Compound coastal marine-terrestrial heatwaves associated with humid-heat stress in Europe.

European coastal regions - home to dense populations, and climate-sensitive ecosystems - are vulnerable to the compounding effects of marine-terrestrial heatwaves, defined here as concurrent extreme heat events over land and adjacent ocean areas. Using satellite and station-based observations, we find a nonlinear and accelerating increase in exposure to these compound events across European coastlines over the past two decades, peaking at 78 days in the Mediterranean in 2022. Attribution analysis reveals that greenhouse gas (GHG) forcing accounts for 95% of the risk of the 2022 event, and the CESM1-LE simulations indicate that such an event would be virtually absent without GHG forcing, emphasizing the nonlinear escalation of risk as GHG emissions continue. We further demonstrate that simultaneous marine heatwaves in the adjacent ocean can amplify coastal terrestrial heatwave exposure by up to 3.5 times, transforming short-lived terrestrial heatwaves into prolonged episodes of extreme heat and humidity. During compound events, coastal regions exhibit a pronounced shift toward humid heatwave regimes, characterized by wet-bulb temperatures [Formula: see text]C and elevated specific humidity. Our finding reveals an amplifying effect of marine heatwaves on the persistence of coastal terrestrial heat extremes through enhanced moisture and heat coupling at the land-sea interface, underscoring the growing climate vulnerability of coastal populations.