A recent study questions the likelihood of a striking sea-level rise prediction that, despite being labeled as “low likelihood,” was included in the latest UN climate report due to its forecast that the melting polar ice sheets could cause global sea levels to rise by up to 50 feet by the year 2300. The researchers argue that the model behind this prediction relies on flawed assumptions about how ice sheets recede and disintegrate, while they emphasize that the rapid loss of ice in Greenland and Antarctica remains a serious concern.
In recent years, reports on climate change have painted a grim picture for our planet, featuring unprecedented wildfires, intensified hurricanes, catastrophic floods, and extreme heat waves.
However, a new study led by Dartmouth researchers suggests that one of the worst-case scenarios for global sea-level rise caused by melting polar ice sheets is highly improbable. This research highlights the urgent situation of ice loss in Greenland and Antarctica, although it disputes the severity of certain predictions.
This study takes aim at a worrying forecast found in the latest extensive report by the United Nations’ Intergovernmental Panel on Climate Change (IPCC), which reviews current climate research and anticipates both long- and short-term impacts of climate change. Released in full last year, the IPCC’s sixth assessment report presented a scenario in which the collapse of Antarctica’s ice sheets could significantly boost global sea-level rise—up to twice as much by 2100 compared to estimates from other models, and three times more by 2300.
While the IPCC categorized this prediction as having a “low likelihood,” the alarming possibility of a 50-foot rise in sea levels was included in the report due to its potential implications. Such a rise would submerge many areas of the Florida Peninsula, leaving only a small stretch of high ground from Gainesville to north of Lake Okeechobee above water, with the state’s coastal cities completely underwater.
However, this forecast hinges on a controversial and untested theoretical model regarding the processes by which ice sheets retreat—referred to as Marine Ice Cliff Instability (MICI). The researchers point out that so far, this model has only been evaluated using one low-resolution approach and has not been observed in real-world contexts, as detailed in their publication in the journal Science Advances.
The team instead employed three high-resolution models to more accurately reflect the complexities involved in ice sheet dynamics. They specifically analyzed the Thwaites Glacier in Antarctica—a vast ice mass known as the “Doomsday Glacier” due to its fast melting—which has the potential to contribute more than two feet to global sea levels. Surprisingly, their simulations revealed that even this vulnerable glacier is not projected to collapse quickly within the 21st century, countering the predictions made by MICI.
Mathieu Morlighem, a Dartmouth professor of earth sciences and the paper’s lead author, explained that their findings indicate that the theoretical physics underpinning the extreme IPCC projections are flawed, which can impact real-world decision-making. According to Morlighem, policymakers often rely on high-risk models when contemplating the construction of protective barriers like sea walls or making difficult decisions about relocating residents in at-risk coastal areas.
“These predictions have real consequences for people’s lives. Policymakers and planners are often fixated on high-end risks, as they want to avoid underestimating the threats,” Morlighem stated. “While we’re not saying that Antarctica is safe or that sea levels won’t continue to rise—our own models indicate a significant retreat of the ice sheet—we assert that such extreme predictions over the next century are unlikely.”
Morlighem collaborated with Hélène Seroussi, an associate professor at Dartmouth’s Thayer School of Engineering, along with teammates from the University of Michigan, University of Edinburgh, University of St. Andrews, Northumbria University, and the University of Stirling.
The core idea of MICI is that if an ice shelf—essentially the floating edge of a land-based ice sheet—were to collapse rapidly, it would expose the ice cliffs making up the outer edge of the sheet. If these cliffs are sufficiently tall, they might collapse under their own weight, revealing even taller cliffs and leading to an accelerated meltdown akin to a row of dominoes collapsing inward. Such rapid ice loss would contribute to the dramatic sea-level rise projected by the model.
However, the authors of the Science Advances article argue that the process is neither as simple nor as swift as proposed. “While it is widely accepted that tall cliffs can collapse, the speed at which this occurs remains uncertain,” Morlighem said. “Our research indicates that the retreat of ice is not nearly as rapid as previously assumed based on initial simulations. When we account for more precise physics, evidence of ice cliff instability doesn’t materialize.”
The researchers chose to focus on the Thwaites Glacier due to its recognized potential for collapse as its supporting ice shelf deteriorates. They simulated Thwaites’ retreat over 100 years following a sudden, hypothetical loss of the ice shelf, as well as over 50 years based on the current rate of retreat.
Across all simulations, the team found that the ice cliffs of Thwaites did not retreat at the pace predicted by MICI. Instead, without the ice shelf’s presence to hold back the glacier, the ice began to move towards the ocean more quickly, which spread the glacier away from its interiors. This accelerated movement also thinned the ice along the glacier’s edge, reducing the height of the cliffs and their risk of collapsing.
“We are not disputing the well-established projections that form the basis of the IPCC report,” Seroussi clarified. “We are merely questioning this particular high-impact, low-likelihood projection based on the new MICI process that remains poorly defined. Established processes, such as Marine Ice Sheet Instability (MISI), will still contribute to ice loss in the coming decades and centuries.”
According to study coauthor Dan Goldberg, a glaciologist at Edinburgh who was a visiting professor at Dartmouth during the project, polar ice sheets are indeed vulnerable to known destabilizing effects like MISI. This predicts that glaciers resting on submerged, sloping continents will retreat unpredictably without the stabilizing presence of ice shelves. Such processes are anticipated to heighten ice loss, thus contributing to sea-level rise, Goldberg noted.
“While MICI wasn’t observed during the 21st century, that is partly due to ongoing processes that could lead to MISI,” he explained. “Nonetheless, Thwaites is expected to experience unstable retreat in the upcoming centuries, highlighting the necessity for ongoing research to gain a better understanding of how this glacier responds to ocean warming and the breakdown of its ice shelf through continued modeling and observation.”
The research paper, titled “The West Antarctic Ice Sheet may not be vulnerable to Marine Ice Cliff Instability during the 21st Century,” was published in Science Advances on August 21, 2024. This research was backed by grants from the National Science Foundation (grant no. 1739031) and the Natural Environment Research Council (grant nos. NE/S006745/1 and NE/S006796/1).
