This summer, heat waves are set to increase. On June 9, the global average temperature reached 16.77°C, a record for a month of June since records began in 1980. The same alert for the oceans, which have never been so hot. The Atlantic, in June, reached 1.2°C compared to the average for the period 1980-2022. Jean-Pierre Gattuso, research director at the CNRS, works at the oceanography laboratory in Villefranche-sur-Mer (Alpes-Maritimes). He explains to Le Point the phenomenon of sea heat waves.
Le Point: What is a marine heat wave?
Jean-Pierre Gattuso: It is when, at sea, we go beyond the usual temperature limits for the month or day in question, in the same way as on Earth, with the air temperature. Temperatures are regularly averaged between 1980 and today. Then we look at how the temperature has varied during that time. Between May and the end of September 2022, we had a sea heat wave in the bay of Villefranche-sur-Mer, near Nice. We were 1.1 degrees above the previous temperature record, which was the heat wave of 2003. We experienced 134 days of sea heat wave, which had never happened since we made measurements in the bay of Villefranche. , that is, since the 1950s.
How does this phenomenon occur?
The marine heat wave occurs when the air is particularly hot. It is jointly linked to an air heat wave. Also, if the area lacks wind for a long enough period, the water does not mix vertically. The surface layer therefore remains very hot. In the bay of Villefranche-sur-Mer, with an air temperature of around 25 degrees, all it takes is a blow of the mistral to bring the temperature down to 8 to 9 degrees. Otherwise, the hot air warms the water and therefore the seabed.
Should we expect more sea heatwaves?
The 2019 IPCC Ocean Report showed that marine heat waves have doubled in intensity and frequency since the mid-1980s. It is estimated that their frequency could increase by a factor of 20 to 50 by 2100, according to the greenhouse gas emissions scenario. The increasingly intense and frequent sea heat waves are not at all a surprise. Modeling oceanographers have predicted the phenomenon for a long time.
What are the consequences for the marine ecosystem?
On the biological side, there are two main types of consequences. The first is the mass mortality of vertebrates and plants. The phenomenon of coral bleaching demonstrates the extent of the process. On Australia’s Great Barrier Reef, extreme temperatures are bleaching corals and killing many of them.
This phenomenon is not observed only in the tropics. A study on the Mediterranean and heat waves, dating from last year, shows the association between heat waves and mass mortality at sea. It counts around fifty species that suffer from mortality. Molluscs, including bivalves, oysters, mussels, corals, gorgonians, sea urchins.
The second consequence for organisms and the marine ecosystem is a change in biodiversity. Species move to try to stay in their thermal optimum. The fish of the intertropical zone move north towards Iceland, Norway, which see the arrival of more species in their territorial waters. These movements can amount to several hundred kilometers per decade.
And for the man?
With regard to fishing, the decrease in fish in the intertropical zone is a real problem for the food security of these countries. Then, we can mention the increase in the frequency and intensity of storms and cyclones. Like storm Alex, in 2020, in the Alpes-Maritimes.
The engine of storms and cyclones is the evaporation of the sea. There is a lot of water vapor that rises in the column of air above the sea. It forms extremely water-laden clouds. . If they encounter cold air masses, all the moisture that has accumulated is released very suddenly, creating incredible storms and precipitation.
Is it possible to limit these sea heat waves?
There is only one way to act, and that is to reduce the concentration of greenhouse gases in the air. This requires an energy transition to fuels that emit little or no CO₂. We can also, as a last resort, try to remove carbon dioxide from the atmosphere. But we do not know how to use this technique on a large scale. It’s much better not to release it into the atmosphere than to try to remove it.