It’s winter in the Southern Hemisphere, when ice typically forms around Antarctica. But this year, that growth has been stunted, hitting a record low by a wide margin.

The sharp drop in sea ice is alarming scientists and raising concerns about its vital role in regulating ocean and air temperatures, circulating ocean water and maintaining an ecosystem crucial for everything from microscopic plankton to the continent’s iconic penguins.

“This year is really different,” said Ted Scambos, a senior research scientist at the University of Colorado Boulder, US, and an Antarctica expert at the National Snow and Ice Data Center. “It’s a very sudden change.”

A continued decline in Antarctic sea ice would have global consequences by exposing more of the continent’s ice sheet to the open ocean, allowing it to melt and break off more easily, contributing to rising sea levels that affect coastal populations around the world.

Less ice also means less protection from solar rays, which can raise the water temperature, making it harder for ice to form.

At the end of June, ice covered 11.7 million square kilometres of ocean around the continent, according to NSIDC data. That’s nearly 1 million square miles less than the expected average from approximately 40 years of satellite observations.

The clear departure from previous years is startling, since sea ice around Antarctica has been slower to respond to climate change than ice in the Arctic Ocean.

Antarctic sea ice also set a record low in 2022, but this year’s ice cover is almost 500,000 square miles smaller.

“The Antarctic sea ice extent low in 2023 is unprecedented in the satellite record,” Liping Zhang, a project scientist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, wrote in an email.

The record low might signal a shift in the sea ice system to a new, unstable state where extremes become more common, but Zhang cautioned scientists are still investigating this question.

Sea ice around Antarctica typically freezes from February to August and then melts until the next Southern Hemisphere winter. Several ocean and atmosphere patterns influence how much ice grows or shrinks, and the overlapping interactions between these forces are complicated.

On top of these natural, short-term patterns is the long-term influence of humans burning fossil fuels that add greenhouse gases to the atmosphere. Some researchers suspect that we are finally seeing the effects of this slow burn on Antarctica’s previously resilient sea ice.

This year’s change, within the context of several years in a row with less sea ice, is “very, very concerning,” said Marilyn Raphael, a geography professor and director of the Institute of the Environment and Sustainability at University of California, Los Angeles, US. “That is not within natural variability,” she said.

Raphael has been working to extend the historical record of Antarctic sea ice past the 1970s, when satellite observations began. She and her colleagues recently published a new data set going back to 1905, using weather observations to reconstruct the extent of sea ice during earlier years.

While it’s still limited data, the longer record captures more cycles of natural variability. Raphael and other experts think that the ocean, which warms up more slowly than the atmosphere and has absorbed much of the heat from the burning of fossil fuels, may have reached a point where that heat is affecting Antarctic sea ice.

Sea surface temperatures have broken records this year, and there are currently three patches of unusually warm water around Antarctica. While other factors are also at play, these hot spots line up with the areas on the coast where sea ice has been unusually slow to form, Scambos said.

The sea ice’s decline is causing real consequences both locally and globally.

Both of Antarctica’s native penguin species rely on sea ice. In some parts of the continent, Adélie penguins exclusively eat krill, a tiny crustacean that thrives in icy water. Less sea ice means less krill and less food for Adélies. Larger emperor penguins, recently listed as threatened under the Endangered Species Act, depend even more directly on sea ice: they lay their eggs and raise their young on these floating habitats. When sea ice melts earlier in the season, before emperor penguin chicks develop waterproof adult feathers, the chicks can drown.

Sea ice also serves as a protective, frozen moat around Antarctica — shielding the continental ice sheet and its glaciers, which have already been destabilised by climate change, from the warmer ocean and the eroding force of wind and waves. If this shield disappears, more land ice could flow or fall into the ocean, though some of this loss could be counterbalanced by more snow falling onto the continent. The amount of ice Antarctica loses to the ocean is one of the biggest factors in determining sea level rise.

Even when Antarctic sea ice reaches its maximum extent around September, it could remain at a record low for that time of year, said Xiaojun Yuan, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory, who maintains a seasonal forecast of Antarctic sea ice. Yuan’s forecast shows less sea ice than usual around most of Antarctica at least through early 2024.

NYTNS