A “great wave of marine heat” weakens the western Mediterranean

Europe 1 with AFP 11:38 a.m., July 28, 2022, modified at 11:39 a.m., July 28, 2022

"A great wave of marine heat" has affected the western Mediterranean since the end of May, with "exceptional" temperatures "4 to 5 degrees" higher than normal, threatening marine ecosystems, evolution experts told AFP. climate of this sea.

Exceptional temperatures

"This great wave of marine heat started at the end of May in the Ligurian Sea" located between Italy and northern Corsica, then continued "in June in the Gulf of Taranto", in the south-east of Italy, Karina Von Schuckmann, a German oceanographer with Mercator Ocean International, told AFP.

This non-profit organization based in Toulouse brings together the main institutes specializing in oceanography from France, Italy, Spain, Great Britain and Norway and pilots the European ocean monitoring service, the Copernicus Marine Service (CMEMS). ).

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In July, "from the Balearic Sea (Spain) to Sardinia (Italy), as well as to the east of Corsica and throughout the Tyrrhenian Sea (between Sicily and Corsica), we observe on the surface (..) exceptional temperature values ​​between 28 and 30 degrees Celsius" which are "higher than normal, of the order of +4 to +5°C", added the organization.

One of the world's leading tourist destinations

If for bathers, many around this sea which is one of the world's leading tourist destinations, these temperatures may seem pleasant, they worry scientists and environmentalists.

This marine heat wave can indeed profoundly modify the fauna and flora, leading to "species migrations" towards less warm waters, a possible "mass mortality of species" or a "decrease" of some and "the appearance news", notes Karina Von Schuckmann who is also one of the authors of the reports of the UN Group of Experts on Climate (IPCC).

With cascading socio-economic effects, especially on fishing, she underlines.

Many cases of mass mortality of species

"In the Mediterranean, following the episodes of ocean heat waves in 1999, 2003 and 2006, many cases of massive mortality of species were observed, such as gorgonians (sometimes called bark corals: Editor's note) or Posidonia", indicates a report from the National Center for Scientific Research (CNRS) published in October 2020.

For Charles-François Boudouresque, professor of marine ecology at Aix-Marseille University, the effects of this "marine heat wave" are "under study" but "we can predict a main impact on fixed organisms such as gorgonians and red coral" with "total or partial" mortality.

Fish such as the very colorful "peacock wrasse or barracuda, which have started to move north from the southern Mediterranean, are also likely to be more abundant" in the western Mediterranean, he told AFP.

New species are approaching the French coasts

"Species from the Red Sea, entering the eastern Mediterranean through the Suez Canal" are also approaching the French coast, two cases that could "pose a problem in five to 10 years": rabbitfish and the giant jellyfish Rhopilema.

The first is "an extraordinarily voracious herbivore" that "risks bypassing normal food chains".

Already present off Lebanon, its proliferation in the western Mediterranean could threaten the algal forests that serve as nurseries for other fish.

The giant jellyfish causes serious stings requiring hospitalization and the closure of beaches when it is present, underlines Mr. Boudouresque.

How to fight against sea heat waves?

To fight against these marine heat waves, "we must reduce greenhouse gas emissions" but "even if we stopped emissions today, the oceans, which store 90% of the heat of the Earth system, would continue to heat up" , explains Von Schuckmann.

These marine heat waves have already "doubled in frequency since the 1980s", according to the IPCC report published in August 2021.

Five consecutive years of mass species mortality

Between 2015 and 2019, "the Mediterranean experienced (...) five consecutive years of mass mortality of species" due to these marine heat waves, also underlines an article in the scientific journal Global Change Biology published on July 18 .

"At least since 2003, they have become more regular and they will in the future have a longer duration, take up more space at sea and be more intense and severe", weakening a precious sea in terms of biodiversity, highlights guard Mrs. Von Schukmann.

If the Mediterranean Sea covers less than 1% of the ocean surface of the planet, it is home to "18% of all known marine species", underlines a report by the network of Mediterranean experts on climate change (Medecc) and already presents " the highest proportion of threatened marine habitats in Europe".