The Pine Island Glacier, sometimes called the “soft underbelly” of the West Antarctic Ice Sheet, suddenly started moving about 10 to 12 percent faster between 2017 and 2020, as about 20 percent of its floating ice shelf broke apart in a series of large iceberg calving events. A new study published in Science Advances on Wednesday warns that the rest of the shelf, which holds the glacier onto land, could fall apart in a few decades, rather than the century previously estimated.
The study reinforces other recent research suggesting that some ice shelves could be prone to catastrophic and sudden collapses, and helps fine-tune projections about how fast parts of Antarctica’s huge ice masses will melt and raise sea level. It adds to a growing body of evidence showing that global warming can cause abrupt changes in ice sheets, not just slow and steady melting.
“Typically, when we make projections for this part of the world, the main factors are, what are the ocean and atmosphere going to do for the next decade to century,” said co-author Pierre Dutrieux, a polar researcher with the British Antarctic Survey. “The main thing we find in this study is that, without much change in the ocean or atmosphere over a short time period there are other processes that we were not taking into account.”
The Pine Island Glacier is one of two big ice streams that drains the California-size West Antarctic Ice Sheet, which is more than a mile thick in places and would raise sea level by about 10 feet if it melts completely.
“If this process was to continue, then that would be a problem. That would basically change everything we were predicting in the past,” he added. “But if that was just like a small hiccup, and now the glacier stabilizes again, then we basically go back to saying the ocean and the atmosphere are driving everything. We’re not saying everything has to be thrown away, but it is pointing to something that was unexpected.”
Scientists say the West Antarctic Ice Sheet is especially vulnerable to global warming because much of it is below sea level, and relatively warm ocean water is increasingly melting its floating ice shelf margins.
A big question for researchers is whether the entire West Antarctic Ice Sheet melted about 125,000 years ago, the last time Earth was as warm as it’s projected to be in the coming century. Knowing how fast ice shelves can break up helps answer that. Currently, West Antarctic ice shelves are retreating between .5 and 2 miles per year, but other research suggests that, during periods of global warming millions of years ago, some ice shelves may have retreated 6 miles per year. That rate determines how fast sea level rises.
In the new study, the scientists analyzed thousands of satellite photos and used an ice flow model to show that the loss of the ice shelf drove the accelerating retreat of the glacier, and that the shelf’s disintegration took on a life of its own after big chunks broke off the edge.
Paul Holland, an ocean physicist with the British Antarctic Survey who was not involved with the research, said the study has implications for understanding the rate of future change in Antarctic ice conditions, and the reversibility of those changes if greenhouse gas emissions stop and global temperatures stabilize.
“What really surprised me is how big the calving had been,” he said, referring to the recent disintegration of 20 percent of the Pine Island Glacier’s floating ice shelf. “The fact that after the calving, the ice flow changed, tells you this is not a normal event.” Normal calving, when the flow of ice into the sea is balanced by new ice accumulating at the top of the glacier, doesn’t disrupt the flow, he added.
Antarctica’s frozen cap has persisted in that delicate balance with the climate, shedding some ice as glaciers flow into the sea, and building up new layers high on the ice sheet at about the same rate. Sediment records below the Pine Island ice shelf tell a similar tale, showing that the edge of the shelf remained fairly steady for millennia, with “break-ups happening at approximately the same location until 2017,” Dutrieux said. But then the retreat of the shelf “successively worsened each year until 2020.”
Holland sees a big difference.
“It should be discarding dandruff, but what’s happened here is a big chunk of hair has been pulled out,” he said. “We’ve known at least since the 1990s that the ice shelves are thinning, and that leads to a speed up of the ice streams.”
“The ice isn’t just thinning, it’s cracking apart. This is about the ice failing,” he said. “Once you’ve smashed a piece of glass, you can’t put it back together.”
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But it’s hard to draw global conclusions from the findings, he said, because there are only a small number of ice shelves around the world and each of them is geographically and topographically distinct. Other Antarctic ice shelves face different threats, including warming downslope winds that drive surface melting and the cracking and collapse of giant ice cliffs into the sea. All told, the disintegration of Antarctica’s ice sheets could lead to a worst-case scenario of 4-feet of sea level rise this century.
Since the floating ice shelf holds back the Pine Island Glacier from the sea, the recent weakening of its edge and speedup of its thinning could shorten the timeline for the glacier’s collapse into Pine Island Bay, said co-author Ian Joughin, a University of Washington glaciologist.
The ice shelf appears to be ripping itself apart due to the glacier’s acceleration in the past decade or two,” Joughin said.
He said some research suggests that If the ice shelf falls apart completely in the next few decades, the outflow from the Pine Island Glacier could double. But the difficulty of creating an accurate computer model of how an individual glacier will act in the future leaves many uncertainties, he added.
Overall, he said, the new study is more evidence that global warming impacts on West Antarctica are intensifying.
“Just what we’ve seen over the last 20 to 30 years, that’s pretty rapid on the scale of a glacier,” he said. “They operate on a scale of tens of thousands of years, so to see this much change in a few decades is rather dramatic. The processes we’d been studying in this region were leading to an irreversible collapse, but at a fairly measured pace. Things could be much more abrupt if we lose the rest of that ice shelf.”