Sensitivity of observed changes in compound climate extremes in Europe to event definition

Compound climate extremes pose an increasingly severe threat to our societies, ecosystems, and economies under climate change. While there is a growing body of work on compound extremes, a clear scientific consensus on the appropriate metrics to characterize such extreme events does not yet exist. Here, we investigated the sensitivity of three types of compound events across Europe (compound drought–heatwave, compound heatwave–extreme precipitation, and hydroclimatic whiplash events) using a total of 39 combinations of drought metrics, accumulation periods, and heatwave definitions across the three event types. We assessed observed changes in event frequency, duration, intensity, and timing during 1950–2024 using the E-OBS dataset. For all compound event types considered, our results revealed a high sensitivity of both magnitude and spatial extent of detected changes to the adopted drought index and heatwave definition. In particular, the adoption of drought indices and heatwave definitions accounting for evapotranspiration and nighttime temperature, respectively, yields higher increases in frequency and intensity of compound events compared to drought indices only considering precipitation or heatwave definitions relying on daytime temperatures. Our work revealed the central role of event definitions in shaping the detection, interpretation, and physical understanding of compound extremes, with important implications for climate trend assessment and regional hazard mapping.