Carbon dioxide removal

• 1. Introduction

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  Carbon dioxide removal, known by its initials CDR, sits at a peculiar crossroads: it is simultaneously one of the most promising tools in humanity's climate arsenal and one of the most misunderstood. As of 2023, CDR pulls roughly 2 gigatons of carbon dioxide out of the atmosphere each year. That sounds large, but it equals only about 4 percent of the greenhouse gases humans emit annually. A gap that size raises an immediate question: what would it take to close it, and at what cost to the land, the oceans, and the economies that would have to bear it?

  The IPCC defines CDR as the deliberate removal and durable storage of atmospheric carbon dioxide in geological, terrestrial, or ocean reservoirs, or in products. That word "durable" is where much of the debate lives. A tree absorbs carbon as it grows, but a wildfire can return every molecule to the sky in days. A geological formation, by contrast, can lock carbon away for thousands to millions of years. The difference between those two timescales shapes nearly every argument about which CDR methods are worth pursuing, and which promises are dangerously hollow.

  Critics, scientists, and policymakers have all converged on one point that is easy to state and difficult to honor: CDR is meant to complement deep cuts in emissions, never to replace them. Oceanographer David Ho put the concern plainly in 2023: "We must stop talking about deploying CDR as a solution today, when emissions remain high, as if it somehow replaces radical, immediate emission cuts."

• 2. The Net Zero Equation

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  Net zero emissions is not the same as zero emissions. That distinction is the reason CDR exists as a policy instrument at all. Certain emission sources, including nitrous oxide from agriculture, aviation, and some industrial processes, are technically difficult to eliminate entirely. CDR is the mechanism that counterbalances what cannot be cut.

  All emission pathways that limit global warming to 1.5 degrees Celsius or 2 degrees Celsius by the year 2100 assume CDR operating alongside emission reductions. After net zero is reached, CDR could continue working, gradually drawing down atmospheric concentrations and partially reversing warming that has already occurred. Climate restoration, in fact, depends on CDR entirely, because restoring the climate system means reducing atmospheric carbon dioxide below current levels, not merely stopping its rise.

  The 2018 landscape made this concrete: all analyzed mitigation pathways that would prevent more than 1.5 degrees of warming included CDR measures. A 2019 consensus study report by the National Academies of Sciences, Engineering, and Medicine estimated that CDR methods deployable safely and economically today, excluding ocean fertilization, could remove up to 10 gigatons of carbon dioxide per year if fully deployed worldwide. The gap between the current 2 gigatons and that potential ceiling is where both the opportunity and the uncertainty reside.

• 3. The Permanence Problem

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  Forests, kelp beds, and other living systems absorb carbon dioxide as they grow, binding it into biomass. The IPCC and researchers classify these as volatile carbon sinks because the sequestration cannot be guaranteed over the long term. Wildfires, disease, economic pressures, and shifting political priorities can all release stored carbon back into the atmosphere.

  Geological storage operates on a different timescale entirely. Carbon dioxide removed from the atmosphere can be injected into the Earth's subsurface or stored as insoluble carbonate salts. Those formations are expected to hold carbon for thousands to millions of years, making them the most durable option currently known. Biomass itself can also be buried directly into the Earth's subsurface, extending what would otherwise be a biological and temporary store into a geological and enduring one.

  Biochar illustrates how the line between biological and geological can be blurred productively. Created by heating biomass at high temperatures in low-oxygen conditions, a process called pyrolysis, biochar sequesters carbon for hundreds to thousands of years rather than decades. A study by the UK Biochar Research Centre estimated that biochar could store 1 gigaton of carbon per year at a conservative level, and potentially 5-9 gigatons per year with greater deployment. At current market prices, biochar commands US$200 to $584 per tonne, higher than nature-based solutions, precisely because its storage is more durable.

• 4. Land, Ocean, and the Scale of Ambition

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  Afforestation and reforestation sit at the top of the technology readiness scale, rated 8 to 9 out of a maximum of 9. They are proven, deployable now, and cost less than $50 per tonne, far below most engineered alternatives. Trees use photosynthesis to absorb carbon dioxide, storing it in wood, organic matter, and soils. Forests take approximately 10 years to ramp up to their maximum sequestration rate, and trees reach maturity after roughly 20 to 100 years, after which they store carbon but no longer actively remove it.

  Ocean-based methods introduce a different set of tradeoffs. Ocean fertilization, the purposeful introduction of plant nutrients to the upper ocean, has been one of the more researched approaches. It would only sequester carbon on a timescale of 10-100 years, and while surface ocean acidity may decrease, sinking organic matter remineralizes and increases deep-ocean acidity. A 2021 CDR report assessed medium-high confidence in the technique's efficiency and scalability, estimating a removal potential of 0.1 to 1 gigatonne per year at a cost of US$8 to $80 per tonne.

