A new frontier of climate risk: Compound marine–terrestrial heatwaves and their growing impact on Small Island Developing States

Small Island Developing States (SIDS) are increasingly vulnerable to compound marine–terrestrial heatwaves (CMTHWs),a newly recognized form of compound climate extreme that simultaneously affects marine and terrestrial environments. This study presents a comprehensive assessment of CMTHWs across SIDS using a multi-decadal global gridded dataset (1982–2023). Combining event-based heatwave detection with an Event Coincidence Analysis (ECA) framework, we quantify the spatiotemporal characteristics at 25 km pixel resolution, including the frequency, duration, intensity, and population exposure of CMTHWs. Three major findings emerge: (1) Regional-based CMTHW characteristics across SIDS exhibited pronounced temporal variability during 1982–2023, with mean summer frequency, duration, and intensity reaching maxima during 1998–2010 (15.58 events in 1998, 17.08 days in 2006, and 0.69 °C in 2004) before declining to 4.73 events, 8.0 days, and 0.40 °C, respectively, by 2023; (2) Northern Hemisphere SIDS, particularly those in the Caribbean, exhibited relatively high CMTHW probabilities, with Saint Lucia recording the highest probability (51.15%), indicating persistent land–ocean thermal coincidence; (3) Coastal populations in SIDS, including Grenada, Kiribati, and Niue, experienced increasing exposure to CMTHWs, particularly in regions associated with lower humidity anomaly, such as the southern tropical Pacific. These findings improve understanding of the spatial and temporal characteristics of CMTHWs in SIDS and provide a basis for future assessments of compound climate risks in vulnerable coastal regions.