The Earth system is a very complex and dynamical one basing on various feedbacks. This makes predictions and risk analysis even of very strong events as floods, landslides, or heatwaves a challenging task. In particular, the large-scale concurrence of humid heatwaves substantially escalates the risk of heat-related mortality.
We use a complex network approach applied to climate reanalysis data to show that the observed intensification of humid heatwaves is closely associated with coastal oceanic
warming over the period 1982–2023. This linkage is more pronounced for the large-scale aggregation of extreme humid heatwaves than for the locally confined events. In particular, approximately 50% and 64% of the upward trends in humid heatwave frequency and spatial-aggregation strength over hotspot regions are linked to their adjacent oceans, respectively. Compared with mid-to-high latitudes, the tropics encompass most high-risk areas and
exhibit stronger land–ocean linkages, highlighting the critical role of tropical oceans. Our study provides insights that coastal sea surface temperature can be a crucial precursor of the large-scale aggregation of humid heatwaves.

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