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Why it matters. A “great wave of marine heat” weakens the Mediterranean

“A major marine heat wave” has been affecting the western Mediterranean since the end of May, with “exceptional” temperatures “4 to 5 degrees” higher than normal, threatening marine ecosystems, experts in the climate change of this sea have indicated. What exactly does this mean? What are the consequences of this wave on our country? Explanations.

What is a marine heat wave?

This is one of the effects of climate change. Under the effect of heat waves and increasingly frequent droughts over a large part of the globe, the sea is warming up. According to one of the scenarios of the Intergovernmental Panel on Climate Change (IPCC), the surface temperature of the oceans could increase by 3.2°C by 2100.

Why is the Mediterranean more worried?

The Mediterranean Sea is one of the fastest warming seas. Temperatures there are rising 20% ​​faster than the global average, according to a WWF France report published in June 2021. It is one of the hotspots of climate change: over the past forty years, the surface temperature has increased by 1. 5°C. This means that the Mediterranean Sea has already exceeded the goal of limiting global warming set by the Paris Agreement of 2015.

In addition, if the Mediterranean Sea covers less than 1% of the planet’s ocean surface, 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 has “the highest proportion of threatened marine habitats in Europe”.

When did this wave start?

“This great marine heat wave 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, said Karina Von Schuckmann, German oceanographer with Mercator Ocean International.

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). ).

How many degrees are we talking about?

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 “above normal, of the order of +4 to +5°C”, reveals the Copernicus Marine Service (CMEMS).

Concretely, what are the consequences?

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 the climate (IPCC).

“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.

Does this wave bring species changes?

Yes. And that’s a problem. Fish like the very colorful “peacock wrasse or the barracuda, which have started to move north from the southern Mediterranean, are likely to be more abundant” in the western Mediterranean, according to Charles-François Boudouresque.

“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 Charles-François Boudouresque.

Is there an impact on our daily life?

Yes. The socio-economic effects are expected to be “cascading”, especially on fishing, underlines Karina Von Schuckmann. The whole industry is in danger.

But also on our climate. If the sea temperature is high, especially at the end of the hot season, in September-October, the possibilities of Cevennes episodes increase. These heavy autumn rains can generate medicanes, mini-cyclones that feed on the heat of the sea. “In a few years, perhaps even this year, these phenomena can become very problematic because they will cause violent storms on the Mediterranean coast,” worries meteorologist Guillaume Séchet. The entire Mediterranean region could be affected: Languedoc-Roussillon, Cévennes, Provence, southern Alps and Corsica.

Is the impact only local?

No. The oceans absorb 30% of carbon dioxide – the main greenhouse gas – produced by humans. Admittedly, this “damped” the effects of global warming, but it destroys habitats and flora, then fauna and, ultimately, humans.

“The link between the ocean and climate change is indisputable,” explains Peter Thomson, special envoy of the United Nations Secretary-General for the oceans. The ocean is the planet’s largest carbon sink and has absorbed more than 90% of the excess heat attributed to anthropogenic greenhouse gas emissions. Ocean acidification rates, which correspond to the dissolution of CO2 in seawater, are currently about 10 times higher than anything we have experienced in the last 300 million years. The effects of this process on marine ecosystems such as coral reefs are devastating, which is very worrying for the future of marine biodiversity, since these reefs are home to around 25% of all marine life. »

Is this the first heat wave in the Mediterranean?

No. These marine heat waves have already “doubled in frequency since the 1980s”, according to the IPCC report published in August 2021.

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 Karina Von Schukmann.

How to cure it ?

Unsurprisingly, “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 Karina Von Schuckmann. However, the establishment of marine protected areas could help. 30% of the sea surface should be protected by 2030.

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