Results of melting Antarctic ice

West Antarctic ice-shelf melting did the opposite of what was expected 

As West Antarctica lost ice during past warm periods, icebergs dumped large amounts of iron into the Southern Ocean, but algae growth didn’t surge as expected. Scientists studying ancient ocean sediments discovered a surprising link between the shrinking of West Antarctica’s ice and the Southern Ocean’s ability to absorb CO2. Ocean-driven melting of floating ice-shelves in the Amundsen Sea is currently the main process controlling Antarctica’s contribution to sea-level rise. Using a regional ocean model, a comprehensive suite of future projections of ice-shelf melting in the Amundsen Sea is presented. It was found that rapid ocean warming, at approximately triple the historical rate, is likely committed over the twenty-first century, with widespread increases in ice-shelf melting, including in regions crucial for ice-sheet stability. When internal climate variability is considered, there is no significant difference between mid-range emissions scenarios and the most ambitious targets of the Paris Agreement. These results suggest that mitigation of greenhouse gases now has limited power to prevent ocean warming which could lead to the collapse of the West Antarctic Ice Sheet.

A new study finds that shifts in the West Antarctic Ice Sheet (WAIS) closely followed changes in marine algae growth in the Southern Ocean during past ice ages. However, the relationship did not work in the way scientists long assumed. The link centers on iron-rich sediment carried into the ocean by icebergs breaking away from West Antarctica. Iron typically acts as a nutrient which supports algae growth. But when researchers examined a sediment core collected in 2001 from the Pacific sector of the Southern Ocean, taken from more than three miles below the ocean surface, they found something surprising. Even when iron levels were high, algae growth did not increase. “Normally, an increased supply of iron in the Southern Ocean would stimulate algae growth, which increases the oceanic uptake of CO2,” says lead author Torben Struve of the University of Oldenburg. Struve worked as a visiting postdoctoral research scientist in 2020 at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School.

In waters surrounding Antarctica, iron is often the nutrient which limits algae growth. Previous research has shown that during glacial periods, strong winds carried iron-rich dust from continental landmasses into the ocean. In areas north of the Antarctic Polar Front, a boundary where cold Antarctic waters meet warmer waters to the north, that dust helped fertilize algae. As algae growth increased, the ocean absorbed more CO2 from the atmosphere. This additional carbon uptake contributed to global cooling as ice ages began. The new study focuses instead on waters south of the Antarctic Polar Front. There, sediment evidence shows that iron delivery peaked during warmer periods rather than during glacial phases. The size and composition of particles in the core also revealed that the primary source of iron was not windblown dust, but icebergs calved from West Antarctica. “This reminds us that the ocean’s ability to absorb carbon isn’t fixed,” says co-author Gisela Winckler, a professor at the Columbia Climate School and a geochemist at the Lamont-Doherty Earth Observatory.

All scenarios exhibit significant and widespread future warming of the Amundsen Sea and increased melting of its ice shelves. The spatial distribution of trends for the Paris 2 °C scenario show mid-depth temperature (200–700 m mean), the water which directly affects the ice-shelf cavities. Indeed, trends in mid-depth temperature significantly correlate with trends in ice-shelf basal mass loss. In reality, basal mass loss will also depend on other factors we cannot account for in our simulations, such as changes in ice-shelf geometry. To explain the mismatch, the research team examined the chemical makeup of the sediment delivered by icebergs. Their analysis showed that much of the iron was highly “weathered,” meaning it had been chemically altered over long periods of time. During earlier warm phases, when more ice broke off from West Antarctica and drifted northward, much of the iron reaching the ocean was in this poorly soluble form. Because algae struggle to use this type of iron, the increased supply did not lead to higher biological productivity. Based on these findings, the researchers conclude that continued shrinking of the West Antarctic Ice Sheet could reduce the Southern Ocean’s ability to absorb CO2 in the future.

Future warming and melting are markedly stronger than historical trends, with ensemble mean future warming trends ranging from 0.8 to 1.4 °C per century compared with the historical mean of 0.25 °C per century. Even under the most ambitious mitigation scenario, Paris 1.5 °C, the Amundsen Sea warms three times faster than in the twentieth century. Comparison shows that local atmospheric changes are the main driver of Amundsen Sea warming, with remote ocean forcing playing a non-negligible secondary role. The findings also shed light on how sensitive the West Antarctic Ice Sheet is to rising temperatures. According to Struve, several recent studies suggest that this region experienced large-scale ice retreat during the last interglacial period about 130,000 years ago, when global temperatures were similar to today. “Our results also suggest that a lot of ice was lost in West Antarctica at that time,” says Struve. As the ice sheet, which reached several miles thick in some areas, broke apart, it produced large numbers of icebergs. These icebergs scraped sediment from the rock beneath the ice and released it into the ocean as they drifted north and melted. The sediment core indicates especially high iceberg activity at the end of glacial periods and during peak interglacial conditions.

“What matters here is not just how much iron enters the ocean, but the chemical form it takes,” says Winckler. “These results show that iron delivered by icebergs can be far less bioavailable than previously assumed, fundamentally altering how we think about carbon uptake in the Southern Ocean.” The researchers believe that beneath the West Antarctic Ice Sheet lies a layer of very old, heavily weathered rock. When the ice sheet retreated during earlier interglacial periods, icebergs carried large amounts of these weathered minerals into the nearby South Pacific. Despite the increased iron supply, algae growth remained low. “We were very surprised by this finding because in this area of the Southern Ocean, the total amount of iron input was not the controlling factor for algae growth,” Struve says. As global warming continues, further thinning of the West Antarctic Ice Sheet could recreate conditions similar to those of the last interglacial period. “Based on what we know so far, the ice sheet is not likely to collapse in the near future, but we can see that the ice there is already thinning,” says Struve. Continued retreat could speed up the erosion of weathered rock by glaciers and icebergs. This process could further reduce carbon uptake in the Pacific sector of the Southern Ocean compared with today, a feedback which could further amplify climate change in future ahead.