“A new year of record heat for the oceans”. The title of the study published on January 11, 2023 in the journal Advances in atmospheric sciences could not be clearer: the waters of the globe never stop beating temperature records.
In 2022, the amount of heat contained in our oceans and seas increased by 10 zettajoules (i.e. the equivalent of 100 times the world’s electricity production), and thus beat the previous record set… in 2021. Records had also been beaten in 2020 and 2019.
A contribution to sea level rise
This structural warming has a first consequence, palpable even on our coasts: it contributes to raising the level of the seas and oceans.
Because if the melting of continental ice is responsible for part of this rise, the rise in sea temperature is responsible for the other part, due to a phenomenon called ” thermal expansion “, which causes the water to take up more space when it heats up.
“Molecules subjected to heat tend to agitate and move away from each other. Consequently, the volume they occupy in equal numbers is greater when the temperature rises., detailed Futura Sciences . However, this greater volume of water can technically only result in a rise in sea level.
More intense extreme phenomena
This increased heat also results in an intensification of certain extreme weather phenomena.
“The accumulation of heat in the oceans feeds convection and the formation of cyclones”thus explains to West France Catherine Jeandel, oceanographer and geochemist at the National Center for Scientific Research (CNRS). In other words: a warmer ocean provides more ” fuel “ to cyclones, typhoons and other hurricanes, making them more severe and intense.
Episodes of extreme rainfall are also intensified by the warming of the seas and oceans. This is for example the case of the Mediterranean episodes, mentioned by Catherine Jeandel. “If the Mediterranean is warmer than usual at the end of the summer, if it has taken a lot of heat, evaporation will be greater and the air mass that will be pushed by the winds towards the Cévennes and the Alps will be more loaded with water. And when it encounters a mass of cold air, it will be done with more intensity. » The rains that will fall on this occasion will therefore also be more intense.
Suffering marine biodiversity
The trend rise in ocean temperature also has a very strong impact on marine biodiversity.
To illustrate the risks in this area, Catherine Jeandel once again takes up the example of the Mediterranean and the sea heat wave it suffered this summer. Temperatures “6°C higher than average” were then noted there, notes the researcher, explaining that such heat had, for certain species of Mediterranean coral, the effect of a ” fire “.
With each wave of marine heat, biodiversity thus takes “a real blow, with the weakening of endemic species, ecosystems and fishing environments”she summarizes.
The modification of ecosystems does not happen without a collapse
In the longer term, the warming of the oceans also pushes certain fish to migrate, and allows the arrival of other invasive species, native to traditionally warmer waters. An arrival that harms local ecosystems.
“There are more than 900 species that have arrived via Suez and flourish in the Mediterranean, especially in the eastern Mediterranean”, lists Catherine Jeandel. But, she recalls, “invasive species are a major cause of the collapse of biodiversity”. “The modification (of the type of species present in a sector) does not occur without a collapse: for example, if the lionfish and the rabbitfish graze very quickly on the Posidonia meadows (sea grasses typical of the Mediterranean ), it will be a real disaster for all the species that are housed there. »
And such a weakening of ecosystems also has a strong economic impact, notes the researcher: “In the Gulf of Lion, the arrival of blue crabs that eat eels forces us to switch from one economy to another, since the first eat the second. »
Warmer oceans are less efficient carbon sinks
Reheating of the oceans also puts the planet in a vicious circle. Because if they constitute a tremendous carbon sink (they have absorbed 93% of the excess heat emitted by humanity since 1970), the world’s seas are less and less able to play this role when they warm up.
The warmer the ocean, the less it is able to absorb CO2
“Gases are less soluble in warmer water,” reminds LCI Laurent Bopp, research director at the CNRS and specialist in the link between climate and the ocean carbon cycle. So, “the warmer the ocean, the less it is able to absorb CO2 and the more a significant part of our emissions remain in the atmosphere”.
If it is by nature less palpable than that of the atmosphere, the warming of the oceans must therefore be monitored just as much. If not, more.