There are growing fears of an alarming shift in Antarctic sea ice

The sea ice that encircles Antarctica has reached near-record low levels for the third year in a row, raising concerns that the ice has undergone a permanent “regime shift” driven by climate change – with alarming consequences for ice shelves, Antarctic ecosystems and the global climate. Researchers say it remains unclear whether such a shift has occurred, but we may see more evidence in a matter of months.

Antarctic sea ice has long defied expectations. Modellers projecting its decline were flummoxed when sea ice cover increased up to 2015, even as sea ice in the Arctic declined. Then, sea ice cover fell sharply below average the following year, reaching a record low early in 2017. That started to look like a trend as the ice set another record low in 2022, then another in early 2023. Researchers were shocked when the ice failed to recover during the Antarctic winter in the middle of last year, remaining so far below average that “our statistical models didn’t work anymore”, says Edward Doddridge at the University of Tasmania in Australia.

In 2024, sea ice cover has again shrunk to a near record low, reaching a minimum extent of just 1.99 million square kilometres on 20 February – the second lowest figure on record, according to the National Snow and Ice Data Center in the US. Now, “all eyes are on winter sea ice”, says Doddridge. “If 2024 is like last year, there’s going to be a lot of evidence suggesting that Antarctic sea ice has changed, potentially irreversibly.”

However, whether the persistently low sea ice represents a permanent shift driven by climate change remains unclear, says Catherine Walker at Woods Hole Oceanographic Institution in Massachusetts. “Is it a trend, or is it an elongated blip, or a regime shift?”

One source of uncertainty is the short satellite record of Antarctic sea ice cover, which extends back only to 1979. That limited view makes it difficult to know the degree to which the current fluctuation in sea ice is part of natural variability in the Antarctic or a response to climate change. Caroline Holmes at the British Antarctic Survey says the magnitude of the change in sea ice suggests it is a combination of both.

Researchers also aren’t sure precisely how the atmosphere and the ocean are driving changes in sea ice, an essential part of understanding why those changes are happening. Extreme temperatures and changes in wind patterns did contribute to the startlingly low ice levels observed last winter, says Walker. But a recent study by Doddridge and his colleagues found these changes in the atmosphere aren’t sufficient to explain the changes in sea ice over the past decade.

Changes in ocean temperature and salinity also drive variations in sea ice cover, but assessing the complex interplay of these factors is a “rabbit hole” that current models can’t resolve, says Zachary Kaufman at Stanford University in California. And there are other hard-to-model factors to take into account, such as rapid melting of large icebergs that normally anchor the sea ice.

The low level of sea ice cover may also have led to a warming feedback, as open water absorbs radiation that would normally be reflected by ice. This heat “memory” may persist for as long as three years in the Southern Ocean, compounding heat each year, according to results Doddridge presented at the American Geophysical Union Ocean Science Meeting in Louisiana in February, where a possible regime shift was a key topic of discussion.

If the ice has shifted for good, the consequences could be significant for Antarctica and for the global climate. Locally, sea ice is an essential component of many Antarctic ecosystems, says Holmes. For instance, openings in sea ice, called polynyas, provide nutrient-rich zones that support the entire food web, from plankton to penguins to predators.

The loss of sea ice also means that the ice shelves extending out into the sea from the margins of the Antarctic ice sheet are more exposed to ocean waves, leading to additional fracturing and melting, and subsequent sea level rise. “You make something more susceptible if you’re bashing it with waves,” says Holmes.

The loss of sea ice could even have knock-on effects throughout the global climate by altering the flow of cold water from Antarctica to the global ocean. Sea ice helps drive this circulation by generating dense water that sinks near Antarctica, as well as by adding freshwater when it melts away from the continent. “Slowing that circulation down or shutting it down could have drastic effects on the climate everywhere,” says Walker.