Future Warm–Dry and Warm–Wet Compound Climate Extremes in Mediterranean Metropolitan Areas under Climate Change

The Mediterranean region is widely recognized as a climate change hotspot, as anthropogenic warming is projected to substantially increase air temperatures by the end of the 21st century, together with longer periods of reduced rainfall. The region is likely to experience warmer and drier conditions with significant consequences for human societies, while the intensification of heatwaves is likely to trigger cascading hazards. At the same time, heavy precipitation events during hot periods may become more common, increasing the likelihood of urban flash floods, especially in densely populated metropolitan areas.

Instead of focusing only on single climate extremes,, compound extremes offer a complementary perspective for assessing future climate risks. We analyze two compound climate indices: Warm/Dry (WD) and Warm/Wet (WW) days. The analysis focuses on representative Mediterranean metropolitan areas characterized by high population density and climatic relevance.

The indices are derived from daily mean temperature and precipitation data obtained from an ensemble of CMIP6 climate model simulations. Annual and seasonal frequencies of compound extremes are evaluated for the mid-century (2041–2060) and late-century (2081–2100) periods, relative to a 1995–2014 reference period, under the SSP2-4.5 and SSP5-8.5 scenarios. Results indicate a robust increase in the frequency of Warm/Dry days across all future scenarios, suggesting that Mediterranean climates will increasingly experience concurrent warming and drying. In contrast, Warm/Wet days are scenario-dependent. These findings highlight a dual climate risk for Mediterranean cities, where more frequent prolonged hot and dry conditions coexist with a higher chance of compound heat and heavy precipitation events under high-emission scenarios.