The Zero Emission Commitment Model Intercomparison Project (ZECMIP) contribution to CMIP6: Quantifying committed climate changes following zero carbon emissions

Abstract. The amount of additional future temperature change following a complete cessation of CO 2 emissions is a measure of the unrealized warming to which we are committed due to CO 2 already emitted to the atmosphere. This " Zero EmissionsCommitment" (ZEC) is also an important quantity when estimating the remaining carbon budget – a limit on the total amount of CO 2 emissions consistent with limiting global mean temperature at a particular level. In the recent IPCC Special Report on Global Warming of 1.5 °C, the carbon budget framework used to calculate the remaining carbon budget for 1.5 °C included the assumption that the ZEC due to CO 2 emissions is negligible and close to zero. Previous research has shown significant uncertainty even in the sign of the ZEC. To close this knowledge gap, we propose the Zero EmissionsCommitment Model Intercomparison Project (ZECMIP), which will quantify the amount of unrealized temperature change that occurs after CO 2 emissions cease and investigate the geophysical drivers behind this climate response. Quantitative information on ZEC is a key gap in our knowledge, and one that will not be addressed by currently planned CMIP6 simulations, yet it is crucial for verifying whether carbon budgets need to be adjusted to account for any unrealized temperature change resulting from past CO 2 emissions. We request only one top priority simulation from comprehensive general circulation Earth SystemModels (ESMs) and Earth SystemModels of Intermediate Complexity (EMICs) – a branch from the 1 % CO 2 run with CO 2 emissions set to zero at the point of 1000 PgC of total CO 2 emissions in the simulation – with the possibility for additional simulations, if resources allow. ZECMIP is part of CMIP6, under joint sponsorship by C4MIPand CDRMIP, with associated experiment names to enable data submissions to Earth System GridFederation. All data will be published and made freely available.