The new year started with the familiar refrain of climate extremes, as scientists with the National Snow and Ice Data Center reported Jan. 3 that the sea ice around Antarctica dropped to its lowest extent on record for early January.
“The current low sea ice extent … is extreme, and frankly we are working to understand it,” said Antarctica expert Ted Scambos, a senior research scientist with the Earth Science and Observation Center at the University of Colorado at Boulder.
Scambos said the sea ice extent is about 270,000 square miles less than the previous low, set in 2018. That’s an area just a bit bigger than Texas, and the measurements reflect a persistent, strong trend toward lower-than-average Antarctic sea ice extent that started in 2016 and shows no signs of letting up, he added.
At this point, researchers don’t have a good explanation for the dramatic decline, and the links to global warming are still unclear. For now, he said, persistent wind patterns around Antarctica seem to be the main cause of this year’s rapid decline, while other scientists said climate warming can’t be ruled out as a contributing factor.
The Antarctic region has warmed more slowly than much of the rest of the planet, but recent research shows that the Southern Ocean, encircling Antarctica and extending northward to 60 degrees south latitude, stored an estimated 45 to 62 percent of the global increase in ocean heat between 2005 and 2017 despite comprising only 6.25 percent of the planet’s total ocean surface area.
Scambos said the poleward contraction of a belt of westerly winds around Antarctica, combined with the effects of other persistent regional winds, created “a pattern that favors slowly nudging the ice northward into warmer air and ocean conditions.”
Melting sea ice does not raise sea level, because the ice is already floating in the ocean. But the sea ice helps keep Antarctica’s land-based glaciers and ice sheets from flowing into the ocean to drive sea level rise. A 2022 study in Nature Geoscience showed how important that stabilizing role can be. If sea ice rapidly retreats from Antarctica’s coastline, it could allow more relatively warm ocean water to flow beneath the ice shelves that extend on to the ocean from the glaciers on land, speeding the flow of land-based ice toward the sea and accelerating sea level rise.
Sea ice in the Arctic and Antarctic regions also acts as a shiny shield that reflects incoming solar energy back into space. The more the sea ice melts, the more of that energy penetrates and warms the oceans, which, in turn, can melt even more of the ice.
Sea ice extent, and the timing of its formation and melting, also plays an important biological role by controlling the production of plankton, which forms the base of the ocean food chain. In the Southern Ocean, plankton is food for tiny shrimp-like crustaceans called krill, which are eaten by larger animals. Big changes in the timing of plankton blooms can disrupt feeding for birds, fish, seals and whales.
Sea ice also protects coastal areas from storm swells by dampening waves. And, in the Southern Ocean, openings in the sea ice, called polynyas, interact with winds to play a key role in maintaining the circulation of a global ocean current that distributes cold and warm water between the Northern and Southern Hemispheres, said Torsten Albrecht, a climate scientist with the Potsdam Institute for Climate Impact Research.
A Backdrop of Climate Change
Even if this year’s sudden meltdown of Antarctic sea ice is most directly linked to wind patterns, the changes “must include the effects of global warming,” said Penn State geoscientist Richard Alley.
“We have changed the climate significantly, and everything happens within that changed climate,” he said. “For a whole lot of features of climate and weather, the direct effects of warming are clear. They were predicted and are occurring.”
Warmer air, he pointed out, allows the atmosphere to hold more moisture to produce more rain in certain conditions, turns some snow to rain and favors more record high temperatures. The warming increases temperature contrasts between the air and sea, reducing snow cover and the extent of Arctic sea ice. These changes affect weather patterns, such as where the storms travel and what types of storms form.
But figuring out how all those puzzle pieces are connected in Antarctica is still a challenge, he said. There is still plenty of cold air close to the continent, where a lot of the sea ice is frozen and then blown outward, leaving space to grow more.
But if the winds blow too hard, the sea ice moves to warmer areas and melts, reducing the extent of the sea ice. If the winds don’t blow hard enough, the sea ice stays near Antarctica, which leaves less room for new ice to grow, and also results in less total sea ice, he said.
“Which way the winds blow, whether they’re piling sea ice up or spreading it out, when the winds blow, and how cold or warm it is, are all important,” he said. Long-term climate models have projected slightly different trajectories for sea ice in the Arctic and Antarctic regions, he added.
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“For decades, models have projected that warming would reduce Arctic sea ice, first with impacts mainly in summer, with more warming reducing wintertime sea ice, and those projections have proven to be accurate,” he said.
Climate models for Antarctica are equally clear that the region’s sea ice will decline with steep temperature increases in the long term, but some of the models suggest that Antarctic sea ice could also increase in the short term, with more rain and melting of land-based ice spilling into ocean, where the fresh water tends to float on top of the salt water and freezes more easily.
“With so much complexity,” he said, “my understanding is that the effect of global warming on Antarctic sea ice remains uncertain, or at least lacks the pound-on-the-table high confidence that can be applied to many other aspects of global warming.”
Eric Rignot, a glaciologist at the University of California, Irvine, explained the complex puzzle of Antarctic sea ice another way. In recent decades, climate warming and the ozone hole above Antarctica strengthened the westerly winds around the continent, which helped slightly expand Antarctic sea ice by spreading it across a larger area, a trend that was observed in the early 2000s.
“But the warming of the Southern Ocean from greenhouse gases is a competing effect that melts sea ice,” he said, noting that the most recent measurements highlight a trend. “It looks like since about 2016, the warming has been catching up, which resulted in the spectacular collapse of the perennial sea ice cover.”