2023-07-08 08:27:00
After a historic melt in February, the Antarctic sea ice is struggling to recover despite the arrival of winter in the southern hemisphere, a phenomenon that could accelerate global warming and threaten many species in the southern ocean.
Some 2.5 million km², i.e. five times metropolitan France: this is the sea ice deficit recorded by the European Copernicus observatory at the end of June, compared to the 1991-2020 average.
On February 16, the Antarctic sea ice, which forms by the freezing of salt water in the ocean, had already reached its lowest extent since satellite measurements began 45 years ago, with an area total of 2.06 million km².
Since then, it has been reforming at an unusually slow rate, despite the onset of winter in the southern hemisphere. The area of sea ice in June thus amounted to 11.5 million km2 (17% less than the average). An “extraordinarily small” extent, according to Ed Blockley, who heads the Polar Climate Group of the Met Office, the British meteorological service.
“An unprecedented and worrying event”, confirms Jean-Baptiste Sallée, oceanographer and climatologist at the French National Center for Scientific Research (CNRS). “We are in something unheard of, with an ice floe that is not growing at the natural rate. The question is: have we entered a new regime? But it is still too early to answer that”.
Until recently, Antarctic sea ice seemed to escape the effects of global warming. For 35 years, it had thus remained stable, even had increased slightly, even beating in September 2014 a record of extent, at more than 20 million km2, for the first time since 1979.
“In 2015, everything turned around: we lost in 2-3 years what we had gained in 35 years”, says François Massonnet, climatologist at the Catholic University of Louvain, in Belgium. “Since 2016, we have broken records almost every year and it seems that these records are not independent of each other.”
One hypothesis would be, according to him, that it is a self-perpetuating phenomenon: the ocean heats up more strongly in summer, for lack of pack ice. Then, “when winter returns, you first have to release all the excess heat before you can form sea ice”, explains Mr. Massonnet. This thinner ice also melts more quickly once summer returns.
– “Refuge area” –
This retreat of the sea ice “is consistent with climate change which is beginning to impact the Antarctic sea ice”, notes Mr. Sallée. But researchers are reluctant to establish a formal link with global warming, as climate models have struggled in the past to predict changes in Antarctic sea ice.
Be that as it may, sea ice reduced to the bare minimum risks aggravating global warming. The ocean, which is darker, reflects the sun’s rays less than the white pack ice: it will therefore store more heat.
By melting, the pack ice will also lose its role as a buffer between the waves and the polar ice cap on the Antarctic continent, risking accelerating the flow of freshwater glaciers towards the ocean.
Finally, the retreat of sea ice threatens the rich ecosystem it supports. Because, far from being a frozen desert, “the pack ice forms terraces, tunnels, labyrinths, which serve as refuges where animals can hide from predators”, explains Sara Labrousse, researcher in polar ecology at the CNRS.
In particular, it is home to krill, a crustacean similar to a shrimp, which grazes on ice algae, before being itself eaten by many predators such as whales, seals or penguins.
“The pack ice is also a resting, moulting and breeding area for many mammals and seabirds,” adds Sara Labrousse.
When the pack ice breaks up too early in the season, young seals with little fat and insufficiently waterproof fur can die of hypothermia when they fall into the water, according to the researcher.
The retreat of the sea ice “may endanger populations”, she warns.
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