France has seen searing temperatures in successive heatwaves over the past few weeks, but it’s not only on land that temperatures are insufferably high. The Mediterranean Sea’s surface temperature reached a record high 30.7°C in late July, and marine heatwaves are becoming increasingly common because of climate change – with dramatic consequences for biodiversity.
As Europe battles wildfires and record drought on land, rising sea temperatures pose another kind of threat. On July 24, the temperature in the Mediterranean reached a peak of 30.7°C off the coast of Alistro in eastern Corsica, according to the Keraunos meteorological observatory. The next day, in the bay of Villefrance-sur-Mer – an idyllic beach town a few miles from Nice – a researcher at the local oceanographic laboratory recorded a temperature of 29.2°C.
“It’s unprecedented,” said the researcher, Jean-Pierre Gattuso. The Mediterraean’s temperature is usually between 21° and 24°C at this time of year.
“What we’re seeing is a marine heatwave,” Gattuso said. “Like the heatwaves we get on land, it’s characterised by unusual temperatures for the season and can go on for several days or even weeks.”
In this case, Gattuso said, record temperatures have been continuing since the end of June and are affecting the entire western Mediterranean, from the heel of the Italian boot to Spain.
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This climatic anomaly is linked to the successive heatwaves that have ravaged southern and western Europe in recent weeks.
“The temperature in the atmosphere and the temperature in the ocean work in tandem,” said oceanographer Carole Saout-Grit at Paris’s CNRS research institute. “When we talk about global warming, we’ve got to remember that 90 percent of the heat that has accumulated since the pre-industrial era has been absorbed by the ocean.”
“When you’ve got excess heat in the atmosphere, the ocean will try to suck it out, so that can cause the water to overheat,” Saout-Grit continued. But for the sea to overheat, there must be no wind. And that is “precisely the situation in the Mediterranean at the moment – otherwise, a gust of wind would allow the water at the surface to mix with the cooler water at the bottom, and the overall temperature would drop”.
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These marine heatwaves don’t just happen in the Mediterranean. “The Pacific Ocean, particularly the North Pacific … has already been affected by this phenomenon,” Gattuso noted. Marine heatwaves have also been observed in the South Atlantic and even the Arctic.
These sudden, atypical spikes in temperature – which come on top of the long-term trajectory of the oceans warming – have disastrous consequences for aquatic fauna and flora. “With a team of 70 scientists, we’ve studied the impact in the Mediterranean for the period 2015-19. We found out that 90 percent of the area had been affected and that around 50 species had suffered deaths on a large scale,” Gattuso said.
On the other side of the world, marine heatwaves are also contributing to the bleaching of the Great Barrier Reef, turning the coral white. According to an Australian government report published in May, 91 percent of the reef has suffered bleaching due to a prolonged heatwaves during the southern hemisphere’s summer season.
Around 50 percent of the world’s coral reefs are considered to be under threat from climate change.
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This article was translated from the original in French.
