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What you need to know about carbon dioxide removal » Yale Climate Connections

Carbon dioxide removal (CDR) from the atmosphere continues to be a hot topic. In its newest report, the Intergovernmental Panel on Climate Change (IPCC) concluded that the Paris Climate Agreement targets cannot be met without substantial efforts to remove some of the more than three-trillion tons of carbon dioxide already in Earth’s atmosphere, about one-third of which originates from humans’ burning of  fossil fuels.

Financial services company Stripe recently teamed up with several other companies, including Google parent Alphabet and Facebook parent Meta, to create a Frontier Fund that commits nearly $1 billion to purchase CDR from startups. In mid-April, Jane Flegal left her position as Senior Director for Industrial Emissions in the White House’s Office of Domestic Climate Policy to work on the Frontier Fund, and Stripe has also hired climate scientist Zeke Hausfather, a past regular contributor to this site.

Many climate advocates express concerns that governments and businesses will use CDR as a diversion from efforts to transition away from fossil fuels. Indeed, the IPCC report is clear that maximally ambitious efforts to both mitigate emissions and remove carbon from the atmosphere are needed to meet the Paris targets. To pursue both avenues aggressively, a 2019 paper suggested that governments set separate targets for emissions cuts and for carbon dioxide removal, and the European Union has done just that in its proposed European Climate Law. As Hausfather has noted, delaying emissions reductions today and relying instead on CDR later would be exceptionally expensive:

How much CDR is needed?

The IPCC report noted that CDR can serve three purposes over different timescales.

In the short-term, it can reduce net human greenhouse gas emissions. In the medium term, CDR can offset hard-to-abate emissions from certain industrial activities, agricultural practices, and long-distance transport to achieve the goal of reaching net zero emissions. In the long term, it can draw down the amount of carbon in the atmosphere to gradually reduce global temperatures. Note that the CDR process is distinct from carbon capture and storage (CCS), which captures carbon from point sources like smokestacks in an effort to prevent it from ever entering the atmosphere.

In a 2019 report, the National Academy of Sciences (NAS) estimated that approximately 10 to 20 billion tons (gigatons, or GT) of greenhouse gases come from sources that would be very difficult or expensive to eliminate. That report concluded that approximately 10 GT of CDR per year would be needed by 2050, and perhaps 20 GT per year by 2100. A separate analysis by CDR experts was more optimistic about the feasibility of reducing emissions, especially from the industrial sector: Report authors Andrew Bergman and Anatoly Rinberg concluded that toward the end of the century only approximately 3 GT of hard-to-abate greenhouse gas emissions might remain if decarbonization efforts are highly successful in all sectors.

In short, a reasonable goal to help achieve the Paris targets and establish the possibility of eventually drawing down atmospheric carbon levels and global temperatures would be in the ballpark of 10 gigatons of carbon dioxide removal by 2050.

How to remove carbon from the atmosphere?

CDR falls into two broad categories: natural and technological. On the natural side, Earth’s soils and plants already store more than 3 trillion tons of carbon. So expanding natural carbon storage offers the opportunity to remove a significant additional amount from the atmosphere by leveraging the photosynthesis process.

The 2019 NAS report estimated that given current technology and understanding, about 10 GT of carbon dioxide per year could now be removed from the atmosphere safely through natural solutions globally at a cost of less than $100 per metric ton; however, “achievable limits could be smaller by a factor of two or more … because human behavior, logistical shortages, organizational capacity, and political factors can also limit deployment.”

A $100 per-ton price for CDR is considered to be relatively cost-effective. The NAS estimated that the U.S. could increase its natural carbon storage by about 1 GT per year annually at that price point.

CDR categories. (Source: National Academy of Sciences)

Two other recent papers, led by The Nature Conservancy’s Joseph Fargione in 2018 and by the University of Virginia’s Stephanie Roe in 2021, had findings consistent with those of the NAS regarding the total natural CDR potential globally and specifically for the U.S., but the studies differed in assessing the potential size of the three major individual natural CDR systems: forests, agriculture, and bioenergy.

Forest and agricultural CDR

Forest CDR can be enhanced by reducing deforestation, planting new forests (afforestation), replanting depleted forests (reforestation), or improving forest management. Curbing deforestation is an effective solution in countries where the practice is a problem, for instance in Brazil and Indonesia, but is not relevant in most developed countries such as the U.S. where forestry is better regulated.

Afforestation and reforestation could increase CDR in the U.S. by about 150 million tons (MT) of carbon dioxide per year at less than $100 per ton, according to the NAS and Roe et al., or by 250 MT per year in the Fargione et al. analysis. Improving forest management in the U.S. – for example by harvesting older trees and logging in a manner resulting in lower forest impacts – could remove an additional 40 MT of carbon dioxide per year, according to Roe et al., 100 MT according to the NAS, and 250 MT in the Fargione et al. analysis.