  Ocean alkalinity enhancement, which involves grinding and dissolving minerals such as olivine, limestone, silicates, or calcium hydroxide on the ocean floor, carries an estimated removal potential also in the range of 0.1 to 1 gigatonne per year, but at a higher cost of US$100 to $150 per tonne. Electrochemical methods such as electrodialysis can remove carbonate from seawater but cost between US$150 and $2,500 per tonne in isolation; when combined with desalination, where salt and carbonate are removed simultaneously, preliminary estimates suggest the cost could be offset in large part by the sale of the desalinated water produced as a byproduct.

• 5. Direct Air Capture and Its Cost

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  Direct air carbon capture and storage, abbreviated DACCS, sits toward the lower end of the technology readiness scale, meaning it is validated but not yet proven at mass scale. Its costs range from US$94 to $600 per tonne, a wide band that reflects the early state of the technology. What DACCS offers that forests cannot is a dramatically smaller land footprint relative to the carbon it removes.

  A DAC plant capturing 1 million tonnes of carbon dioxide per year requires only 0.4 to 1.5 square kilometers of land. Achieving the same carbon capture rate through trees would require approximately 3,098 to 4,647 square kilometers of forest, the difference between a modest industrial facility and an area the size of a small country. For nations with limited land available for new forests, that ratio matters enormously.

  Land scarcity is not a hypothetical concern. The combined land requirements of removal plans submitted under the global Nationally Determined Contributions in 2023 amounted to 1.2 billion hectares, an area equal to the combined size of all global croplands. Some mitigation pathways proposing high rates of CDR assume that hundreds of millions of hectares of cropland would be converted to growing biofuel crops. That assumption is not a technical problem so much as a social and political one, since the same land grows food.

• 6. Finance, Policy, and the Market Gap

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  As of early 2023, financing for high-tech CDR methods fell well short of what would be needed for them to contribute significantly to climate change mitigation. The increase in available funds that did occur came predominantly from voluntary private-sector initiatives. A private-sector alliance led by Stripe, with prominent members including Meta, Google, and Shopify, revealed in April 2022 a nearly $1 billion fund to reward companies able to permanently capture and store carbon.

  Nan Ransohoff, a senior Stripe employee, described the fund as "roughly 30 times the carbon-removal market that existed in 2021. But it's still 1,000 times short of the market we need by 2050." The gap between what private actors are willing to commit and what the scale of the problem demands has prompted concern among researchers who note that voluntary markets have historically proven orders of magnitude smaller than those driven by government policy.

  Government action has been increasing. Sweden, Switzerland, and the United States all increased their support for CDR as of 2023. In June 2022 the US government issued a Notice of Intent to fund the Bipartisan Infrastructure Law's $3.5 billion CDR program, and the Inflation Reduction Act of 2022 included the 45Q tax credit designed to enhance the CDR market. In Europe, as of 2021, CDR was not yet covered by the EU Allowance system, but the European Commission was preparing carbon removal certification and considering carbon contracts for difference. CDR might also be added to the UK Emissions Trading Scheme, a step that would place removal on the same regulatory footing as reduction for the first time.

• 7. Risk, Moral Hazard, and Hard Limits

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  Reliance on large-scale future CDR deployment was identified in 2018 as a "major risk" to achieving the goal of less than 1.5 degrees Celsius of warming. The concern is that if governments and industries believe that CDR will eventually clean up excess emissions, they may reduce near-term efforts to cut those emissions in the first place. This dynamic has a name in the ethics literature: moral hazard.

  The 2019 NASEM report addressed this directly, concluding that "any argument to delay mitigation efforts because negative emissions technologies will provide a backstop drastically misrepresents their current capacities and the likely pace of research progress." CDR methods that are safely and economically deployable right now collectively reach a ceiling of around 10 gigatons per year, still far below what unchecked emissions would require them to absorb.

  Equitable distribution adds another layer of complexity. Many countries have insufficient land to contribute an equitable share of global CDR, and others lack adequate geological storage capacity. Strategies that rely less on CDR and more on sustainable energy use carry less systemic risk, but even those strategies assume some level of CDR to handle residual and hard-to-abate emissions. The Oxford Principles for Net Zero Aligned Carbon Offsetting call on organizations to gradually increase the percentage of carbon removal offsets they procure, committing to sourcing exclusively carbon removals by mid-century, a benchmark that, given current financing levels, remains a considerable distance away.