A precautionary approach to carbon dioxide removal needs

A central finding of the most recent reports of the Intergovernmental Panel on Climate Change (IPCC) is that net zero CO2 emissions are required for stabilizing CO2-induced global average surface temperature increase. However, given substantial uncertainties of the earth system response after reaching net zero CO2, outcomes of long-term declining, or continuously increasing global temperatures after achieving and maintaining net zero CO2 emissions cannot be excluded. At the same time, nearly all emission reduction pathways  assessed by the IPCC, which are consistent with global climate goals, require large-scale deployment of carbon dioxide removal (CDR) to balance remaining positive emissions and to draw down temperatures after a peak. Using evidence from Earth System Models and Simple Climate Models we show that peaking global mean temperature may potentially require net CO2 removals of the order of several hundred gigatonnes (Gt) sustained over multiple decades in case of strong positive earth system feedbacks. We also estimate the contribution of scenario uncertainty (e.g., reduction in emissions of different greenhouse gases) to such net CO2 removal figures. A precautionary approach to ensure a very high chance of peaking of global mean temperature may therefore require the availability of substantial amounts of CDR. Our findings have direct implications for CDR needs in order to achieve the long-term temperature goal of the Paris Agreement. The potential risk of strong earth system feedback outcomes have to be carefully considered when discussing temperature reversal in case of a (temporary) overshoot above the 1.5°C threshold, in particular as there are well-documented biophysical, technological and sustainability limits to CDR deployment.