France has seen scorching temperatures in successive heat waves in recent weeks, but it’s not just on land where temperatures are unbearably high. The surface temperature of the Mediterranean Sea reached an all-time high of 30.7°C in late July, and marine heat waves are becoming more common due to climate change, with dramatic consequences for biodiversity.
As Europe battles wildfires and record drought on land, rising sea temperatures pose a different 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 seaside town a few kilometers from Nice, a researcher from the local oceanographic laboratory recorded a temperature of 29.2°C.
“It is unprecedented,” said the investigator, Jean-Pierre Gattuso. The temperature of the Mediterranean usually ranges between 21° and 24°C at this time of year.
“What we are seeing is a marine heat wave,” Gattuso said. “Like the heat waves we get on land, it is characterized by unusual temperatures for the season and can last for several days or even weeks.”
In this case, Gattuso said, record temperatures continue from 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 heat waves that have devastated southern and western Europe in recent weeks.
“The temperature in the atmosphere and the temperature in the ocean work together,” said oceanographer Carole Saout-Grit of the CNRS research institute in Paris. “When we talk about global warming, we must remember that 90 percent of the heat accumulated since the pre-industrial era has been absorbed by the ocean.”
“When there is excess heat in the atmosphere, the ocean will try to suck it up, which 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 on the surface to mix with the cooler water on the bottom, and the overall temperature would drop.”
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These marine heat waves do not only occur in the Mediterranean. “The Pacific Ocean, particularly the North Pacific… has already been affected by this phenomenon,” Gattuso said. Marine heat waves have also been observed in the South Atlantic and even in the Arctic.
These sudden and atypical temperature spikes, adding to the long-term trajectory of ocean warming, have disastrous consequences for aquatic fauna and flora. “With a team of 70 scientists, we have studied the impact on the Mediterranean for the period 2015-19. We found that 90 percent of the area had been affected and around 50 species had suffered large-scale die-offs,” Gattuso said.
On the other side of the world, marine heat waves 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 suffered from bleaching due to prolonged heat waves during the southern hemisphere summer season.
About 50 percent of the world’s coral reefs are considered to be threatened by climate change.
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This article was translated from the French original.