For the second year in a row, Antarctic sea ice has reached near-record low levels. This reinforces concerns that human-caused climate change has initiated a lasting “regime shift” in the amount of ice that forms in the Southern Ocean each year.
“Last year we were talking about whether Antarctic sea ice is undergoing a regime shift. Not anymore,” says at the Institute for Marine and Antarctic Studies in Australia. “Antarctica has pretty definitively answered that question for us. Now we are talking about what the impacts of that regime shift will be.”
In early 2023, researchers were shocked when the sea ice encircling Antarctica smashed the previous record low set in February 2022 in the southern summer. It remained far below average levels as it expanded through the southern winter of 2023. That same pattern has repeated in 2024. In February this year, the ice reached the second lowest extent on record and it has stayed around 8 per cent below average levels.
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Though there has generally been more ice than 2023, on 7 September, its coverage dipped below last year’s levels. It set a new record low for that day with an extent of just 17 million square kilometres, according to a from the Australian Antarctic Program Partnership. That is about 1.4 million square kilometres below the long-term average for that time of year – an expanse of missing ice roughly twice the size of Texas.
The coverage has expanded a bit over the past few days, but the persistently low levels suggest a more lasting change is occurring, says at the British Antarctic Survey. “The evidence is building for something like a regime shift,” she says.
Distinguishing natural variability from the influence of human-caused climate change is difficult due to unreliable sea ice models and a relatively short 45-year satellite record of sea ice extent. However, it is “plausible that this is the start of a climate change signal in Antarctic sea ice”, says Holmes.
A sustained increase in ocean heat in particular appears to be driving the change, with waters too warm to freeze, says at Stanford University in California. However, he says the “jury is still out” on whether the response of the ice is a permanent change or just a blip.
Either way, the lack of sea ice could have consequences for Antarctic ecosystems. It affects animals like seals and penguins that live on the ice, as well as the photosynthetic plankton that form the base of the food web.
“If you have repeating seasons like this, then it’s hugely impactful for populations that depend on sea ice for [their] habitat and breeding cycle,” says at the Woods Hole Oceanographic Institution in Massachusetts.
Less sea ice could also impact global ocean circulation by reducing the amount of cold, dense Antarctic “bottom water”, says Walker. “That was expected at some point, but not this soon.”
A more speculative effect is how the loss of sea ice might boost the by the Southern Ocean. During the southern winter, some parts of the ocean contain higher concentrations of CO2 than the atmosphere does, but sea ice can act like a “lid” that stops the greenhouse gas from being released, says Doddridge. “Such massive reductions in wintertime sea ice raise really big questions about the future of the Southern Ocean carbon cycle,” he says.