While the Fargione team was the most bullish on forestry solutions, Roe et al. saw the most potential in agricultural CDR. This category can be enhanced by applying regenerative agriculture practices such as cover cropping, no-till farming, agroforestry (incorporating trees and shrubs into farms), applying compost and biochar, and rotational grazing on grasslands. The NAS and Fargione et al. estimated that if implemented in the U.S., these practices could achieve an additional 250 to 350 MT of CDR per year for less than $100 per ton.

Roe et al. concluded that biochar (a charcoal-like substance that’s made by burning organic material from agricultural and forestry feedstocks) alone in the U.S. could achieve 260 MT CDR per year. The study also estimates that shifting to lower-impact grazing practices on managed pastures could achieve a further 146 MT of CDR, plus another 76 MT from agroforestry and 65 MT from cover cropping and no-till farming. Concerning biochar, Fargione’s team noted that “current adoption is negligible due to a variety of cultural, technological, and cost barriers.” Development of facilities to produce both biochar and biofuels may help overcome these obstacles.

Bioenergy and natural CDR challenges

Bioenergy with carbon capture and storage (BECCS) is another potential natural solution. The process involves burning biowaste for energy (from agriculture, forestry, and municipal sources) or from purpose-grown crops like corn that could be used as the feedstock, and then capturing and storing the carbon from the smokestack. BECCS is a favorite solution of climate modelers because it can replace fossil fuel energy and also achieve CDR, since the captured carbon is removed from the atmosphere by plants, unlike carbon capture from burning fossil fuels, which would at best be carbon-neutral.

The NAS report estimated that BECCS could achieve about 4 GT per year of CDR globally and 500 MT in the U.S. for less than $100 per ton. The Roe study put the global number around 2.5 GT if BECCS is able to widely replace fossil fuel energy production. But BECCS so far has been implemented at just one facility, in part because burning vegetation is roughly only half as efficient as burning coal, and because carbon capture technology presents an added cost, so biomass power plants don’t apply it.

Land availability poses another challenge for many natural CDR solutions. For bioenergy, collecting sufficient biowaste is logistically difficult, and devoting suitable agricultural land to growing crops to be burned for energy reduces the land available to grow crops for food. Devoting land to afforestation raises the same land competition obstacle.

Carbon storage permanence poses another challenge. Carbon captured in soils through regenerative agricultural practices can subsequently be released back into the atmosphere if farming practices change. Forestry CDR can similarly be reversed if the trees are killed, for example by climate-worsened wildfires or by bark beetle outbreaks. Scientists from the World Resources Institute recently estimated that global forests declined by 62 million acres in 2021, including 9 million acres of old growth tropical forests that released 2.5 GT of carbon dioxide, about 17% of which were burned by wildfires.

Technological and ocean CDR

Technology-based direct air capture (DAC) can be more reliably permanent if the carbon is stored in stable geologic formations. The DAC process generally involves using fans to blow air across a filter that can capture the carbon, but is very energy intensive and currently expensive.

Climeworks has the only existing commercial DAC machine, in Iceland, where it captures just 4,000 tons of carbon dioxide annually at a reported cost of $600 per ton. The IPCC envisions that direct air capture could achieve 5 to 40 GT per year of CDR globally, but that approach faces significant barriers like currently high costs and the availability of extra clean energy to run the fans.

There are also a number of potential ocean CDR processes. For example, iron fertilization might enhance phytoplankton growth, which could draw carbon from the atmosphere. Large-scale kelp farming could achieve similar ends. But the CDR effectiveness of these methods remains uncertain, as do the impacts on local marine ecosystems. A 2021 NAS report recommends further research into proposed ocean CDR solutions.

The bathtub analogy

Earth’s atmosphere can be thought of like a bathtub that’s close to overflowing. The amount of water the tub can hold represents the Paris agreement’s target carbon budget. The faucet represents human emissions, and CDR is the drain. The water level is already so high that avoiding damage from overflow requires both turning down the faucet and opening the drain as quickly as possible. Neither alone can be done quickly enough to avoid overflow.

(Credit: M. May/Helmholtz-Zentrum Berlin)

Natural CDR solutions could cost-effectively remove 10 GT of carbon dioxide per year globally in the coming years, 10% of which could be achieved by the U.S., but may reach just half those levels given various practical constraints like land availability and politics. Reaching a CDR goal of around 10 GT per year by 2050 would thus require substantial development of new CDR techniques like direct air capture, as is the goal of the Frontier Fund.

But advocates are correct to warn that CDR efforts should not distract or divert resources from efforts to reduce greenhouse gas emissions. The bathtub water level will continue to rise until the faucet is turned off, and right now it’s on full blast.

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