Carbon dioxide removal (CDR), sometimes called negative emissions, encompasses a range of techniques that pull CO₂ out of the ambient atmosphere and lock it away on timescales relevant to the climate system. CDR is distinct from point-source carbon capture and storage (CCS), which intercepts CO₂ at smokestacks before it enters the air, and from solar radiation management, which alters Earth's energy balance without removing greenhouse gases.
Methods commonly grouped under CDR include:
- Afforestation and reforestation — growing forests to sequester carbon in biomass and soils.
- Soil carbon sequestration — farming practices that increase organic carbon in agricultural soils.
- Bioenergy with carbon capture and storage (BECCS) — burning biomass for energy while capturing the resulting CO₂.
- Direct air capture (DAC) — chemical processes, often with sorbents or solvents, that extract CO₂ directly from ambient air.
- Enhanced rock weathering — spreading crushed silicate minerals that react with CO₂.
- Ocean-based approaches — including ocean alkalinity enhancement and macroalgae cultivation.
- Biochar — pyrolyzed biomass added to soils.
The IPCC's Sixth Assessment Report (AR6, 2021–2022) concluded that CDR deployment is unavoidable if the world is to reach net-zero CO₂ and limit warming to 1.5°C, since some residual emissions from aviation, agriculture, and heavy industry are extremely difficult to abate. The Paris Agreement's Article 4.1 references a balance between anthropogenic emissions by sources and removals by sinks in the second half of the century, providing the legal hook for CDR in international climate policy.
CDR raises governance challenges around measurement, reporting, and verification (MRV), permanence of storage, land-use competition, energy demand (DAC is highly energy-intensive), and the risk of mitigation deterrence — the concern that promises of future removals weaken near-term emissions cuts. Article 6 of the Paris Agreement and emerging voluntary carbon markets are key arenas where CDR crediting rules are being negotiated.
Example
In 2024, Microsoft signed multi-year offtake agreements with direct air capture firms including Stockholm Exergi and 1PointFive to purchase hundreds of thousands of tonnes of CDR credits toward its 2030 carbon-negative pledge.
Frequently asked questions
CCS captures CO₂ at industrial point sources before it reaches the atmosphere, while CDR removes CO₂ already in the ambient air. CCS reduces new emissions; CDR creates negative emissions.
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