Rising sea surface temperatures in coastal waters are driving 50 to 64 percent of the increase in large-scale humid heatwaves according to new research.

The study, from researchers at the Potsdam Institute for Climate Impact Research (PIK), Princeton University and Sun Yat-sen University, concluded that coastal sea surface temperatures could be a potential early warning indicator for widespread humid heat extremes.

Humid heatwaves have intensified rapidly in recent decades and are projected to worsen, raising the risk of heat-related mortality. Previous research has indicated that even healthy people can succumb to wet bulb temperatures – a measure combining heat and humidity – above 31.5°C, when the body is no longer able to cool itself by sweating. However, the widespread occurrence of humid heatwaves across large regions, like the 2023 heatwave in Asia, has so far remained poorly understood.

“We see a strong link between warming coastal waters and clustered hot, humid extreme events, especially in the tropics, where oceans supply more moisture to the atmosphere, which is then transported to land, amplifying the heat”, said lead author and PIK scientist Fenying Cai. “In regions further from the equator, combined land-ocean warming, linked to large-scale atmospheric wave patterns, also plays an important role.”

Analysing climate data from 1982 to 2023 using a complex network approach, the international research team uncovered strong land-ocean linkages. For example, warming Indian Ocean waters are closely tied to rising humid heat risks in South Asia and the Middle East, while tropical North Atlantic Ocean warming intensifies risks in northern South America. The study also shows that these ocean influences are stronger for large-scale events than for isolated local heatwaves.

“Better understanding these land-ocean interactions can help to improve climate adaptation strategies. Crucially, coastal sea surface temperatures emerge as a potential early warning indicator for widespread humid heat extremes,” added co-author and PIK scientist Jürgen Kurths.

The warning comes after the World Meteorological Organization (WMO) issued its State of the Global Climate report 2025 which concluded the Earth’s climate is more out of balance than at any time in observed history, as greenhouse gas concentrations drive continued warming of the atmosphere and ocean and melting of ice. These rapid and large-scale changes have occurred within a few decades but will have harmful repercussions for hundreds – and potentially thousands – of years.

It added last year, ocean heat content (to a depth of 2,000 metres) reached the highest level since the start of records in 1960, exceeding the previous high set in 2024.

Over the past nine years, each year has set a new record for ocean heat content.

The rate of ocean warming over the past two decades, 2005–2025, is more than twice that observed over the period 1960–2005 – and is about 11.0–12.2 Zetajoules per year – about 18 times the annual human energy use per year.

Despite La Niña conditions, around 90% of the ocean surface area experienced at least one marine heatwave in 2025.

“Ocean warming has far-reaching consequences, such as degradation of marine ecosystems, biodiversity loss and reduction of the ocean carbon sink,” The WMO explained. “It fuels tropical and subtropical storms and exacerbates ongoing sea-ice loss in the polar regions.”