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Marine heatwaves are becoming more severe and more frequent, and they’re lasting longer. These prolonged periods of abnormally high ocean temperatures can hamper marine ecosystems, and now, new research suggests they may be making life more sweltering on land, too.

Leiqiu Hu, an atmospheric scientist at the University of Alabama in Huntsville, found that major coastal cities around the world are experiencing higher air temperatures and humidity when there is a marine heatwave in the adjacent ocean. This link is stronger in the summer, and more prominent for coastal cities at higher latitudes. People in coastal cities need to be aware of this when considering strategies to mitigate the damaging effects of heatwaves, Hu says.

Heatwaves are among the most dangerous natural hazards, according to the World Health Organization. Between 1998 and 2017, heatwaves killed more than 166,000 people, including more than 70,000 during the 2003 heatwave in Europe. Hot and humid conditions are particularly uncomfortable and dangerous because the moisture in the air makes it harder for the body to cool by sweating.

While techniques like urban greening can help reduce the urban heat island effect, which makes cities hotter than the surrounding countryside, it is harder to control the temperature of the ocean. “There is not one strategy for all,” Hu explains.

Hu discovered the link between terrestrial and marine heatwaves after analyzing satellite and ground-based measurements of ocean surface and air temperature for 38 cities, identifying marine and urban heatwaves that occurred between 1982 and 2019. She grouped the cities into four latitudinal zones: north temperate (from Helsinki, Finland, to Tokyo, Japan), north subtropics (Osaka, Japan, to Taipei, Taiwan), tropics (Hong Kong, China, to Rio de Janeiro, Brazil) and south subtropics (Cape Town, South Africa, to Melbourne, Australia).

The analysis showed that coastal marine heatwaves are now lasting longer, occurring more often, and are more intense than four decades ago. These increases were particularly pronounced in the north temperate zone, where most large coastal cities are located. There, the duration, frequency, and intensity of marine heatwaves all almost doubled over the study period. Hu found that marine heatwaves overlap, in almost all cases, with higher air temperature and humidity in nearby cities.

The heat index, a measure of how hot it feels that combines air temperature and relative humidity, also increased in almost all the cities during marine heatwaves. On average it rose by 1.9 °C, but again this was more pronounced in the north temperate zone, with average increases of 2.3 °C. And in some places, it was even higher. In Seoul, South Korea, the heat index increased by an average of 4 °C during coastal marine heatwaves, compared with a usual summer day, Hu found.

Hu suspects that wind blows heat from the ocean onto land, though she can’t say precisely how the ocean is making coastal cities hotter. “This study provides evidence there is strong linkage between the coastal cities and the oceans, but the mechanism could be complex,” Hu says.

Erich Fischer, an expert on climate and weather extremes at ETH Zurich in Switzerland, says this finding is important as it brings together different fields looking at heatwaves on land and in the ocean, capturing the area in the middle: the coast.

With the evolution of bigger, stronger marine heatwaves, it will likely be the case that the air blowing in off the ocean is increasingly going to be warm and humid—bad news for coastal dwellers used to relying on the cool ocean breeze to keep them sane when the mercury spikes.