In the Atlantic Ocean, an unseen heat wave source of mass deaths

Less dramatic than the fires in Canada, the unprecedented heat wave currently battering the waters of the Atlantic Ocean will, according to scientists, cause an invisible slaughter of marine species, an extreme phenomenon that is likely to recur as global warming worsens.

Between March and May, the average ocean surface temperature reached an all-time high in 174 years of measurements, exceeding the 20th century average by 0.83 degrees Celsius, according to data from the US Ocean Administration NOAA.

This marine heat wave did not spare the Atlantic Ocean, which in June experienced particularly severe heat waves from southern Iceland to Africa, with temperature differences of more than 5 degrees Celsius off the British Isles.

“Such temperature differences in this part of the North Atlantic are unheard of,” Daniela Schmidt, professor of earth sciences at the University of Bristol, quoted in the British Media Science Centre.

“Anomalies are very strong, startling and very disturbing,” confirms Jean-Baptiste Sale, an oceanographer and climatologist at the National Center for Scientific Research.

This marine heat wave, with temperatures exceeding 23°C in the North Atlantic, doesn’t exactly surprise scientists, who know that the oceans absorb 90% of the heat generated by global warming. Thus this type of event is destined to become more frequent and intense under the influence of global warming.

“Surprisingly, things are moving so quickly,” notes Jean-Pierre Gattuso, director of research at the CNRS (National Center for Scientific Research) and co-editor of a report by Giec (the climate experts from the ‘United Nations).

Several hypotheses have been put forward to explain this extreme phenomenon, such as the reduction of wind-borne desert dust or sulfur emissions from ships, two types of aerosols that usually have a cooling effect on the atmosphere.

But it “remains in the hypotheses,” Mr. Sally estimates.

As for El Niño, it appears to be too far behind to have an effect on the North Atlantic. “We prefer to expect an impact next spring,” explains Juliette Minot, an oceanographer at the IRD (Institute for Development Research).

The researcher envisions a possible “modulation of sea currents” or a meteorological phenomenon that could be superimposed on global warming.

Whatever the origin of this ocean heat wave, scientists expect it to cause a “mass die-off” of marine species, including corals and invertebrates. “But as it happens beneath the surface of the ocean, it will go unnoticed,” Ms. Schmidt laments.

During heat waves in the Mediterranean, about fifty species (corals, gorgonians, sea urchins, molluscs, bivalves, posidonia, etc.) – Author of an article on the topic.

Other species prefer to migrate towards the poles. According to the researcher, “the waters of Norway and Iceland, for example, will become more suspicious,” at the expense of the countries of the tropical region.

Through warming, the ocean, which captures a quarter of the carbon dioxide emitted by humans, could eventually lose part of its role as a carbon pump. This would then have an “amplifying effect on atmospheric warming”, Ms Minot asserts, noting the “tipping point”.

“These tipping points, we know they exist, but it’s hard to know what level of warming they’ve caused,” asserts Mr. Sally. “We know that potentially, between 2 and 3 degrees Celsius of warming, tipping points can occur.”

By the end of the century, the Intergovernmental Panel on Climate Change projects a 50-fold increase in the frequency of ocean heatwaves in its most pessimistic scenario, with episodes that will multiply in intensity by ten.

But “we can limit the damage,” Mr. Gattuso reassures. He asserts that “if greenhouse gas emissions follow a trajectory consistent with the Paris Agreement, we can completely halt ocean warming and acidification.” “All is not lost.”