathways to deliver reductions in CO 2 emissions on a life-cycle basis. Several commercial initiatives aim to replace the lime-based ordinary Portland cement—which currently dominates the global market—with alternative binders such as steel-slag based systems46 or geopolymers made from aluminosilicates47.

CO 2 -EOR

Enhanced oil recovery using CO 2 currently accounts for around 5% of the total US crude oil production48. Conventionally, operators aim to maximize both the amount of oil recovered and the amount of CO 2 recovered (rather than CO 2 stored) per tonne of CO 2 injected; between 1.1 and 3.3 barrels (bbl) of oil can be produced per tonne of CO 2 injected under conventional operation and within the constraints of natural reservoir heterogeneity49. However, in principle—and depending on operating conditions and project type—CO 2 -EOR can be operated such that, on a life-cycle basis, more CO 2 is injected than is produced upon consumption of the final oil product50.

More than 90% of the world’s oil reservoirs are potentially suitable for CO 2 -EOR51, which implies that as much as 140 Gt CO 2 could be used and stored in this way5. We estimate a 2050 utilization rate of around 0.1 to 1.8 Gt CO 2 yr−1. If EOR was deployed to maximize CO 2 storage—rather than oil output—then genuine CO 2 emission reductions are possible, depending on the emissions intensity of the counterfactual and on the relevant inefficiencies (Box 1).

At oil prices of approximately $100 bbl−1, EOR is economically viable if CO 2 can be sourced for between $45 and $60 per tonne of CO 2 49,51, implying a breakeven cost of CO 2 of −$60 to −$45 per tonne of CO 2 . These cost estimates (realistically or unrealistically) assume $100 bbl−1 oil prices and are specific to the United States, where the business model is mature.

Non-conventional utilization pathways

The five non-conventional utilization pathways that we review here are BECCS, enhanced weathering, forestry techniques, land management practices, and biochar. Previous reviews18,28,52,53,54 have shown that these pathways offer substantial CO 2 removal potential: a recent substantive scoping review28 gives values of 0.5 to 3.6 Gt CO 2 yr−1 for afforestation/reforestation, 2.3 to 5.3 Gt CO 2 yr−1 for land management, 0.3 to 2 Gt CO 2 yr−1 for biochar, and 0.5 to 5 Gt CO 2 yr−1 for BECCS. Enhanced weathering offers a removal potential of 2 to 4 Gt CO 2 yr−1 at costs28 of around $200 per tonne of CO 2 . Not all of this potential involves utilization of carbon dioxide resulting in economic value, but the approximate scale of CO 2 utilized that is described below could be considerable. The breakeven costs per tonne of CO 2 utilized that we estimate here are low and are frequently negative.

BECCS

BECCS involves the biological capture of atmospheric carbon by photosynthetic processes, producing biomass used for the generation of electricity or fuel, before CO 2 is captured and removed. Although there is substantial uncertainty regarding the total quantity of available biomass55—particularly in light of concerns over competition for land use with food crops—100 to 300 EJ yr−1 of primary energy equivalent of biomass could be deployed by 2050.

BECCS provides two distinct services: bioenergy, and atmospheric CO 2 removal. Although several cost estimates exist in the literature—for example, around $200 per tonne of CO 2 28—these typically assign all costs to the CO 2 removal service, and thus implicitly assume that no revenue is received for the bioenergy services that are generated. By approximating those revenues using a basket of wholesale electricity prices across countries that are suited to host BECCS systems56, we estimate breakeven costs of between $60 and $160 per tonne of CO 2 utilized.

Enhanced weathering

The use of terrestrial enhanced weathering on croplands could increase crop yields28. This yield enhancement is unlikely to originate directly from increases in soil carbon, but from nutrient uptake that is facilitated by pH effects57. However, under our broad definition, there may still be an as-yet-unquantified CO 2 utilization potential associated with the increase in net primary productivity.

Forestry techniques

In afforestation/reforestation, atmospheric CO 2 is removed via photosynthesis and the carbon is stored in standing forests. If used for sustainable forestry, a portion of that carbon enters production processes and, after minor energetic losses, becomes wood products. Both wood products and standing forests provide economic value, and can be seen as CO 2 utilization (standing forests provide ecosystem services, which are not quantified here). The utilization of CO 2 in wood products will occur in addition to the direct removal of CO 2 by forests under certain highly specific circumstances; sustainable harvesting can maintain carbon stocks in forests while providing a source of renewable biomass58,59.

We estimate that, of the volumes of CO 2 sequestered via afforestation/reforestation in 2050, between 0.07 and 0.5 Gt of the CO 2 utilized per year may flow into industrial roundwood products, at approximate breakeven costs of between −$40 and $10 per tonne of CO 2 utilized. An optimistic scenario might also consider the volumes of wood products that are sustainably harvested from existing forests and plantations. Yearly inflows of carbon used as wood products are estimated to be around 1.8 Gt CO 2 in 2050. Of these, 0.6 Gt CO 2 may arise from the portion of those flows that are industrial roundwood products sustainably harvested for use in the construction industry (Supplementary Materials); this leads to a top-end estimate of 1.1 Gt CO 2 utilized per year from afforestation/reforestation and sustainable forestry techniques.

Wood products have potential as long-term stores of carbon—particularly when used in long-lived buildings, the lifespans of which can be conservatively estimated at 80–100 years59. We estimate that around half of the carbon in the wood-product pool might continue to be stored beyond the usable life of the products (the non-decomposed fraction of the portion of total wood products that are presently committed to landfill (around 60%) is approximately 77%60). The remainder of the carbon in the wood-product pool will return to the atmosphere as a fraction (about 0.5 Gt CO 2 yr−1) of the 5 Gt CO 2 yr−1 land-use change flux that is depicted in Fig. 1.

Soil carbon sequestration and biochar

CO 2 in land management and biochar pathways can be considered to be utilized if it enhances economically valuable agricultural output. The CO 2 taken up by land ultimately becomes either CO 2 utilized (with increased output) or CO 2 removed (stored in soils), but not both. We estimate that around 0.9 to 1.9 Gt CO 2 yr−1 may be used by soil carbon sequestration techniques on croplands and grazing lands by 2050; approximate breakeven costs are estimated at between −$90 and −$20 per tonne of CO 2 utilized, owing to yield increases that are associated with increases in soil organic carbon stock. We tentatively estimate that approximately 0.2 to 1 Gt CO 2 yr−1 may be utilized via yield increases after the application of biochar on managed lands, at approximate breakeven costs of between −$70 and −$60 per tonne of CO 2 utilized. These estimates are based on currently reported yield increases (of 0.9% to 2% associated with soil carbon sequestration techniques61,62 and 10% associated with biochar63) from sparse literature, using crop production as a proxy for net primary productivity. Impacts on yield are likely to be highly variable—for example, according to climatic zone64. Crop productivity increases are important not only for economic returns for operators but also for land-use requirements. For instance, if the application of biochar led to an increase in tropical biomass yields of 25%, the associated reduction in land requirements would equate to 185 million hectares, and would result in a cumulative net emission benefit from those increased yields of 180 Gt CO 2 to 210065.

Table 2 presents breakeven cost ranges and estimated volumes of CO 2 utilized or removed per year in 2050.