The planet’s widest glacier is melting “more aggressively” than previously projected, meaning its impacts on global sea level rise could be double current estimates, according to new research by a team of glaciologists. Thwaites Glacier in West Antarctica — nicknamed the Doomsday Glacier because of the catastrophe its collapse would trigger — is a huge wildcard when it comes to forecasting global sea level rise. The Florida-sized glacier, which lies in one of the least accessible places on Earth, has been retreating for decades and its melting ice currently contributes around four per cent to global sea level rise. But the mechanisms behind its retreat are exceptionally complex, making it tricky to precisely map out its future. It’s a vital task, however, as Thwaites holds enough water to raise sea levels by more than two feet if it were to collapse. Because it also acts as a cork holding back the vast Antarctic ice sheet, it could ultimately lead to around 10 feet of sea level rise, devastating coastal communities from Miami to Manila. Glaciologists from the University of California, Irvine and NASA’s Jet Propulsion Laboratory have developed a new ice sheet model they say better reflects what’s happening underneath this ice giant. Thwaites’ rapid retreat is driven by processes at its “grounding line” — the point at which the ice rises up from the seabed and starts to float on the ocean as an ice shelf. But while typical ice sheet models have tended to view the grounding line as something fixed and mostly stable, “we have come to acknowledge a different arrangement beneath Earth’s major ocean-terminating glaciers,” said the report’s lead author Mattia Poinelli, a project scientist at UCLA who was an Earth system science researcher at UC Irvine at the time of the study. Satellite data reveals the grounding line is actually a “grounding zone,” sometimes miles wide, where seawater constantly flushes in and out with the tides, lifting and lowering the thick ice in a process known as “tidal pumping.” This allows relatively warm ocean water to penetrate “deep underneath the ice sheets and causing vigorous melting,” Poinelli said. When the scientists adjusted models to include the concept of a grounding zone, they found Thwaites could contribute up to 1.2 inches of sea level rise by 2100 — twice previous estimates — even without further global warming, according to the findings published Monday in the journal Nature Communications. The land on which Thwaites sits slopes downward, meaning as the glacier melts, more ice is exposed to warm ocean water. The topography is complicated, however, by two key underwater ridges, which “pin” the ice, holding it back from flowing into the ocean more rapidly. The melting processes at Thwaites’ grounding zone are accelerating the “unpinning” of ice from a large ridge called the Mouginot Ridge, the study found. There is a 60% chance that it will be unpinned by the end of the century, the study found, after which the glacier is likely to retreat quickly down the sloping bedrock toward the next major ridge. The findings suggest the impacts of losing Thwaites might have been underestimated. A sea level rise of 1.2 inches may not sound much but it’s enough to greatly magnify dangers to coastal populations. The results “predict greater risk of and exposure for coastal flooding than anticipated in the coming decades,” said Eric Rignot, a report author and professor of Earth system science at UC Irvine. Ted Scambos, a senior research scientist for the Earth Science and Observation Center at the University of Colorado Boulder, who was not involved in the research, said the findings could be “a real gamechanger for sea level forecasting from Thwaites.” But he and other experts caution that more research is needed to understand the impacts of different grounding zone melting processes, not least by getting direct data from underneath the glacier. “The reality is that there is a lot we still need to find out about grounding zone processes to be able to make confident predictions about how fast ice will be lost,” said Robert Larter, a marine geophysicist at British Antarctic Survey, who was also not involved in the study. The environment beneath Thwaites, which is very difficult to access, is even more complex than the paper describes, he added. Getting observational data from Thwaites’ grounding line is the next step, Rignot said. The scientists are planning to travel there in 2028 to deploy instruments able to document the flushing in and out of seawater. “We have been aware of the impacts of climate change on our planet’s sensitive ice sheets for decades,” Rignot said, “but a mounting realization gained in recent years of what is really happening on the undersides of glaciers like Thwaites is leading to an even greater sense of alarm.” He said the world must cut planet-heating pollution now to limit the impacts but warned, “even if we do, we can expect higher sea levels in the future resulting from the damage that has already been done.”
