Mitigation choices impact carbon budget size compatible with low temperature goals

Global-mean temperature increase is roughly proportional to cumulative emissions of carbondioxide (CO2). Limiting global warming to any level thus implies afinite CO2 budget. Due to geophysical uncertainties, the size of such budgets can only be expressed in probabilistic terms and is further influenced by non-CO2 emissions.We here explore how societal choices related to energy demand and specificmitigation options influence the size of carbon budgets formeeting a given temperature objective.We find that choices that exclude specific CO2mitigation technologies (like CarbonCapture and Storage) result in greater costs, smaller compatible CO2 budgets until 2050, but larger CO2 budgets until 2100. Vice versa, choices that lead to a larger CO2mitigation potential result in CO2 budgets until 2100 that are smaller but can bemet at lower costs. Inmost cases, these budget variations can be explained by the amount of non-CO2mitigation that is carried out in conjunction withCO2, and associated global carbon prices that also drivemitigation of non-CO2 gases. Budget variations are of the order of 10% around their central value. In all cases, limitingwarming to below 2 °C thus still implies that CO2 emissions need to be reduced rapidly in the coming decades.