The oceans have reached the limit of their capacity to store heat resulting from human activity. No longer able to fulfill their role as a CO2 pump, they are warming inexorably, and the consequences of this rise in temperature, which is damaging marine biodiversity, could be underestimated. For scientists, this trend is typical of the impact of human activity on the oceans and global warming.
According to data from the US Oceanic and Atmospheric Observing Agency, NOOA, the average ocean surface temperature in April, excluding the poles, was 21.1 ˚C. Although temperatures have since fallen slightly due to a natural cycle linked to the end of the Austral winter, they tend to remain above seasonal averages, and the consequences of this warming will not only affect marine fauna.
According to CNRS oceanologist Jean-Baptiste Sallée, changes in ocean temperature are leading to a multitude of regional marine heat waves, acting like underwater fires that can irreversibly degrade thousands of square kilometers of underwater forests, such as Posidonia meadows and coral reefs. He also points out that the most spectacular temperatures are found in the Pacific along Central America, but the anomaly also affects various areas in the Northern Pacific Ocean and the Atlantic coasts of Southern Europe and West Africa.
Under normal circumstances, the ocean acts as a sponge, absorbing over 90% of the increased heat generated by human activities. Oceanologist and CNRS research director Catherine Jeandel points out that the ocean becomes a bit like a time bomb as it heats up, and that warming translates into increased evaporation and a high risk of more intense cyclones, with possible consequences for ocean currents.
She explains that warmer waters act as a barrier that slows down gas exchanges, resulting in the ocean’s oxygen pump and CO2 pump not working as well, which in turn limits the absorption of man-made greenhouse gases. Frédéric Hourdin, a specialist in climate models and deputy director of the CNRS dynamic meteorology laboratory, points out that such record-breaking temperatures are normal in the context of a long-anticipated situation that requires a profound transformation of our ways of operating.
However, it is difficult to talk of underestimating global warming, given the natural variability of the climate, which means that warming can continue for months or even years. These recent record temperatures may have been amplified by the end of the cooling effect of surface waters after three consecutive years of La Niña, combined with the arrival of the inverse El Niño phenomenon and long-term warming. For climatologist David Ho of the University of Hawaii, it’s not the current surface temperatures that are worrying, as they are in line with those usually generated by El Niño, but it’s the long-term trend that should give cause for concern.
Having absorbed around 90% of excess terrestrial heat from human activities over the course of the industrial era, scientists believe that the oceans contain a colossal amount of energy in their depths, estimated at 10 zettajoules in 2022, or 10×1021 joules, the equivalent of a hundred times the world’s electricity production. It is possible that ocean surface temperatures will stabilize more or less rapidly as greenhouse gas emissions decline. However, Karina Von Schuckmann, an oceanographer at Mercator Ocean, the French Center for ocean analysis and Forecasting, points out that the deep ocean adjusts over centuries or millennia, and that projections estimate that historical ocean warming is irreversible in this century.