If all our fear and uncertainty over climate change could be distilled into a single statistic, then arguably it was delivered to an emergency summit on the future of the Antarctic last month.
at the Australian National University, Canberra, opened her presentation with a slide headlined “Antarctic sea ice has declined precipitously since 2014, and in July 2023 exceeded a minus 7 sigma event”.
At times, the noise of the 500 researchers who gathered for in Tasmania was overwhelming, as they tried to make sense of the unprecedented shifts underway at the bottom of the globe. But as Abram’s slide sunk in, it was as if the whole room was holding its breath.
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Put simply, a minus 7 sigma event, meaning seven standard deviations below the average, should be all but impossible, says at the Australian Antarctic Program Partnership, who works with Abram.
It is “actually really hard to convey just how extreme this difference was, how extreme the low sea ice extent was”, he says. One way is to liken it to the concept of a one-in-100-year flood, for example. “If you run those sorts of statistics for Antarctic sea ice last year, you get a number somewhere between one in 7.5 million years and one in 700 billion years,” says Doddridge.
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Of course, given this event has occurred, something must be wrong with our models. They just aren’t able to predict how radically and possibly permanently the Antarctic environment has changed, he says.
The entire Antarctic research community is now scrambling to understand what will happen to the sea ice in the coming years and what the implications will be for everything from local ecosystems to global weather.
So just how bad is it? According to the US National Snow and Ice Data Centre, the 7 sigma event was the lowest maximum since records began in 1979. This year was the second lowest, with Antarctic sea ice “stalling out” at a maximum extent of 17.16 million square kilometres, or just 200,000 square kilometres more than last year.
Remarkably, that is 1.55 million square kilometres below the expected average extent. In other words, in the past two years an area of ice nearly 6.5 times the size of the UK has disappeared. Another way to imagine it is that the ring of sea ice that forms every winter around the entire Antarctic continent has contracted by an average of 120 kilometres, says Abram.
While much of the world’s focus has been on the loss of winter sea ice in the Arctic, Abram told the conference, in just a decade the decrease in the Antarctic is equivalent to that lost in the northern hemisphere in the past 45 years. Even more alarming is the projection that Antarctica could possibly experience summers that are essentially free of sea ice before even the Arctic, which is projected to reach that point before 2050.
“It’s really serious,” says Abram. “This is a major part of the Earth’s system that is changing, and it’s changing incredibly quickly. We don’t fully understand why it’s changed so quickly, and that worries me in terms of the sort of surprises that might be ahead.”
Of particular concern is that what happens in Antarctica doesn’t stay in Antarctica. The Southern Ocean around the continent has currents that drive the distribution of heat, oxygen, nutrients and carbon dioxide as they are transported around the globe, she says. “There is now modelling evidence and observational data that points to a slowdown of that overturning circulation around Antarctica.”
What that means not just for the world’s climate but also for the wildlife that relies on this circulation is still unknown, however, researchers expect impacts throughout the food chain, from krill up to whales and penguins. There have already been mass deaths of emperor penguin (Aptenodytes forsteri) chicks after the early break-up of sea ice.
Nearby continents such as Australia are already beginning to see the impacts of climate disruptions in the form of dry, hot spells as well as winter blasts of cold when the southern polar vortex is pushed north. at the University of Tasmania says stormier weather in the Southern Ocean will also result in more waves, which will probably lead to greater damage to the less solid outer edges of the sea ice, known as the marginal ice zone – a worrying feedback loop.
Even the sea ice still present in Antarctica may be in worse shape than even the loss of coverage suggests, says Doddridge, as researchers suspect it is thinning. “As it gets thinner, it gets weaker and it becomes more susceptible to wind break-up, to wave break-up and to not growing back. We may be rapidly getting to the threshold where it all smashes,” he says.
Beyond fears over sea ice, an even bigger concern is looming – what will happen to the kilometres-thick ice sheets that cover Antarctica’s landmass? Normally, sea ice plays a crucial role in preserving these sheets, both by forming a protective apron around the continent that keeps ocean waves away and by keeping water around the continent cooler.
“If we start to break up or to disintegrate these ice shelves, the ice on the continent can start to flow more quickly into the ocean,” says , also at the University of Tasmania. “So we get a much larger magnitude sea level rise, more quickly.”
Thompson spent the last Antarctic summer at the Denman-Shackleton ice shelf and says there is growing evidence that it is being weakened by warmer water intruding under the areas where the glacier extends over the ocean. That is bad news – if the Denman glacier were to completely melt then it would contribute 1.5 metres to global sea level rise.
To investigate, her team drilled through 200 metres of ice to reach the ocean below, measuring the 800 metres of water there for the first time. Unfortunately, the results were sobering.
“We knew that warmer modified circumpolar deep water was in the vicinity, but we had no idea whether it was actually getting into the cavity under the ice shelf or not,” says Thompson. “Our results now show that it is.”
While it is just one data point, she says it does appear that there is significant melting of the glacier on its underside.
“We think that at that site, we’re probably getting about 2 metres of melting at the base of this ice shelf on an annual basis,” she says. The fear is that number may grow as sea ice retreats and warmer waters intrude further under the shelf. “As soon as we remove the sea ice cover, you have more absorption of solar radiation into the ocean. The winds and the swell can have a greater impact on the ice shelf itself, so there are a number of positive feedbacks that happen as soon as you remove that ice cover.”
“You can get these really big ecological tipping points because of loss of sea ice,” says at the University of Wollongong, Australia. “You can bring all those things together and there’s a chance that we don’t notice the connections until the system has really gone catastrophically bad.”
What unfolds for the global climate in the coming decades is, to a significant extent, going to depend on whether supposed one-in-a-billion-year events like the 7 sigma sea ice contraction continue to happen every year.
The consensus of researchers at last month’s summit is that, while we may see years with less or more sea ice, a tipping point has been passed. This means there is an extreme urgency in understanding all the consequences of the regime change underway.
“I don’t think it’s going back to normal,” says Abram. “I think the evidence is pointing to the rapid loss of sea ice because the ocean is taking up heat. Once you get that heat into the ocean, it holds on to it.”
Doddridge says that something may have permanently shifted, and the maximum sea ice extent is unlikely to return to pre-2023 levels. “We played that game in 2024, waiting for sea ice to return to normal, and it didn’t happen.”
He sums up the situation with one very simple, very unscientific word. “Grim.”
