An invisible heat wave source of mass mortality in the Atlantic Ocean

Less spectacular than the fires in Canada, the unprecedented heat wave currently hitting Atlantic waters will cause an invisible massacre of marine life, according to scientists, an extreme event that global warming will worsen and repeat.

Between March and May, the average sea surface temperature reached 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 experienced strong heat waves from Iceland south to Africa in June, with temperature anomalies of more than 5 °C outside the British Isles.

“Such temperature anomalies are unheard of in this part of the North Atlantic,” said Daniela Schmidt, professor of geosciences at the University of Bristol, as quoted by the British Center for Science Media.

“Very strong anomalies, very striking and worrying”, confirms Jean-Baptiste Sally, oceanographer and climatologist at the CNRS.

This ocean heat wave, with temperatures exceeding 23 degrees Celsius in the North Atlantic, did not completely surprise scientists who know that the oceans absorb 90% of the heat generated by the greenhouse effect. These types of events should become more frequent and intense under the effects of global warming.

“One surprise is that things are happening so fast,” notes Jean-Pierre Cattuso, director of research at the CNRS (National Center for Scientific Research) and co-author of Giec’s report. UN).

Several hypotheses have been proposed to explain this extreme phenomenon, including the reduction of wind-borne Saharan dust or sulfur emissions from ships, two types of aerosols that generally have a cooling effect on the atmosphere.

But “it is at the level of hypotheses,” said Mr. Sully assesses.

As for the El Niño phenomenon, it appears to be underdeveloped enough to impact the North Atlantic. “We expect an effect next spring,” explains IRD (Institute for Research in Development) oceanographer Juliet Mignaud.

A meteorological phenomenon that the researcher attributed to possible “reversal of ocean currents” or global warming.

Whatever the origin of this ocean heat wave, scientists expect it to cause “massive die-offs” of coral reefs and marine invertebrates. “But because it happens beneath the surface of the ocean, it goes unnoticed,” laments Ms. Schmidt.

During heat waves in the Mediterranean Sea, about fifty species (corals, gorgonians, sea urchins, molluscs, bivalves, posidonia, etc.) suffer from “massive mortality between the surface and 45 meters deep”, says Mr. Gattuso. – Author of an article on the topic.

Other species migrate towards the poles instead. “The waters of Norway and Iceland, for example, will become overfished,” the researcher says, harming countries in the tropics.

By warming, the ocean, which captures a quarter of the CO2 emitted by humans, will eventually lose part of its role as a carbon pump. This would then have an “amplifying effect on global warming,” Ms. Mignot underlines and refers to the “tipping point”.

“These hotspots, we know they exist, but it’s hard to know to what extent they’re causing warming,” Mr. Sully confirms. “We know that between 2 degrees Celsius and 3 degrees Celsius of warming, tipping points can be triggered”.

By the end of the century, the IPCC predicts that the frequency of these ocean heat waves will increase by a factor of 50.

But “we can minimize the damage,” assures Mr. Gattuso. “If greenhouse gas emissions follow a path consistent with the Paris Agreement, ocean warming and acidification can be halted completely,” he promises. “All is not lost.”