A sea full of jellyfish in the near future? Ocean heatwaves, which affect almost half of the world’s marine waters, threaten to weaken and profoundly transform marine ecosystems already affected by global warming.

What are the dimensions of ocean heatwaves?

This July, 44% of the world’s oceans are experiencing heat waves, a record high since 1991, according to the US National Oceanic Administration (NOAA), which estimates that this figure could reach 50% by September-October.

The global ocean temperature reaches 21°C, close to the absolute record (21.1°C). The North Atlantic reached 24.5 degrees Celsius, due to the persistent heat that hit the Bay of Biscay and the sea area off Portugal since the end of May.

In the Mediterranean, temperatures locally reach up to 30°C (4°C above normal) in the area between Sicily and Naples, according to Thibault Guinaldo, a researcher in Space Oceanography at the Center for Satellite Meteorology Studies (CEMS) in Lanyon.

How are they explained?

“There are many cases. But the basic premise is that the ocean temperature is increasing with global warming,” explains the researcher.

Globally, marine heat waves have doubled in frequency since pre-industrial times, according to climate experts at the Intergovernmental Panel on Climate Change (IPCC/GIEC).

To this must be added “the natural variability” of the climate, according to Timbo Guinaldo. In the North Atlantic, a weak wind has reduced the mixing of surface waters as part of an anticyclonic phenomenon that favors warming, with fewer clouds reflecting solar radiation.

In the Pacific Ocean, it is probably the rise of the periodic El Niño climate phenomenon that is contributing to warming waters.

How do ecosystems react?

“The succession of heat waves tends to weaken ecosystems. This can cause the dramatic reduction of populations or shifts of populations to colder waters”, estimates Roland Séférian, climatologist at the National Center for Meteorological Studies (CNRM) in Toulouse.

During the 2015-2019 heat waves in the Mediterranean, about fifty species (corals, urchins, molluscs, bivalves, posidonia, etc.) experienced mass mortality between the surface and up to a depth of 45 meters, according to an article published in July 2022 in the scientific journal Global Change Biology.

Some species, such as fish, will migrate poleward in search of cooler waters, while others will “migrate into the depths,” according to the researcher, who nevertheless highlights the “grey areas associated with the adaptability, resilience or migration capacity” of marine ecosystems.

“There are many things we don’t know. It is very difficult to predict mass mortality events,” adds the researcher. Thus, “for corals, if the heat waves are not too intense and close together,” “some degree of resilience is produced,” he notes, adding that global warming “tends to transform ecosystems deeply.”

What life for the oceans in the future?

Under the influence of global warming, heat waves could be 20 to 50 times more frequent by the end of the century than in the pre-industrial era, according to the IPCC, which also predicts a worsening of their intensity and extent.

“On the other hand, there are species that will benefit from this change. Jellyfish are organisms that will benefit from a warmer ocean, they will multiply,” notes Roland Seferian. “Maybe the ocean of the future will be an ocean full of jellyfish.”

Is the ecosystem already changing? During an operation in the Bay of Biscay, Ifremer scientists were surprised to observe large concentrations of swimmer crabs, as well as “large populations of planktonic organisms, mainly salps”.

“We observe these crabs every year, but we have never seen them in such large numbers and organized,” explains Erwan Duhamel, fisheries researcher and co-leader of the Pelgas campaign.

“The presence of these crabs, jellyfish and salps in large concentrations indicates a possible change in the ecosystem,” he adds. “A few years ago, only one out of four years was considered a jellyfish year. Now it’s actually every year.”