The study questions a recent scenario presented in the United Nations' Intergovernmental Panel on Climate Change (IPCC) sixth assessment report. This scenario proposed that Antarctica's melting ice sheets could contribute to a sea-level rise twice as high by 2100 and three times as high by 2300 compared to other models. Though marked as "low likelihood" by the IPCC, the extreme possibility of sea levels rising by up to 50 feet was still included in the report, raising concerns about its potential impact.
This scenario is based on a hypothetical mechanism known as Marine Ice Cliff Instability (MICI), which has yet to be observed and has only been tested with a single low-resolution model. However, the Dartmouth-led team employed three high-resolution models to simulate the retreat of the Thwaites Glacier-an Antarctic ice sheet nicknamed the "Doomsday Glacier" due to its rapid melting and significant potential to raise global sea levels.
The results from these models suggest that even Thwaites, one of the most vulnerable glaciers, is unlikely to undergo the rapid collapse predicted by the MICI scenario within this century. "These projections are actually changing people's lives. Policymakers and planners rely on these models and they're frequently looking at the high-end risk. They don't want to design solutions and then the threat turns out to be even worse than they thought," Morlighem said.
Morlighem emphasized that while the study's results don't imply that Antarctica is safe or that sea levels won't rise, it is crucial for high-end projections used in coastal planning to be physically accurate. "We're not reporting that the Antarctic is safe and that sea-level rise isn't going to continue-all of our projections show a rapid retreat of the ice sheet," he continues. "But high-end projections are important for coastal planning and we want them to be accurate in terms of physics. In this case, we know this extreme projection is unlikely over the course of the 21st century."
The research team, which included scientists from Dartmouth, the University of Michigan, the University of Edinburgh, the University of St. Andrews, Northumbria University, and the University of Stirling, focused on how MICI might influence the retreat of the Thwaites Glacier. Their findings indicate that while cliff failure can occur, the retreat rate of ice cliffs is far slower than initially assumed in the extreme scenarios. "Everyone agrees that cliff failure is real-a cliff will collapse if it's too tall. The question is how fast that will happen," Morlighem said. "But we found that the rate of retreat is nowhere near as high as what was assumed in these initial simulations. When we use a rate that is better constrained by physics, we see that ice cliff instability never kicks in."
Helene Seroussi, an associate professor at Dartmouth's Thayer School of Engineering, clarified, "We're not calling into question the standard, well-established projections that the IPCC's report is primarily based on. We're only calling into question this high-impact, low-likelihood projection that includes this new MICI process that is poorly understood."
The study also highlighted that other mechanisms, such as the Marine Ice Sheet Instability (MISI), are still expected to play significant roles in the loss of polar ice sheets over the coming decades and centuries. According to Dan Goldberg, a glaciologist at the University of Edinburgh and coauthor of the study, "While we did not observe MICI in the 21st century, this was in part because of processes that can lead to the MISI. In any case, Thwaites is likely to retreat unstably in the coming centuries, which underscores the need to better understand how the glacier will respond to ocean warming and ice-shelf collapse through ongoing modeling and observation."
Research Report:The West Antarctic Ice Sheet may not be vulnerable to Marine Ice Cliff Instability during the 21st Century
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