Sea ice around Antarctica has dropped to the lowest level seen in four decades of observations, a new analysis of satellite imagery reveals.
On Tuesday (22), the ice covered 1.94 million km² around the coast of Antarctica, less than the previous record of 2.1 million km², observed in early March 2017. The information comes from an analysis of the National Snow and Ice Data Center in Boulder, Colorado.
“It’s really unprecedented,” said Marilyn N. Raphael, a professor of geography at UCLA (University of California at Los Angeles) and a scholar of Antarctic sea ice. For her, higher ocean temperatures may have contributed, “but there are other factors that we will seek to unravel in the coming months”.
The extent of Antarctic sea ice is highly variable from year to year, but overall it has been increasing slightly, on average, since the late 1970s, when satellite observations began. In contrast, the extent of Arctic sea ice, which is warming at a rate three times faster than other regions, has declined by more than 10% per decade over the same period.
The two regions are different. The Arctic Ocean covers high latitudes, including the North Pole, and is surrounded by land masses. In the southern hemisphere, Antarctica covers the pole. The Antarctic Ocean, which surrounds the continent, starts at much lower latitudes and is open to the north.
While the accelerating warming of the Arctic is largely responsible for the shrinking sea ice in that region, the effect of climate change on Antarctic sea ice is much less clear.
Climate scientist Edward Blanchard-Wrigglesworth of the University of Washington said many scientists predict that global warming will eventually lead to a reduction in Antarctic sea ice. But at the moment, he said, “it’s really hard to link the two, especially in terms of one-off events like this.”
Instead, there is a complex set of factors at work with regard to Antarctic sea ice. Large-scale atmospheric patterns, often occurring far from the mainland, in addition to local winds and sea currents, can all increase or decrease the area covered by sea ice.
For example, Blanchard-Wrigglesworth said, some research suggests that a strong El Niño in 2015 and 2016, when surface ocean temperatures in the tropical Pacific rose above normal, resulted in a sharp drop in the area covered by sea ice in 2016.
Ted Scambos, a senior researcher at the Center for Earth Science and Observation at the University of Colorado, said in an email that higher than normal sea surface temperatures in some areas around Antarctica may have contributed to the current low.
For Raphael, the winds may also have had an effect, especially in the Amundsen Sea region on the western side of the continent. A region of low atmospheric pressure that develops regularly over the sea was especially strong this year, she said, and that resulted in stronger winds that could have driven more ice further north into warmer waters where it would have melted more. quickly.
Although the total extent of sea ice has increased only slightly since the late 1970s, the increase began to accelerate in 2000, and in 2014 the ice reached its record extent. But then something unexpected happened, Raphael said. The next three years saw a drastic drop, and in 2017 the ice extent reached the lowest level ever seen.
Since then the ice extent has grown again, the scientist said, and by 2020 it had returned to more or less the average level.
Normally, she said, levels would have remained at or above average for several years. But this year’s new sharp drop came before that. “It was really quick,” she commented.
“That’s what makes this shrinkage unusual,” she went on. After 2017 “the ice returned to normal, but it didn’t stay that way”.
Blanchard-Wrigglesworth said that to understand why the ice extent is so low now, scientists will have to analyze how conditions may have changed in the past year. “It won’t surprise me if we find that this shrinkage is the result of changes in winds over the last three to six months,” he said.
Low sea ice extent has been notable in the Weddell Sea, east of the Antarctic Peninsula, which, because of its circular current, retains much more ice from year to year than other parts of the Antarctic coast.
A group of scientists and explorers encountered relatively mild ice conditions this month when they ventured out to sea to search for the remains of the Endurance, explorer Ernest Shackleton’s ship that sank during a 1915 Antarctic expedition.
The area covered by ice could decrease even further this year, depending on the weather, but it is expected to increase again soon as temperatures start to drop, with the arrival of the Antarctic autumn and winter. The area covered by ice reaches a maximum each year around the end of September. The average maximum over four decades has been over 18 million square kilometers.
According to Blanchard-Wrigglesworth, events like this and the previous record low offer scientists an opportunity to better understand the connection between climate change and Antarctic sea ice.
“A valid new question for research is whether these are early indications of a reversal in long-term trends,” he said.
Translation by Clara Allain.
