Looking out from the lounge window at Scott Base, you can trace a line where Antarctica’s largest ice shelf meets the frozen ocean. Solid waves rise at this juncture between sea ice and the much thicker Ross Ice Shelf, which carries a large part of the outflow from the West Antarctic Ice Sheet (WAIS) hundreds of kilometres out to sea. Where these ice masses come up against the beach outside Scott Base, they buckle into towering, jagged pressure ridges.
But some 1200km south from Scott Base, at the continental edge of the WAIS where it lifts off the ground to become the floating Ross Ice Shelf, the view is very different. In every direction, it expands endlessly across the flat, smooth, snow-covered ice. Sometimes, when clouds descend low, the horizon line between the greys of the ice and the sky becomes barely distinguishable.
This is where one of the most ambitious Antarctic research projects has set up camp for its first season this summer. SWAIS2C – short for Sensitivity of the West Antarctic Ice Sheet to 2°C – is an international collaboration aiming to decipher how the WAIS reacted to warming in the past, including during the last interglacial period some 125,000 years ago when Earth was about 1.5°C warmer than pre-industrial temperatures – similar to the target set in the Paris Agreement.
“Glaciologists have been suggesting for a long time that we’re going to lose the ice sheet if we exceed 1 or 1.5 degrees,” says Richard Levy, a geologist at GNS Science and Victoria University of Wellington/Te Herenga Waka and a co-leader of the SWAIS2C project.
Antarctica is a complex continent. Its larger eastern ice sheet is about 4000m thick and covers a massive mountain range. In contrast, West Antarctica is an archipelago, enveloped by an ice sheet that drops below sea level between the islands. Because of these marine sections of the WAIS, this part of the continent is more sensitive to warming oceans. For almost 50 years, scientists have been warning this ice sheet could collapse if Earth’s average temperature rises by a degree or more.
Extreme heatwave
For the longest time, Antarctica seemed to go against the trend of vanishing ice observed in the Arctic during the past decades of rising temperatures. But that has changed dramatically in recent years. In March 2022, an area in East Antarctica known as Dome C recorded temperatures 39°C above normal – reaching -10°C – in the most extreme heatwave ever documented. In September 2023, the area covered by sea ice shrank to its lowest level in the 40-year satellite record. And ever-warming ocean waters continue to erode parts of West Antarctica from below, pushing some glaciers, including the Pine Island and Thwaites glaciers in the Amundsen Sea, closer to a point of no return. Across the continent, Antarctica has been losing 150 billion tonnes of ice per year since 2002.
The sleeping giant has been awoken, UN Secretary-General António Guterres said during his visit to the Antarctic Peninsula in November, appealing for action to end the “climate anarchy”.
The SWAIS2C camp site is as close to the grounding line of the Ross Ice Shelf as possible to set up a sediment coring drill rig. Only about 50m of ocean separates the bottom of the 600m-thick ice shelf from the seafloor. The team used hot water to thaw a hole through the ice to reach the ocean floor, where layers of mud and rock have been accumulating for millennia. These sediment layers represent one of Earth’s memory banks, recording environmental conditions at the time they were deposited – similar to tree rings. The deeper the team drill to extract cores of sediment, the further back in time they travel. “We’ll try to recover a sequence of rocks that tell us what the West Antarctic Ice Sheet did during glacial and interglacial cycles in the past – times when it was colder than today, and when it was a bit warmer than today,” Levy says. “If we drill back in time and find sediments that contain marine algae, then it tells us that the ice shelf was gone, and likely the ice sheet itself had retreated, with open marine conditions.”
If floating ice disintegrates, it makes no difference to sea levels, but the Ross Ice Shelf acts as a buttress, holding the WAIS in place. If the shelf were to go in a warming world, the WAIS, or at least parts of it, would likely follow, lifting sea levels by up to 5m. The resulting impact is hard to fathom.Coastlines on every continent would be drastically reshaped and no part of the globe would be spared from extreme weather.
The big question is what would happen if the WAIS collapsed fully under temperatures we’re likely to reach in the next decade or two. “We can’t avoid talking about tipping points in this context,” says Tina van de Flierdt, a geochemist at Imperial College London and a SWAIS2C co-leader with Levy. “We’ll get the opportunity to look at intervals where Earth was one, two or three degrees warmer, maybe even more. We don’t quite know how much ice exactly was lost when, and that’s what we really need to figure out.”
Recent research suggests that increasing flows of warm deep water will speed up the melting of the WAIS over the coming decades, regardless of future anthropogenic greenhouse gas emissions. This would mean that even if we reach global net-zero targets, ocean warming could push the WAIS towards runaway melting, which would lock in rising sea levels for centuries.
Telltale octopus
Other evidence suggesting an open seaway in West Antarctica during the last interglacial comes from biology. Deep in the DNA of the Turquet’s octopus – a species that has been living around the Antarctic continent for about four million years – lie genetic memories of intermingling between different populations that are currently separated by the ice sheet. There are clear signs that about 125,000 years ago, some octopus populations that currently live on opposite sides of the WAIS were mixing. The most likely explanation for this exchange is the complete collapse of the ice sheet and an opening of an ocean passage between the south Weddell Sea and the Ross Sea, the two large embayments that pinch West Antarctica, which are currently covered by floating ice shelves. “If we find evidence for open marine conditions, we can support the octopus argument and the arguments made by scientists in the 1970s,” Levy says.
