A normalised framework for the Zero Emission Commitment: competing controls by thermal and carbon processes

Climate models reveal a range of global surface temperature responses after net zero, generally a slight cooling, but sometimes a slight continued warming. This post emission response is affected by a range of processes including carbon uptake by the land and ocean, planetary heat uptake and time-varying climate feedback. To reveal their relative importance, a normalised framework is set out for the Zero Emissions Commitment (ZEC),  connecting the change in surface temperature (normalised by the change at net zero)  to changes in the atmospheric carbon inventory, radiative forcing, planetary heat uptake and climate feedback. Whether the temperature decreases or continues to rise after net zero is controlled by opposing contributions from (i) a weakening in radiative forcing due to a decrease in atmospheric carbon from the uptake by the land and ocean carbon sinks versus (ii) a strengthening in the surface warming due to a decline in ocean heat uptake and sometimes augmented by time-varying climate feedbacks. Inter-model differences in the post emission temperature response for the ZEC Model Intercomparison Project scenario are primarily determined by differences in the ocean uptake of heat and the land uptake of carbon, followed by differences in the ocean uptake of carbon and time-varying climate feedbacks.