After a historic melt in February, the Antarctic ice pack is struggling to recover despite the onset of winter in the southern hemisphere, a phenomenon which could accelerate global warming and threaten many species in the Southern Ocean.
A deficit of some 2.5 million km2 was recorded by the European Copernicus observatory at the end of June, compared with the 1991-2020 average.
On 16 February this year, the Antarctic sea ice, which forms by freezing ocean salt water, had already reached its lowest levels since satellite measurements began 45 years ago, with a total surface area 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. In June, the ice pack covered 11.5 million km2 (17% less than the average). An "extraordinarily low" area, says Ed Blockley, who heads the Polar Climate Group at the Met Office (the British meteorological service).
Oceanographer and climatologist at the French National Centre for Scientific Research (CNRS), Jean-Baptiste Sallée, confirmed the phenomenon as "an unprecedented and worrying event". He described it as "something that has never been seen before, with an ice pack that is not growing at its natural rate."
Breaking records year on year
Until recently, the Antarctic ice pack seemed to have escaped the effects of global warming. For 35 years, it had remained stable or even increased slightly. In September 2014 it even broke a record for its size, at more than 20 million km2, for the first time since 1979.
"In 2015, everything turned upside down: in 2 to 3 years we lost what we had gained in 35 years," says François Massonnet, a climatologist at Belgium's UCLouvain university. "Since 2016, we have been breaking records almost every year, and it seems that these records are not independent of each other."
One hypothesis, Massonnet believes, is that this is a self-perpetuating phenomenon: the ocean warms up more in summer due to the lack of sea ice. Then, "when winter returns, all the excess heat has to be released before sea ice can form". This thinner ice also melts more quickly once summer returns.
The reduction in Antarctic pack ice is "consistent with climate change", stresses Sallée. But researchers are reluctant to establish a formal link with global warming, given the difficulty climate models have previously had in predicting changes in the ice pack.
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Reduced sea ice is likely to exacerbate global warming. The darker ocean reflects fewer sunrays than the white pack ice, so it stores more heat. By melting, the ice will also lose its role as a buffer between the waves and the polar ice cap on the Antarctic continent, and may accelerate the flow of freshwater glaciers towards the ocean.
Finally, the melting threatens the rich ecosystem that the ice shelters. Far from being a frozen desert, "the pack ice forms terraces, tunnels and labyrinths that serve as refuges where animals can hide from predators", explains polar ecology researcher at the CNRS, Sara Labrousse.
In particular, it is home to krill (a shrimp-like crustacean that grazes on ice algae before being eaten by numerous predators such as whales, seals and penguins). "The pack ice is also a resting, moulting and breeding area for many marine mammals and birds," Labrousse adds.
When the pack ice breaks up prematurely in the season, young seals with little fat and insufficiently waterproof fur can die of hypothermia by falling into the water, and this melting can therefore "endanger populations", she warns.
