Fair carbon removal obligations under climate response uncertainty

Deploying carbon dioxide removal (CDR) is considered unavoidable to meet global climate goals. However, current assessments of the potential role of CDR tend to overlook uncertainty in the Earth System response to our emissions. Here, we assess the level of ‘preventive’ CDR needed to draw warming down to 1.5°C in case of a stronger-than-median Earth System response. Using the ‘1.5°C with no or limited overshoot’ ensemble of pathways assessed by the Intergovernmental Panel on Climate Change (IPCC), we estimate that around 323–787 Gt CO2 (interquartile range) of additional CDR (beyond the 418–763 Gt CO2 (interquartile range) already deployed in these pathways) may be required after net zero CO2 for a very likely (> = 90%) chance of reaching 1.5°C in 2100. We cannot know now whether a net zero society will need to utilize the preventive capacity, but the option must be available to them. Feasibility and sustainability concerns associated with large-scale CDR deployment raise fundamental questions over reducing potential future CDR reliance in light of Earth System uncertainty. Our analysis shows that reducing residual emissions from long-lived (e.g. CO2 and N2O) and short-lived climate forcers (e.g. CH4) can significantly reduce the scale of preventive CDR required. We also explore an illustrative approach to equitably allocate global preventive CDR needs. North America is allocated a per-capita removal responsibility of 13 t CO2/capita annually between 2020 and 2100 in a pathway with limited residual emission cuts, which is more than halved in another with deeper residual emission cuts. Our results underscore the importance of limiting so-called ‘hard-to-abate’ emissions in addition to rapid near-term cuts in emissions as preventive measures to avoid over-reliance on unsustainable levels of preventive CDR.