State Dependence of Zero Emissions Commitment (ZEC) in Multi-Model TIPMIP Simulations

The Zero Emissions Commitment (ZEC) - the change in global temperature after CO2 emissions cease - plays a key role in quantifying remaining carbon budgets and assessing the reversibility of global temperature under carbon removal. ZEC has often been assumed to be close to zero in policy-relevant assessments. However, emerging single-model studies suggest that ZEC is not a fixed quantity, but may vary substantially with global warming level (GWL).

We present the first coordinated multi-model assessment of ZEC state dependence using results from the TIPMIP protocol. This analysis extends previous single-model studies by applying a consistent framework across Earth System Models (ESMs) to evaluate post-emissions temperature evolution following a common emissions-driven ramp-up to multiple GWL targets. We combine multi-century ESM simulations with a two-layer energy balance model to attribute ZEC to the evolving balance between committed ocean heat uptake warming and carbon-sink-driven cooling from land and ocean.

Preliminary intercomparisons suggest that models show relatively similar post-emissions temperature behaviour at lower GWLs (≤2K), remaining close to zero ZEC, whereas responses at higher GWLs are more varied, with most models continuing to warm. This coordinated analysis will deliver new understanding of the processes driving ZEC state dependence, with direct implications for TCRE assessments, IPCC carbon budget estimates, and the design of CO2 removal pathways.