Satellite images show that two important glaciers in the Antarctic are sustaining rapid damage at their most vulnerable points, leading to the breaking up of vital ice shelves with major consequences for global sea level rise.
The Pine Island and Thwaites glaciers, which sit side by side in West Antarctica on the Amundsen Sea, are among the fastest changing glaciers in the region, already accounting for 5% of global sea level rise. Scientists say the glaciers are highly sensitive to climate change.
A new study, published in the Proceedings of the National Academy of Sciences on Monday, found that the glaciers are weakening at their foundations and this damage over the past few decades is speeding up their retreat and the possible future collapse of their ice shelves.
The researchers, led by Stef Lhermitte, satellite expert at Delft University of Technology in the Netherlands, used satellite data to document the growth of the damaged areas from 1997 to 2019. The images showed highly crevassed areas and open fractures in the glaciers.
“We knew they were sleeping giants and these were the ones losing a lot of miles (of ice), but how far and how much still remains a large uncertainty,” Lhermitte said. “These ice shelves are in the early phase of disintegration, they’re starting to tear apart.”
Thwaites Glacier is one of the largest and most unstable ice streams in Antarctica. It’s a giant mass of more than 192,000 square kilometers (74,000 square miles) – an area similar in size to the US state of Florida, or Great Britain.
The two glaciers effectively act as arteries connecting the West Antarctic ice sheet to the ocean. At their base are permanent floating ice shelves that act as a buttress to the fast-flowing ice behind it. The region holds enough ice to raise global sea levels by 1.2 meters (4 feet) according to NASA.
So what’s happening to the glaciers now?
Human-induced warming of our oceans and atmosphere because of the increasing release of heat-trapping greenhouse gases is weakening the planet’s ice shelves.
This ocean warming has increased the melting and calving (the breaking off of ice chunks) of Pine Island and Thwaites glaciers, studies show, while declining of snowfall means the glaciers can’t replenish themselves.
The damage researchers found pointed to a weakening of the glaciers’ shear margins – areas at the edges of the floating ice shelf where the fast moving ice meets the slower moving ice or rock underneath.
“Typically the ice shelf acts like slow traffic. It’s floating on the ocean but it buttresses the ice traffic behind it,” Lhermitte said. “So if you weaken this slow car, then the ice discharges more rapidly.”
That’s exactly what the researchers observed – and they believe these severely weakening parts of the glacier will accelerate mass ice loss. The study makes the case that this process should be included in models that project sea level rise, which it’s not currently a part of.
Researchers found that while the tearing of Pine Island Glacier’s shear margins has been documented since 1999, their satellite imagery shows that damage sped up dramatically in 2016.
Similarly, the damage to Thwaites Glacier began moving further upstream in 2016 and fractures rapidly started opening up near the glacier’s grounding line, which is where the ice meets the rock bed.
Researchers warn the process is creating a feedback loop – where the weakening ice shelf is speeding up the damage to the glacier’s vulnerable shear margins, which in turn leads to more damage and disintegration of the ice shelf.
Isabella Velicogna, Professor of Earth System Sciences at the University of California Irvine, who wasn’t involved in the study, said that, “with a process of weakening of the ice shelf included in models, it is likely that the glacier speed up will occur sooner and will be larger in magnitude, which means that sea level will rise faster than currently projected.”
Velicogna said that there are other processes that play “a much larger role” in glacier evolution, such as “the rate of retreat of the grounding line forced by a warmer ocean.”
Glaciers in trouble
The cavities hidden beneath the ice shelf are likely to be the route through which warm ocean water passes underneath the ice shelf up to the grounding line, they said.
Over the past three decades, the rate of ice loss from Thwaites and its neighboring glaciers has increased more than five-fold. If Thwaites were to collapse, it could lead to an increase in sea levels of around 25 inches (64 centimeters).
And there’s more bad news for glaciers on the other side of the world. On Monday, scientists announced that a 44-square-mile chunk of ice, about twice the size of Manhattan, has broken off the Arctic’s largest remaining ice shelf in northeast Greenland in the past two years, raising fears of its rapid disintegration.
The territory’s ice sheet is the second biggest in the world behind Antarctica’s, and its annual melt contributes more than a millimeter rise to sea levels every year.
These recent findings from Antarctica show that the glaciers are “weakening from all angles,” Lhermitte said.
“Most of the weakening in this part of Antarctica is coming from below,” he said. “Warm ocean water gets to the (glaciers’) base and weakens them. What we observed is that this becomes so weakened, that they speed up and once they speed up, the shear margins speed up and start to break.”
Velicogna said the research “points to another Achilles’ heel of the system conducive to faster retreat, and triggered by climate change.”
“It seems that the more we look at these systems evolve, the more we see reasons for them to disappear more rapidly than we thought,” she said. “We have to act quickly on controlling climate change to preserve our future. The time to act is now.”