Sea ice around Antarctica has bottomed out in four decades of sightings, a new analysis of satellite imagery shows.
According to analysis by the National Snow and Ice Data Center in Boulder, Colo., ice covered 750,000 square miles around the Antarctic coast on Tuesday, below the previous record low of 815,000 square miles in early March 2017.
“It’s really unprecedented,” said Marilyn N. Raphael, a professor of geography at the University of California, Los Angeles, who studies Antarctic sea ice. Warmer ocean temperatures may have played a role, she said, “but there are other factors we’ll be working on in the coming months to find out.”
The extent of Antarctic sea ice varies widely from year to year, but has generally increased very slightly on average since the late 1970s, when satellite observations began. In contrast, the extent of Arctic sea ice, which is warming about three times faster than other regions, has declined by more than 10 percent per decade over the same period.
The two regions are very different. The Arctic Ocean occupies high latitudes, including the Arctic itself, and is hemmed in by land masses. In the southern hemisphere, Antarctica covers the pole. The Southern Ocean, which surrounds the continent, begins at much lower latitudes and is open to the north.
While rapid warming in the Arctic is largely responsible for the shrinking sea ice there, the effect of climate change on Antarctic sea ice is much less clear.
Edward Blanchard-Wrigglesworth, a climate scientist at the University of Washington, said many scientists expect global warming to eventually lead to declines in Antarctic sea ice. But right now, he said, “it’s really hard to connect the two, especially when it comes to separate events like this one.”
Instead, a complex group of factors come into play when it comes to Antarctic sea ice. Large-scale atmospheric patterns, often occurring far from the continent, as well as local ocean currents and winds can all increase or decrease sea ice cover.
dr. Blanchard-Wrigglesworth, for example, said some studies suggest that a strong El Niño in 2015 and 2016, when sea surface temperatures in the tropical Pacific were higher than normal, led to sharply lower sea ice cover in 2016.
Ted Scambos, a senior researcher at the University of Colorado Earth Science and Observation Center, said in an email that warmer-than-normal sea temperatures in some areas around Antarctica may have played a role in the current minimum.
and dr. Raphael said wind may also have had an effect, especially in the Amundsen Sea area on the continent’s western side. An area of low-pressure air that regularly develops over the sea was particularly strong this year, she said, resulting in stronger winds that may have pushed more ice further north, in warmer waters where it would melt faster.
While the total extent of sea ice has only increased slightly since the late 1970s, the increase started to accelerate in 2000, reaching an all-time high in 2014. But then something unexpected happened, said Dr. Raphael. It dropped drastically over the next three years, reaching its low point in 2017.
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Since then, the extent of the sea ice has recovered, said Dr. Raphael, and returned it to roughly average levels by 2020.
Normally, she said, levels would then have been average or above average for several years. But the new sharp decline this year came earlier. “It happened so fast,” she says.
“That’s what makes this one unusual,” she added. After 2017, “the ice returned to normal, but it didn’t stay that way.”
dr. Blanchard-Wrigglesworth said that to understand why the ice is so low now, researchers will need to examine how conditions shifted last year. “I wouldn’t be surprised if we find that this is a result of wind changes over the past three to six months,” he said.
The extent of the low sea ice is noticeable in the Weddell Sea, east of the Antarctic Peninsula, which retains much more ice from year to year than the other parts of the Antarctic coast because of its circular flow. A group of scientists and explorers encountered relatively mild ice conditions this month when they set out to sea this month in search of the wreck of Ernest Shackleton’s ship Endurance, which sank during an Antarctic expedition in 1915.
This year’s amount of ice could drop even lower, depending on the weather, but should soon begin to increase as temperatures begin to plummet towards the Antarctic fall and winter. Ice cover reaches a maximum around the end of September each year. The average maximum over four decades is more than 7 million square miles.
dr. Blanchard-Wrigglesworth said events like this and the previous record low offered researchers the opportunity to better understand the link between climate change and sea ice in Antarctica. “A valid new research question could be: are these the first few indications that long-term trends are beginning to reverse?” he said.