The oceans are warm too. Although it may seem less spectacular than the Canada fires, an unprecedented heatwave is currently hitting Atlantic waters and will result in an unseen slaughter of marine life, according to scientists. Like their terrestrial counterparts, these extreme phenomena, which are likely to repeat themselves as global warming intensifies, can last for several days or even several months and affect thousands of square kilometers.
When can one speak of a marine heat wave?
A marine heatwave occurs when seawater temperatures exceed seasonal limits for at least five days. “Many marine heat waves originate from terrestrial heat waves, but that’s not the only factor: favorable physical conditions are also required.” in Villefranche-sur-Mer and co-author of the Special Report of the Intergovernmental Panel on Climate Change (IPCC). on the oceans and cryosphere, published in 2019.
Clear skies, high outside temperatures and no wind to cool the water… Here is an explosive cocktail that could lead to an ocean heatwave. In normal times, the ocean is able to regulate outside temperatures: in winter it mitigates the cold and in summer it mitigates the heat by hiding the warmth of the atmosphere in its depths. Experts estimate that since human-induced climate change began, 90% of excess heat has been trapped by the oceans. However, during an ocean heatwave, this thermostat effect is drastically reduced: surface water remains frozen and cold water at depth no longer rises.
This month of June, therefore, the Atlantic Ocean is experiencing particularly intense heat waves from southern Iceland to Africa, with temperature anomalies of more than 5°C off the British Isles. “Unheard of” in this part of the North Atlantic, where the temperature exceeds 23°C, says Daniela Schmidt, professor of geosciences at the University of Bristol, quoted by the Science Media Center, a British organization. The event commemorates last year’s historic heatwave in the Mediterranean Sea, during which the ocean temperature surpassed 30 degrees in places. A breathtaking temperature that can be reached in calm and flat areas.
Is the heat wave in the sea surprising?
Since the oceans regulate the global climate, it’s no wonder their temperature is rising. According to the United States Observation Agency (NOAA), between March and May, the average sea surface temperature reached an all-time high in 174 years of measurement, exceeding the 20th-century average by 0.83 degrees. This value is worrying, although it is below the temperature of +1.2 degrees observed on land. The oceans have a great deal of inertia: it takes more energy to heat a volume of water than air. For example, the season changes are less pronounced there than on Earth. The same applies to the consequences of climate change. “Increasing the average temperature of the oceans by a few tenths of a degree on a surface that covers 70% of the planet requires an enormous amount of energy,” says CNRS oceanographer and climatologist Jean-Baptiste Sallée. At the beginning of April, the average surface temperature of the oceans even exceeded 21°C. Unimaginable since the surveys began in 1981.
Why does this anomaly occur so early in the year?
However, the very strong anomaly currently occurring in the North Atlantic surprised researchers who did not expect to observe it so soon. As part of the El Niño natural weather phenomenon, which officially began in June and is characterized by the warming of a vast reservoir of surface water stretching from the central Pacific to the coasts of Peru and Ecuador, some of the heat buried in the deep ocean is being spewed out, leading to heat spikes. But the “Terrible Child of the Pacific,” which occurs for nine to 12 months on average every three to seven years, still seems too dormant to have any impact on the North Atlantic. “We prefer to expect an effect next spring,” Juliette Mignot, an oceanographer at the Research Institute for Development (IRD), tells AFP.
Several hypotheses have been put forward to explain the extent of this marine heatwave. For example, the researchers are thinking of a possible change in ocean currents – the large current rotating clockwise and involving the Gulf Stream has an important impact on ocean temperature. Heat from the atmosphere would interfere with global warming. Also highlighted is the reduction of windborne Saharan dust or sulfur emissions from ships, two types of aerosols that normally have a cooling effect on the atmosphere. But the question is not yet settled.
What are the consequences for biodiversity and humans?
Whatever the origin of this ocean heatwave, scientists predict it will lead to a “mass extinction” of marine species, including corals and invertebrates. “But since it happens under the sea surface, it will go unnoticed,” says Daniela Schmidt of the University of Bristol. During the heat waves in the Mediterranean, around fifty species (corals, gorgonians, sea urchins, molluscs, bivalves, posidonia, etc.) were affected by “massive deaths between the surface and 45 meters deep”, reports Jean-Pierre Gattuso.
Other species migrate more toward the poles. “The waters of Norway and Iceland, for example, are becoming richer in fish,” to the detriment of the countries in the inner-tropical zone, the researcher explains. Therefore, fisheries could be affected by these fish movements. Without forgetting that the ocean, which absorbs a quarter of the CO2 emitted by humans, could eventually lose part of its role as a carbon pump, as could terrestrial forests, carbon sinks whose health is steadily deteriorating. In fact, due to the lack of water and the high temperatures, the trees grow more slowly, which slows down the absorption of CO2.
According to experts, this loss of efficiency would have an amplifying effect on global warming. How can we reach a tipping point, those warming thresholds, beyond which parts of the climate system transition into a new, often irreversible, state that has major impacts on climate, ecosystems, and human societies? “We know that there is a potential for tipping points to be triggered between 2 and 3°C of warming,” answers Jean-Baptiste Sallée. By the end of the century, the IPCC, in its most pessimistic scenario, predicts a fiftyfold increase in the frequency of these ocean heatwaves, with episodes increasing in intensity tenfold. “But we can limit the damage,” assures Jean-Pierre Gattuso. If greenhouse gas emissions follow a trajectory compatible with the Paris Agreement, we can stop ocean warming and acidification completely. All is not lost.”