Knowing what we know now: predicting ZEC with observables in a simple climate model

The Zero Emissions Commitment (ZEC) is understood to be the result of two evolving processes in the time after net zero: cooling due to carbon uptake by the land and ocean, and warming due to decreasing heat uptake by the ocean. The balance between these warming and cooling effects is what determines whether we can expect additional global temperature change after emissions stop, or whether zero emissions marks the point of temperature stabilisation. But are there observables prior to net zero that can predict which way this balance will fall?

Using the simple climate model MAGICC, we find ZEC to be a function of a handful of variables in the years leading up to net zero: global surface temperature, carbon uptake, ocean heat uptake and effective radiative forcing. This simple regression performs well for predicting additional global temperature change 50 years after net zero, with reasonable predictability 100, 200 and 1000 years after emissions stop. We find that higher warming at net zero increases the probability of a positive ZEC. We test the predictability of this model in FAIR, and assess the agreement with ESM and EMIC results from ZECMIP. We investigate the potential for constraining ZEC using this model and observables available today.