2023-06-22 03:24:59
Less spectacular than the fires in Canada, the unprecedented heat wave currently hitting the waters of the Atlantic will cause, according to scientists, an invisible slaughter of marine species, an extreme phenomenon likely to repeat itself with the worsening of global warming.
Between March and May, the average ocean surface temperature hit an all-time high in 174 years of measurements, exceeding the 20th century average by 0.83°C, according to data from the US Oceanographic Administration NOAA. .
This marine heat wave did not spare the Atlantic Ocean, which in June suffered particularly strong heat waves from the south of Iceland to Africa, with temperature anomalies of more than 5°C in off the British Isles.
“Such temperature anomalies in this part of the North Atlantic are unheard of,” said Daniela Schmidt, professor of Earth Sciences at the University of Bristol, quoted by the British Science Media Center.
“Extremely strong anomalies, quite striking and worrying”, confirms Jean-Baptiste Sallée, oceanographer and climatologist at the CNRS.
This marine heat wave, with a temperature exceeding 23°C in the North Atlantic, does not completely surprise scientists, who know that the oceans absorb 90% of the heat generated by the greenhouse effect. This type of event is therefore destined to become more frequent and intense under the effect of global warming.
“What is a surprise is that things are going extremely fast”, notes however Jean-Pierre Gattuso, director of research at the CNRS (National Center for Scientific Research) and co-editor of a report by the Giec (climate experts from the ‘UN).
Several hypotheses have been put forward to explain this extreme phenomenon, such as the reduction of Saharan dust transported by the wind or that of sulfur emissions from ships, two types of aerosols which normally have a cooling effect on the atmosphere.
But “it remains in the state of hypotheses”, estimates Mr. Sallée.
As for the El Niño phenomenon, it seems too underdeveloped to have an impact on the North Atlantic. “We would rather expect an effect next spring,” explains Juliette Mignot, oceanographer at the IRD (Research Institute for Development).
The researcher envisages a possible “modification of sea currents” or a meteorological phenomenon that would be superimposed on global warming.
Whatever the origin of this ocean heat wave, scientists expect it to cause “mass mortalities” of marine species, including corals and invertebrates. “But as it happens under the surface of the ocean, it will go unnoticed,” regrets Ms. Schmidt.
During the heat waves in the Mediterranean, about fifty species (corals, gorgonians, sea urchins, molluscs, bivalves, posidonia, etc.) were affected by “massive mortalities between the surface and 45 meters deep”, according to Mr. Gattuso, co-author of an article on the subject.
Other species will rather migrate towards the poles. “The waters of Norway and Iceland will for example become more fishy”, to the detriment of the countries of the intertropical zone, according to the researcher.
By warming up, the ocean, which captures a quarter of the CO2 emitted by humans, could even eventually lose part of its role as a carbon pump. This would then have “an amplifying effect on atmospheric warming”, underlines Ms. Mignot, referring to a “tipping point”.
“These tipping points, we know that they exist, but it is difficult to know at what level of warming they are triggered”, confirms Mr. Sallée. “We know that potentially, between 2°C and 3°C of warming, tipping points can be triggered”.
By the end of the century, the IPCC predicts a 50-fold increase in the frequency of these ocean heat waves in its most pessimistic scenario, with episodes whose intensity will be multiplied by ten.
But “we can limit the damage,” reassures Mr. Gattuso. “If greenhouse gas emissions follow a trajectory compatible with the Paris Agreement, we can completely stop the warming and acidification of the ocean”, he assures. “All is not lost.”
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