Understanding the Mechanisms Behind Zero Emissions Commitment (ZEC) at Different Warming Levels

The Zero Emissions Commitment (ZEC) describes the climate response following the cessation of emissions and is critical for understanding long-term climate projections and remaining carbon budgets. Using simulations from the UK Earth System Model (UKESM), we explore ZEC behaviour across stabilized warming levels (WLs) following the protocol developed for TIPMIP. UKESM simulations reveal a strong dependence of ZEC on WL: while ZEC is near zero for WL <=2K, it becomes increasingly positive at higher WLs. This behaviour underscores the importance of disentangling the contributions of the different underlying processes to understand the mechanisms driving ZEC variability.

To explore the drivers of this behaviour, we analyse ZEC across a range of WLs focusing on both the thermal response and carbon cycle dynamics. We find that changes in physical feedbacks dominate the WL dependence of ZEC. However, the carbon cycle response still exhibits notable dynamics: land carbon uptake saturates after a few decades, while ocean uptake persists for centuries, shifting the balance between land and ocean contributions over time. While the climate response is approximately linear during the ramp-up phase, we hypothesize that ZEC is influenced by both the magnitude and duration of warming, reflecting a dependence on the system’s distance from equilibrium. These results highlight the critical role of WL-dependent responses in shaping long-term climate commitment and provide new insights into the mechanisms driving the variation in ZEC across scenarios.