Carbon dioxide removal (CDR)

Working Definition

Carbon dioxide removal (CDR) is any deliberate human activity that extracts CO₂ from the open atmosphere and stores it durably in soils, terrestrial biomass, geological formations, the oceans, or long-lived mineral products. Two operational criteria define the concept: the removal must be additional to natural carbon-cycle sinks, and the storage must have verifiable permanence measured in decades to millennia. In popular shorthand, CDR is often conflated either with “planting trees” or with giant fan walls sucking carbon out of the sky; technically, it spans a spectrum of methods from afforestation, soil carbon sequestration, and biochar, through bioenergy with carbon capture and storage (BECCS) and enhanced rock weathering, to direct air capture with geological storage (DACCS). Crucially, CDR is neither point-source carbon capture at a smokestack (CCS)—which merely prevents a new emission from entering the air—nor solar radiation management (SRM), which reflects sunlight without reducing atmospheric greenhouse gas concentrations.

Genesis and Global Lineage

The concept emerged from carbon-budget arithmetic rather than industrial engineering. When integrated assessment modelers in the late 1990s began calculating emission trajectories compatible with stabilizing greenhouse gas concentrations at low levels, they discovered that after decades of delayed mitigation the equations could only be closed by introducing negative emissions in the second half of the twenty-first century. Around the turn of the millennium, several research clusters almost simultaneously formalized bioenergy with carbon capture and storage (BECCS): Kenneth Möllersten and Jinyue Yan in Stockholm, David Keith in Pittsburgh, and an international group centered at IIASA in Laxenburg, Austria, whose letter “Managing Climate Risk” appeared in Science in October 2001. In parallel, physicist Klaus Lackner began developing direct air capture (DAC) in the late 1990s. The decisive institutional turning point was the absorption of negative emissions into IPCC scenarios: in the Fifth Assessment Report (AR5, 2014), the majority of pathways holding warming below 2^\circC relied heavily on BECCS, and the Special Report on Global Warming of 1.5°C (SR15, 2018) made CDR an indispensable feature of every single 1.5^\circC trajectory. SR15 also separated CDR from solar geoengineering, under which it had previously been grouped, and the Sixth Assessment Report (AR6, 2022) codified its modern taxonomy. In public circulation, however, the term has been partially captured by voluntary carbon markets, where avoided-emission credits and fragile forestry offsets are routinely marketed as “carbon removal.”

Central European Lineage

A key founding text of the negative-emissions literature grew out of the research ecosystem of Vienna and Laxenburg, Austria. Michael Obersteiner, lead author of the October 2001 Science letter “Managing Climate Risk,” completed his graduate training in Vienna—at the University of Natural Resources and Life Sciences (BOKU) and the Institute for Advanced Studies (IHS)—and from 1993 worked in the Forestry Program at the International Institute for Applied Systems Analysis (IIASA) in Laxenburg outside Vienna, where he later directed the Ecosystems Services and Management program. Co-authored with Christian Azar, Kenneth Möllersten, Keywan Riahi, and others, the 2001 letter framed BECCS not as a silver bullet replacing mitigation, but as a risk-management insurance policy: if climate sensitivity turned out to be high or near-term abatement proved politically sluggish, humanity would need a technical lever capable of physically drawing down atmospheric carbon. Laxenburg was a natural incubator for that synthesis: since the 1970s, IIASA had housed both global energy-systems modeling and global forestry and land-use modeling under one roof—the exact two halves that BECCS couples together. While parallel work was underway in Stockholm and Pittsburgh, the Vienna–Laxenburg group played a central role in embedding negative emissions inside the integrated assessment models that subsequently shaped IPCC scenarios.

Why Now

Three developments between 2018 and 2026 thrust CDR from modeling appendices into the center of climate and industrial policy. First, the IPCC’s 2018 1.5^\circC report and 2022 Sixth Assessment Report made explicit that every pathway limiting warming to 1.5^\circC or 2^\circC requires removing hundreds of gigatons of CO₂ this century—both to balance “hard-to-abate” residual emissions (aviation, agriculture, cement) and to reverse temperature overshoot. Second, national and corporate net-zero pledges shifted massive weight onto the word “net,” creating immediate demand for a removal industry that barely existed. Third, public subsidies and advance market commitments began to flow: in 2022 the private buyers’ coalition Frontier committed $925 million to permanent carbon removal, Climeworks switched on its Orca (2021) and Mammoth (2024) direct-air-capture plants in Iceland, and the 2022 US Inflation Reduction Act raised the Section 45Q tax credit for DAC with geological storage to $180 per ton. Yet The State of Carbon Dioxide Removal (2023, updated in 2024 and 2026) found that humanity removes roughly 2 GtCO₂ per year—almost entirely through conventional forestry—while novel high-permanence methods (BECCS, biochar, DACCS) account for only about 0.1%.

Explanatory Power

The concept makes clear that “net zero” is an equation with two variables, not one. First, when a national strategy projects 10% or 20% residual emissions in 2050, CDR forces the audit question: “Which physical sink will remove that tonnage every year, who will pay for it, and how long will it stay stored?”—exposing whether a net-zero plan is an engineering budget or a wish list. Second, in corporate claims, CDR draws a bright line between avoided emissions (paying not to cut down an existing forest) and physical removal (extracting CO₂ from ambient air and storing it for centuries); because only the latter neutralizes ongoing fossil-fuel combustion, most cheap voluntary carbon credits fail the net-zero test. Third, in coal-dependent regions (such as Polish debates over retrofitting plants like Bełchatów to burn biomass), CDR reframes the question from “keep open or shut down?” to “does a sustainable biomass supply and geological sink exist to make this net-negative?” Finally, CDR is the only physical mechanism by which early industrializers could clean up their historical stock of cumulative emissions.

Axes of Debate

Underappreciated Angle

The hardest problem in carbon dioxide removal is not chemical engineering, but institutional longevity. Every ton of CO₂ stored in a forest, a peatland, or agricultural soil requires a legal and administrative custodian to keep it from returning to the atmosphere over a century or more—longer than the lifespan of almost all private corporations and most constitutional regimes. Even geological storage requires long-term monitoring and liability rules for wellhead leaks a hundred years after the injection company has dissolved. CDR is therefore at bottom a problem of intergenerational law and insurance: who remains the debtor to the atmosphere long after the upfront payment has been spent? Here the historical experience of Central and Eastern Europe offers a sobering lesson: state forests in Poland or the Czech Republic—today counted as national LULUCF carbon sinks—changed sovereign borders, property regimes, and political systems four times over the twentieth century alone. Seen from this angle, the decisive question about CDR is not “how many gigatons can we capture?” but “how long a contract can human institutions credibly enforce?”