Ocean warming weakens the sea–land breeze in coastal megacities

The sea–land breeze (SLB), driven by thermal contrasts between sea and land, forms a key circulation system in coastal cities which mitigates urban heat islands, improves air quality and supports urban liveability. Yet its response to rising sea-surface temperatures (SST) under global warming remains unclear. Here we simulate SLB evolution across 18 major coastal megacities under varying SST. The results show that historical SST increases have reduced SLB days by 3–45% in 67% of these cities, with mid-latitude cities experiencing the largest declines (29–45%) as a result of a more than 5% reduction in diurnal sea–land thermal contrast. A further 0.52 °C (~2%) SST increase relative to historical levels induces a 4.5-fold reduction in SLB days in high-impact regions and heightened sensitivity in moderate-impact regions under high-emission scenarios. This erosion of SLB days, primarily driven by ocean warming, poses a critical yet overlooked threat to coastal liveability. The sea–land breeze acts to counter urban heat in many coastal cities. Here the authors simulate how this circulation changes with warming ocean water, showing that it decreases in most of them, adding heat stress to urban areas.