Preventive carbon dioxide removal under climate response uncertainty

Carbon dioxide removal (CDR) is now commonly considered an unavoidable part of a mitigation portfolio to meet global climate goals, complementing rapid and sustained cuts in existing emissions. However, current mitigation assessments of the potential role of CDR have tended to ignore the uncertainty in the Earth System response to our emissions. Here, we assess the level of “preventive” CDR to hedge against a stronger-than-median Earth System response. Using the C1 (“1.5°C with no or limited overshoot”) set of pathways assessed by the Intergovernmental Panel on Climate Change (IPCC), we show that the potential preventive CDR, in addition to CDR already deployed in these pathways, for a very likely (>= 90%) chance of reaching 1.5°C in 2100 may be 323 - 787 Gt CO₂ (interquartile range). This is of a similar order of magnitude, and additional to the pathways’ existing assumed deployment, potentially exacerbating existing concerns over large-scale CDR deployment. We show that scenarios that limit residual emissions, both from long-lived (e.g., CO₂ and N₂O) and short-lived climate forcers (e.g., CH₄), can significantly reduce the scale of required preventive CDR. Ensuring preventive CDR capacity is available at scale, if needed after net-zero, will require additional near-term investments. We cannot know now whether a net zero society will need to utilize it but emphasize that the option must be available to them. Our results suggest the need to rethink the role of so-called “hard-to-abate” emission sectors – limiting the emissions in these sectors in addition to rapid near-term cuts in emissions may be crucial to mitigate the worst climate impacts and avoid unsustainable CDR deployment.