Human-induced escalation of compound coastal marine–terrestrial heatwave associated with humid-heat stress on European coasts

European coastal regions—home to dense populations, critical infrastructure, and climate-sensitive ecosystems—are increasingly vulnerable to the compounding effects of marine–terrestrial heatwaves, defined here as events in which a marine heatwave fully encompasses a terrestrial heatwave within paired coastal land and adjacent ocean grid cells. Using satellite-based sea surface temperatures and station observations, we show a non-linear and accelerating rise in exposure to compound heatwave days across European coastlines over the past two decades. Of the analyzed regions, the Mediterranean exhibits the steepest increase, reaching nearly 78 compound marine-terrestrial heatwave days by 2022. Attribution analysis indicates that greenhouse-gas forcing (GHG) accounts for ~95% of the risk of the 2022 event (5–95% CI: 93–100%), with the CESM1-LE model estimating its probability of occurrence under a no-GHG-forcing scenario as less than 5% (5–95% CI: 7%–0%). Notably, the increase in exposure time in response to GHG forcing is nonlinear: the rarer the event under a no-GHG-forcing scenario, the greater its frequency amplification under GHG forcing. Results further reveal that coastal marine heatwaves amplify terrestrial heatwave exposure by up to 3.5 times. During compound events, land-based wet-bulb temperatures exceed 25.5 °C and specific humidity surpasses 19 g/kg, marking a clear shift toward humid heatwave regimes in coastal regions. These findings highlight the amplifying effect of marine heatwaves on the persistence of coastal terrestrial heat extremes through enhanced moisture and heat coupling at the land–sea interface. This underscore the urgent need for the development of targeted, region-specific adaptation strategies across Europe’s most vulnerable coastal zones.