CDR (Carbon Dioxide Removal)

Carbon Dioxide Removal (CDR) refers to a portfolio of techniques and technologies that extract carbon dioxide from the atmosphere and store it durably, distinguishing it from emissions reduction (which avoids putting CO₂ into the air in the first place) and from solar radiation modification (which does not address atmospheric CO₂ concentrations at all).

The Intergovernmental Panel on Climate Change (IPCC) treats CDR as essential to most modeled pathways consistent with limiting warming to 1.5°C or 2°C, particularly to counterbalance residual emissions from hard-to-abate sectors (aviation, cement, agriculture) and to achieve net-negative emissions later in the century. The IPCC's Sixth Assessment Report (AR6, 2021–2022) explicitly states that deployment of CDR is "unavoidable" if net-zero targets are to be met.

CDR methods span a wide spectrum of maturity and cost:

• Land-based biological: afforestation, reforestation, soil carbon sequestration, biochar.
• Ocean-based: ocean alkalinity enhancement, ocean fertilization, coastal blue carbon restoration.
• Engineered / geochemical: direct air capture with carbon storage (DACCS), bioenergy with carbon capture and storage (BECCS), enhanced rock weathering, mineralization.

Each method differs in storage durability (decades for soils versus millennia for geological storage), measurement/verification challenges, land and water footprint, energy demand, and cost per tonne. DACCS, for example, currently runs at several hundred dollars per tonne of CO₂, while reforestation can be far cheaper but is more vulnerable to reversal through fire or land-use change.

CDR is governed indirectly through a patchwork of instruments: the UNFCCC and the Paris Agreement (especially Article 6 on cooperative approaches and market mechanisms), the London Protocol (which regulates marine geoengineering), and the Convention on Biological Diversity. Critics warn of a moral hazard: that the prospect of future removals may weaken near-term mitigation ambition. Proponents counter that residual emissions and historical accumulation make some CDR mathematically necessary regardless of mitigation speed.