CO2 and El Niño push oceans towards heat records

(Brest) After storing vast quantities of heat in 2023, the oceans broke new temperature records in February. A phenomenon linked to the thermal inertia of the oceans, coupled with global warming and the El Niño phenomenon.

The average surface water temperature reached 21.06°C in February, an all-time monthly record, according to data from the European Copernicus Observatory.

The one-day temperature record (21.09°C) was also broken on February 28, as summer is in full swing in the Southern Hemisphere, where the largest expanses of ocean are found.

These records are “not surprising” due to the inertia of the oceans, whose temperature varies much more slowly than that of the atmosphere, underlines Thibault Guinaldo, researcher in space oceanography at the Center for Studies in Satellite Meteorology (CEMS). from Lannion, in western France.

“We are continuing with 2023”, a record year, he underlines, with temperature anomalies of 5°C observed in spring, off the coast of Ireland and in the North Sea. “That never happened.”

But this start of 2024, “if we remove the contribution from the previous year”, “is not so exceptional”, adds Mr. Guinaldo.

The main cause of these records remains global warming, caused by greenhouse gas emissions which are not weakening: they reached a new record in 2023, for those linked to energy.

The ocean, which absorbs 90% of the excess heat produced by human activities, stored a colossal quantity of energy last year, enough to boil “billions of Olympic swimming pools”, according to an international study. published in January.

“No acceleration”

These continuous heat peaks, on an ocean surface which covers 70% of the planet, may have raised fears of an acceleration of climate change.

A hypothesis refuted by Éric Guilyardi, oceanographer and climatologist at the National Center for Scientific Research (CNRS), who urges us not to focus on a particular year. “The climate requires taking averages over 20-30 years,” he recalls.

And on a long-term trend, “the temperature follows the cumulative greenhouse gas emissions quite linearly. There is no acceleration or deceleration,” insists the researcher, lead author in 2014 of 5e report from the IPCC (United Nations group of climate experts).

In addition to global warming, the observed temperature anomalies can be explained by natural climate variations, primarily the El Niño phenomenon. Located in the tropical Pacific Ocean, “that is to say on a quarter of the planet’s surface”, “this regional warming has an arithmetic impact on the average temperature of the planet”, underlines Mr. Guilyardi.

After three years of the La Niña phenomenon, “so rather cold”, “we went to El Niño: not only are we no longer cooling, but we are warming,” he explains. This alternation “can explain global temperature variations from one year to the next of up to 0.3 degrees”, according to him.

At the same time, the Atlantic Ocean experienced “an atmospheric effect which added to global warming”, with “less wind therefore less cooling and more direct heating of the atmosphere”, describes Juliette Mignot, oceanographer at the Research Institute for Development (IRD).

Return of La Niña?

Coupled with ocean acidification, this warming is not without consequences on marine ecosystems. The growth of certain organisms, such as shells or corals, is made more difficult. Corals are bleaching and phytoplankton are decreasing in warmer regions, disrupting the fish food chain. And some species migrate towards the poles, in search of colder waters.

At the global level, however, these temperature records could mark time in the coming months, with a weakening of El Niño already underway in the Pacific.

The return of La Niña, with its cooling effect, is even predicted this summer or fall by the American oceanographic agency NOAA. A phenomenon which should, however, not cause temperatures to drop, supported by global warming.

“With an exceptional year 2023, we have reached a milestone,” underlines Mr. Guinaldo.


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