‘Worst-Case’ Scenario for Antarctic Ice Sheets and Sea-Level Rise Less Likely Than Predicted, Study Finds

A new study led by Dartmouth College researchers has found that a “worst-case” projection for sea level rise from the melting of polar ice sheets is highly unlikely, though the accelerating ice loss in Antarctica and Greenland is still alarming.

The study challenges a prediction in a United Nations’ Intergovernmental Panel on Climate Change (IPCC) report that evaluates climate research and projects long- and near-term climate crisis impacts. 

The sixth assessment report from the IPCC proposed the possibility of the collapse of Antarctic ice sheets contributing twice as much to average sea level rise worldwide by 2100 than was projected by other models, with three times the amount by 2300, Dartmouth said in a press release.

“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 this century,” said lead author of the study Mathieu Morlinghem, an Earth Sciences professor at Dartmouth, in the press release.

While IPPC called the prediction “low likelihood,” the model projected the possibility of oceans rising up to 50 feet. This would mean the submersion of the Florida Peninsula, other than a piece of high ground in the interior of the state.

The prediction was based on a new theory of the manner in which ice sheets retreat and break apart. However, the research team said the mechanism — marine ice cliff instability (MICI) — has yet to be observed and has only been put to the test with one low-resolution model.

“[N]o one has observed marine ice cliff instability in action. We don’t know if it will happen, because a lot depends on how quickly the ice collapses,” Morlinghem said in The Conversation. “When the theory of marine ice cliff instability was first introduced, it used a rough approximation of how ice cliffs might collapse once the ice shelf was gone. Studies since then have determined that ice cliffs won’t fail systematically until the ice is about 442 feet (135 meters) high. Even at that point, they would fail more slowly than projected until they became much taller.”

The team used three high-resolution models to test the MICI with an ice collapse rate that “more accurately” captured the complicated dynamics of ice sheets, the press release said. They simulated Antarctica’s Thwaites Glacier — called the “Doomsday Glacier” for its accelerating rate of melt and corresponding potential to raise sea levels — retreating, and found that even Thwaites was not likely to experience rapid collapse this century.

Morlinghem said the study’s findings suggested that the physics in the IPCC report were inaccurate.

“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 in the press release. “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. 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 researchers found that glacial collapse is not simple and does not happen that quickly.

“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 explained. “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.”

The computer model used by the researchers was based on how ice moves and melts. They used it to simulate the retreat of Thwaites for a century after its ice shelf suddenly collapsed, as well as for a half-century using the current actual rate of retreat.

None of the simulations predicted the inland retreat of Thwaites’ ice cliffs at the speed suggested by MICI. Instead, when the ice sheet was not being held back by the ice shelf, the glacier’s movement toward the ocean accelerated rapidly, causing its ice sheet to expand. The accelerated movement thinned ice at the edge of the glacier, reducing cliff height and susceptibility to collapse.

“We’re not calling into question the standard, well-established projections that the IPCC’s report is primarily based on,” said Hélène Seroussi, an associate professor in Dartmouth’s Thayer School of Engineering, in the press release. “We’re only calling into question this high-impact, low-likelihood projection that includes this new MICI process that is poorly understood. Other known instabilities in the polar ice sheets are still going to play a role in their loss in the coming decades and centuries.”