But there is also a more nuanced scenario. A modelling study suggests that the grounding zone of the Ross Ice Shelf actually expanded during the last interglacial, probably because snowfall increased in this region, Levy says. “But the region behind where we are drilling collapsed. The Thwaites Glacier actually collapsed and formed a big marine seaway.”
Overall, according to this modelling study, Antarctica lost enough ice to raise sea levels by 4m during this warmer interval, which “suggests the whole system is really sensitive to small increases in temperature”.
It may be too late to slow or stop the retreat of the WAIS in areas where glaciers and ice shelves are already bathed in warming ocean flows, but Levy says the collapse of the entire ice sheet is not an inevitability – yet – because the cavities below the large ice shelves, including the Ross Ice Shelf, remain cold. In Antarctic terms, this means -1.8°C, the freezing point of polar sea water. “Our big concern is when those cold cavities switch and become warm like the smaller ice shelf cavities along the Amundsen and Bellingshausen seas. At what increase in temperature will the oceans around Antarctica change their flow such that they will come up onto the continental shelf in the Ross Sea and start to cause thinning or retreat of the ice shelf? We don’t know that yet.”
Race against time
Some of the ground-up rocks in the sediment cores the SWAIS2C team is extracting may provide answers, says van de Flierdt. “Every rock on the continent has a certain age and a certain composition, and that leaves a chemical fingerprint. When you erode this material into sand and mud on the ocean floor, this signal is preserved … and you can try to trace back where on the continent it came from.”
The team’s working hypothesis is that in a world that is 1-1.5°C warmer, the Ross Ice Shelf and the West Antarctic Ice Sheet could remain partly intact. “You might lose Thwaites,” Levy says. “Sea level will still rise but the Ross Sea sector stays cold.”
At the end of the second SWAIS2C expedition next year, Levy hopes to have clearer answers, but whatever evidence the team unearths, it will help modellers to improve their ability to simulate how ice sheets react to warming. “It’s all about acquiring environmental data from the past that provide ground truths for some of the models we use to get sea level predictions more accurate.”
But even if the team finds that West Antarctica is not facing a complete collapse, it’s no reason to be complacent about the changes happening in Antarctica, Levy says.
“There are many things that are alarming about what’s happening in Antarctica at the moment. We know that the ice shelves along the Antarctic Peninsula and the Amundsen and Bellingshausen seas are losing mass right now. The more warm water flows up onto the continental shelf, the faster the ice will melt.”
Van de Flierdt agrees. “There are too many unknowns, too many surprises that might hit us. We can’t sit back and wait. We’ve got to get on top of our emissions as fast as we can to keep warming as low as we can so that we don’t expose ourselves to these potential surprises.”
Rhythm of the ice
Vanishing sea ice has been the most visible change in Antarctica in the past year. During the polar winter, the area covered by frozen ocean usually grows to double the size of the continent, but in 2023, it shrank to the smallest seen in 44 years of satellite records – so far outside the normal range that scientists warned of a breakdown in the link between sea ice and the atmosphere, driven by warming. By October, when Antarctic scientists met for an emergency summit in Wellington, the extent of sea ice remained 20% below normal, missing the equivalent of 10 times the size of New Zealand.
“2023 was a red-letter year,” says Natalie Robinson, a marine physicist at Niwa. “We’re just touching on the 1.5°C target now … and with that we’ve seen all sorts of records dramatically broken. Sea ice is dramatic and easy to see but it’s symptomatic of bigger changes.”
Robinson’s research focus is on the role of the ocean as a connector between sea ice and the cavity underneath the floating Ross Ice Shelf. Her team has set up monitoring instruments to collect year-round data to complement her field work during the Antarctic spring. It is too early to know if last year was exceptional or the beginning of a more systematic change, she says, but “we’ve been expecting a change in this direction and to see it so dramatically in such a short time, it’s hard to see how it would recover”.
The annual freeze-thaw rhythm of sea ice has been described as Antarctica’s heartbeat. An ongoing disruption would have many interlinked flow-on effects. The bright white surface of sea ice reflects the sun’s energy back into space, but as it melts, the darker ocean absorbs more heat, in turn causing more ice to melt in a self-perpetuating feedback loop.
Sea ice is also an essential driver of ocean currents that carry oxygen, carbon and nutrients into the deep ocean and around the world. “Water from Antarctica is far reaching,” Robinson says. “It controls how the deep ocean operates and where the heat goes and it is sequestering carbon into the deep ocean.”
All forms of life in the Antarctic food web – from tiny ice algae to penguins – depend on sea ice. Emperor penguins in particular rely on sea ice to raise their chicks. Even with large sea ice floes still in place, surface melting of the snow on top can be fatal for penguin chicks while they still have only their downy feather coats to protect them from the cold, but not from getting wet.
“Whether sea ice forms or doesn’t form is a response to large-scale ocean-atmosphere conditions and to see such a dramatic shift from one year to the next is signalling a big step change and a substantial shift,” Robinson says.
“Every single tenth of a degree of warming that we can avoid is going to have massive benefits. The less action we take, the worse it will be.”
