ANTARCTIC SEA ICE IRREVERSIBLY CHANGED?

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.

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.”

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.