Betting on CDR under uncertain climate sensitivity is bad climate policy

Climate change mitigation strategies consistent with the Paris Agreement’s temperature targets rely heavily on future carbon dioxide removal (CDR). Although such strategies have drawn considerable critique for long, e.g., that they are ‘betting on negative emissions’, the risks from this betting have not been quantified nor addressed properly. We use a lightweight integrated assessment model SCORE to explore possible scenarios using CDR for limiting global warming to 1.5 °C by 2100. Particularly, we quantify the impacts of relying on CDR when accounting for 1) possible under- and overestimation of the cost, potential, and availability (feasibility) of future CDR and 2) the compound effect with uncertainty in climate sensitivity.

All scenario results unquestionably show that aggressive near-term mitigation is required for limiting warming to 1.5 °C by 2100 for all levels of climate sensitivity, but that some amount of CDR is likely required in the future even if climate sensitivity turns out to be extremely low. If uncertainty in climate sensitivity is disregarded, initial assumptions on the CDR feasibility have only minor effects on the total cumulative mitigation cost. However, taking the uncertainty in climate sensitivity into account changes this conclusion. Wrong assumptions on CDR feasibility can, surprisingly, even lead to lower costs under extreme realizations of climate sensitivity, especially in scenarios where CDR feasibility is underestimated. Assuming low feasibility for CDR eliminates the possibility of sky-rocketing costs associated with overestimating CDR feasibility in combination with a high climate sensitivity. Therefore, a prudent climate policy should assume a low feasibility of CDR to reduce the risk of leaving runaway mitigation costs to future generations.