Enhanced response of extreme compound events to cumulative CO₂ emissions.

Abstract

Compound events-such as concurrent hot-wet and drought-heat extremes-are among the most consequential climate hazards on Earth^1-4 and are projected to become more severe under warming. Although the transient mean temperature response to cumulative CO₂ emissions has been well quantified^5-8, the corresponding response of compound events remains less clear. Here we show that the response of the transient compound events to cumulative CO₂ emissions (TCoRE), defined as the change in event frequency per unit of cumulative CO₂ emissions, is strongly dependent on the background frequency of compound events. In particular, we find that historically frequent compound events increase almost linearly with increasing cumulative CO₂ emissions, whereas rarer and more severe events escalate disproportionately. Moreover, the observationally constrained TCoRE is 37-75% higher than the multi-model average, indicating that compound extremes will occur more frequently than Earth system models project. The constraint also reduces model ensemble uncertainty by 37-56%. Applying the constrained TCoRE further suggests that the allowable CO₂ emissions consistent with limiting warming to 1.5 °C and 2 °C are substantially lower when accounting for changes in compound events. We propose the TCoRE as a simple, robust and observationally constrained metric with direct relevance for climate risk assessment and policy development.