A carnage for several species. Such is the risk of the sea heat wave which is raging in the month of June in the Atlantic, from the south of Ireland to Africa. Concretely, this state designates the overheating of the ocean. It is decreed when the temperature at the surface of the water is higher than normal for at least five days. This criterion is raised by the current episode, which is breaking heat records.
Anomalies of more than 5°C have been recorded off the British Isles. “In this part of the North Atlantic, it’s unheard of,” said Daniela Schmidt, professor of Earth Sciences at the University of Bristol, quoted by the British Science Media Center. And the other oceans are not left out. Between March and May, the average surface temperature rose 0.83°C above the 20th century average, according to data from the US Oceanographic Administration NOAA. This is equivalent to a record in 174 years of measurements.
Marine heat waves are linked to climate change. The oceans absorb 90% of the heat generated by the greenhouse effect, which contributes to increasing their temperatures. Ocean currents can also create areas of warm water. These episodes are therefore not unusual, but “what is a surprise is that it goes extremely quickly”, comments for AFP Jean-Pierre Gattuso, director of research at the CNRS (National Center for Scientific Research) and co-editor of an IPCC report (UN climate experts).
Marine species in danger
Several hypotheses have been put forward to explain this extreme phenomenon. First, the reduction of Saharan dust transported by the wind or that of sulfur emissions from ships, or a modification of sea currents. But whatever the cause, the heat wave causes the same consequences each time: the mortality of marine species, in particular corals and invertebrates. A similar phenomenon has already occurred in the Mediterranean. A study conducted in eleven countries and published in the journal Global Change Biology had then shown that between 2015 and 2019, around fifty species had experienced massive mortality.
Worse still: sea heat waves are bound to recur. Because of their link with global warming, the IPCC predicts a 50-fold increase in their frequency in its most pessimistic scenario. This same scenario relies on episodes whose intensity will be multiplied by ten. The trend is already confirmed: between 1925 and 2016, the number of sea heat wave days increased by 54%, according to a study published in NatureCommunications.
By warming up, the ocean could even eventually lose part of its role as a carbon pump. This would then have “an amplifying effect on atmospheric warming”, underlines to AFP Juliette Mignot, oceanographer at the IRD (Research Institute for Development). She thus evokes a “tipping point”. Jean-Pierre Gattuso remains optimistic: “we can limit the damage”, he believes. “If greenhouse gas emissions follow a trajectory consistent with the Paris Agreement, we can completely halt ocean warming and acidification. All is not lost.”