A normalised framework for the Zero Emissions Commitment

Abstract. The Zero Emissions Commitment (ZEC) measures the transient climate response after carbon emissions cease, defined by whether there is a continued rise or decrease in global surface temperature. A normalised framework for the ZEC connects the surface temperature response post emissions to carbon, radiative and thermal processes, involving changes in carbon inventories, radiative forcing, planetary heat uptake and climate feedback. The normalised ZEC, defined by the surface temperature change since the pre industrial divided by the temperature change at the time of net zero, is controlled by opposing-signed contributions: (i) a cooling contribution from a weakening in radiative forcing due to a decrease in atmospheric CO₂ from carbon uptake by the land and ocean versus (ii) surface warming contributions from a decline in the fraction of radiative forcing used for planetary heat uptake augmented by possible amplification by climate feedbacks. From a set of 9 CMIP6 Earth system models following an idealised atmospheric CO₂ scenario, inter-model differences in the post-emission climate response are primarily determined by differences in the ocean heat uptake and the land and ocean uptake of carbon. These inferences as to the controls of the ZEC broadly carry over for diagnostics of a large ensemble, observationally-constrained efficient Earth system model using two different emission scenarios to reach net zero. The large ensembles reveal a partial compensation between the changes in landborne and oceanborne fractions, as well as revealing ensembles with greater range in amplification of warming by climate feedbacks.