Short answers, pulled from the story.
Carbon dioxide removal (CDR) is the deliberate removal of carbon dioxide from the atmosphere and its durable storage in geological, terrestrial, or ocean reservoirs, or in products. Methods range from planting forests and biochar production to direct air capture and ocean alkalinity enhancement. The IPCC defines CDR as including both the enhancement of biological or geochemical sinks and direct air capture and storage.
As of 2023, CDR removes approximately 2 gigatons of carbon dioxide per year, almost entirely through low-tech methods such as reforestation. This equals about 4 percent of the greenhouse gases emitted annually by human activities.
A 2019 consensus study 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.
Carbon capture and storage (CCS) collects carbon dioxide from point sources such as power plant smokestacks and sequesters it, but does not reduce carbon dioxide already in the atmosphere. CDR, by contrast, actively draws down existing atmospheric concentrations and is therefore classified separately by the IPCC.
Direct air capture costs between US$94 and $600 per tonne of carbon dioxide removed. Nature-based solutions such as reforestation cost less than $50 per tonne, while biochar costs between US$200 and $584 per tonne. The higher cost of engineered methods reflects their more durable and verifiable carbon storage.
The concern is that the prospect of large-scale future CDR deployment could reduce near-term motivation to cut emissions, since actors may assume CDR will compensate later. The 2019 NASEM report stated that arguments to delay mitigation because negative emissions technologies will provide a backstop "drastically misrepresent their current capacities and the likely pace of research progress."