Scientists still struggle to predict how much sea levels will rise in the future as a result of global warming. The greatest uncertainty here is the Antarctic Ice Sheet because it is not yet known exactly how it will react to climate change. A study has now shown that the ocean around East Antarctica has warmed significantly since 1930, helping to clarify how vulnerable the East Antarctic Ice Sheet, the largest on Earth, is to ocean warming. The study was recently published in the journal Nature Climate Change.
It was long thought that the East Antarctic Ice Sheet – unlike ice masses in West Antarctica, which are melting rapidly – is shielded from ocean warming by cold, dense seawater that forms on the continental shelf in front of the ice shelves. Data and observations from the last ten years show that this assumption is not (any longer) true. Warm and saline water is increasingly reaching the edge of the East Antarctic Ice Sheet, causing it to melt from below. But the extent of the warming and the processes behind it have not yet been determined.
To fill this gap, oceanographers Laura Herraiz-Borreguero of Australia’s national science agency (CSIRO) and Alberto Naveira Garabato of the University of Southampton, United Kingdom, analyzed records of ocean temperature and salinity along the East Antarctic continental slope and open ocean from 1930 to 1990 and 2010 to 2018. They compared these with satellite data used to map the boundaries of ocean currents.
The two researchers found that circumpolar deep water temperatures have increased by up to 2°C since 1930 and that this trend is accelerating. Circumpolar deep water is a mass of water that moves at a depth of about 500 meters – roughly the depth of the continental shelf – and is warmer compared to the cold surface and bottom waters.
Since the 1990s, ocean warming in the region has actually tripled, with the greatest temperature increase over the East Antarctic continental slope, near Denman, Vanderford, Totten, and other glaciers whose ice shelves have shrunk or thinned most rapidly.
“It was really striking that the largest warming was in these areas where we know the ice sheet is losing ice mass,” Herraiz-Borreguero says.
Previous research suggested that the warming may be related to a southward shift of the Antarctic Circumpolar Current. The current study suggests that this is exactly what happened, transporting warmer water to East Antarctica. Herraiz-Borreguero and Naveira Garabato suggest that this expansion is driven by strong westerly winds that also move poleward during the summer months – a shift that is expected to continue throughout this century.
Understanding how these processes are driving large-scale loss of Antarctic ice mass will help resolve uncertainties in climate models that try to project how ice loss will contribute to future sea level rise, Herraiz-Borreguero says.
But there are also other processes contributing to warming, as Matthis Auger, a physical oceanographer at Sorbonne University in Paris, points out. The influx of fresh meltwater from ice sheets, as well as upwelling of warm water near the Antarctic continent, could further exacerbate ice loss by preventing the formation of cold, dense water that normally shields floating ice shelves, Auger said.
However, most of the observations analyzed for the current study were collected relatively recently and hundreds of kilometers away from the ice shelf front. “What matters for ice-shelf melting is the temperature right at the ice-shelf front,” says Yoshihiro Nakayama, an oceanographer at Hokkaido University in Sapporo, Japan. Collecting more observations of ocean conditions near ice shelves and seeing if existing data match simulations created by ocean models would help oceanographers understand “what’s really happening,” he adds.
As the shift in westerly winds is expected to continue into the 21st century, it is likely that warmer water will flow increasingly into East Antarctica and reach the ice shelf margins. “Then the ice melt would be almost unstoppable,” Herraiz-Borreguero says.
Julia Hager, PolarJournal
Link to the study: Herraiz-Borreguero, L., Naveira Garabato, A.C. Poleward shift of Circumpolar Deep Water threatens the East Antarctic Ice Sheet. Nat. Clim. Chang. 12, 728-734 (2022). https://doi.org/10.1038/s41558-022-01424-3
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