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Research scientists on ships along Antarctica’s west coast said their recent voyages have been marked by an eerily warm ocean and record-low sea ice coverage—extreme climate conditions, even compared to the big changes of recent decades, when the region warmed much faster than the global average.

Despite “that extraordinary change, what we’ve seen this year is dramatic,” said University of Delaware oceanographer Carlos Moffat last week from Punta Arenas, Chile, after completing a research cruise aboard the RV Laurence M. Gould to collect data on penguin feeding, as well as on ice and oceans as chief scientist for the Palmer Long Term Ecological Research program.

“Even as somebody who’s been looking at these changing systems for a few decades, I was taken aback by what I saw, by the degree of warming that I saw,” he said. “We don’t know how long this is going to last. We don’t fully understand the consequences of this kind of event, but this looks like an extraordinary marine heatwave.”

If such conditions recur in the coming years, it could start a rapid destabilization of Antarctica’s critical underpinnings of the global climate system, including ice shelves, glaciers, coastal ecosystems, and even ocean currents. Such radical changes have already been sweeping the Arctic, starting in the 1980s and accelerating in the 2000s.

Data collected during Moffat’s most recent research voyage includes the first readings from temperature and salinity sensors that were deployed a few years ago, which will give the scientists a starting point for comparisons. Moffat said it’s “too early, and difficult” to attribute this year’s conditions to long-term climate change until some peer-reviewed results are published.

“But it seems to me that this might be a really unprecedented event,” he said. “These episodes of relatively rapid ocean warming that can persist for months have been occurring all over the place. They haven’t been common in this region.”

He said ocean temperature readings going back to April 2022 speak to the persistence of the warm conditions off the Antarctic Peninsula. The cruise covered an area more than 600 miles long and crisscrossed waters above the 125-mile wide continental shelf, documenting widespread ocean heating.

“That’s a very significant region,” he said. “We don’t have data going back 30 years for the entire region. But for the parts of the shelf for which we do have that data, it really seems extraordinary. It’s very difficult to warm the ocean, and so when we see these conditions, that really speaks to a very intense forcing.”

A dangerous climate feedback

Greenhouse gases, mostly from burning fossil fuels, are the force behind the warming of the atmosphere and the oceans. The latest reports from Antarctica raise concern that a perilous climate feedback cycle of warmer oceans and melting ice has started around the continent, said Johan Rockström, director of the Potsdam Institute for Climate Impact Research.

“We know the melting of Antarctica is most sensitive to lubrication by water,” he said. “It’s the sea melting the ice from below, it’s not atmospheric melting from above. And this is really, really worrying… and quite surprising, because up until 10 years ago, we were absolutely convinced that the Greenland ice sheet and the Arctic was the more sensitive of the two poles.”

Up until about 2014, science suggested that Antarctica was still gaining ice, but “that has shifted,” he said. An assessment released that year by the Intergovernmental Panel on Climate Change warned that there is likely an Antarctic tipping point between 1.5° and 2° Celsius warming that would trigger irreversible melting of ice shelves and glaciers.

The Paris Climate Agreement to cap warming in that range was signed the following year with the understanding that a vicious climate cycle in Antarctica has global implications, raising sea level faster than expected and contributing to the slowdown of the critical Atlantic thermohaline circulation that moves warm and cold water between the poles. He said research shows that system of currents has been affected by global warming in recent decades, leaving more warm water in the Southern Ocean to drive marine heatwaves.

Instead of flowing northward to the Gulf Stream, the warmer water persists around Antarctica because "that whole system has slowed down by 15 percent,” he said. “So when the circulation slows down, and you have more heat, you get more warm surface water in Antarctica.”

The potential start of an icy death spiral

Antarctica was seen as a frozen redoubt until very recently because its ice sheets average more than a mile thick and cover an area as big as the contiguous United States and Mexico combined, spreading over about 5.4 million square miles, with its center more than 1,000 miles from the ocean.

The continent is also encircled by a swift ocean current—the only one that flows all the way around the world—and an accompanying belt of jetstream winds several miles above it. Both helped buffer Antarctica’s sea ice, as well as its land-based glaciers and floating ice shelves, from the rapid increase of climate extremes seen in most other parts of the world the past few decades.

But the observations from this year’s conditions may bolster several recent studies showing how global warming is eroding that protection. An August 2022 study in Nature Climate Change suggested that “circumpolar deep water” at a depth of 1,000 to 2,000 feet has warmed by up to 2° Celsius, which is in turn related to a poleward shift of the westerly wind belt.

That’s a critical depth where the water creeps up the continental shelf and beneath the floating ice shelf extensions of Antarctica’s huge land-based ice sheets, which poses a threat not only to ice in West Antarctica, already known to be vulnerable, but also to the thick, remote ice on the eastern half of the continent.

Warming through the world’s oceans is projected to persist in coming decades, so “the oceanic heat supply to East Antarctica may continue to intensify, threatening the ice sheet’s future stability,” the authors of the 2022 paper wrote.

Another study, published in June 2022 in Science Direct, showed that the changes to the winds responsible for pushing the warmer water closer to shore will also persist if greenhouse gas emissions continue, so without immediate action to implement global climate policies, the Antarctic system could loop into a death spiral.

A 2016 study outlined a worst-case scenario in which warming would contribute to a rapid break-up of towering ice cliffs near the shore in a process that could speed up sea level rise, raising the water up to seven feet by 2100 and 13 feet by 2150, increases that would be very hard to adapt to.

The water’s rise is already accelerating. In the 1990s, the global average sea level increased at about 3 millimeters per year, but that annual rate increased to 4.5 millimeters in the last five years. Between August 2020 and January 2021, sea level rose 10 millimeters.