Compound events—such as concurrent hot–wet and drought–heat extremes—are among the most consequential climate hazards on Earth1–4 and are projected to become more severe under warming. Although the transient mean temperature response to cumulative CO2 emissions has been well quantified5–8, 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.
