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A tropical force causes ice shelves to tumble into the sea

After a regional cold spell, "we're back into place where these atmospheric rivers could have bigger consequences."

A tropical force causes ice shelves to tumble into the sea
The black swirl shows the path of an atmospheric river (left); a small iceberg calves from a glacier (right) 1, 2

Scientists were stunned when two major ice shelves in Antarctica collapsed in the late ‘90s and early 2000s.

The loss of the Larsen A and B ice shelves was "quite dramatic and quite a surprise to the [scientific] community,” polar climatologist and meteorologist Jonathan D. Wille tells IE.

Researchers knew the ultimate cause was the combination of a regional warm spell and global climate change. What they couldn't figure out was what had happened in the days and weeks leading up to the unexpected events.

Now, two decades later, a team of more than a dozen researchers has cracked the case. After combining several sources of evidence, they realized that streams of warm, humid air from the tropics and subtropics — called atmospheric rivers — made landfall just before the two ice shelves collapsed, triggering a cascade of events that sent the giant chunks of ice into the sea.

Further analysis revealed that atmospheric rivers were responsible for 60 percent of the major ice shelf collapse and iceberg calving events that happened between 2000 and 2020.

These events “have major consequences even today because, without these ice shelves, the glaciers [they helped contain] are flowing into the ocean more freely,” says Wille, who was part of the research team.

Their findings were published Thursday in the peer-reviewed journal Communications Earth & Environment.

Ice shelves maintain global seal levels

Ice shelves quietly play a crucial part in keeping sea levels within a normal range. “An ice shelf is a floating piece of ice that is connected to a landmass. It kind of acts like a cork in a bottle of wine or champagne,” Wille says.

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Ice shelves form when ice flows from land into frigid ocean waters. While they’re found across the coldest regions of the planet, most are in Antarctica. Once formed, an ice shelf can last for millennia. Larsen B, for example, was roughly 10,000 years old when it collapsed in 2002.

“In Antarctica, the glaciers don't move so much if there's an ice shelf in a way, but if you get rid of the ice shelf, then that ice can just flow into the ocean,” he says. Ice shelf collapse is among the several reasons that global sea levels have risen roughly four inches (10 cm) since the 1990s.

There were clear patterns hiding in the data

Wille and his team brought together several streams of data to understand what causes ice shelves to collapse. They used an automatic detection algorithm to comb through reams of meteorological data in search of atmospheric rivers that occurred around the Antarctic peninsula, and they compared those numbers with a climate model packed with “information about melting and snowfall and other atmospheric processes,” he says.

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Then they connected those findings with satellite images showing the collapse of the Larsen A and B ice shelves along with similar events. “[We] created a sort of climatology of how atmospheric rivers affect the stability of the Antarctic Peninsula ice shelves,” Wille says.

“We found a vast majority of the highest temperatures were happening at the same time as atmospheric river landfills,” he says. “That was leading to very intense surface melting on the Larsen A and Larsen B ice shelves.”

That widespread pattern of surface melting led to a series of events that ultimately spelled doom for the ice shelves.

Record-setting temperatures could cause more ice shelves to collapse

After the collapse of Larsen B in 2002, Antarctica entered a cooler period that saw fewer significant ice shelf collapses. But evidence suggests that temperatures are once again rising at the bottom of the world.

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“The all-time [highest] temperature for Antarctica was set in February of 2020 on the Antarctic Peninsula. In this past summer, we've had more high temperature occurrences on the Antarctic Peninsula that seem to be related to these atmospheric rivers,” Wille says.

Just last month, the 450-square-mile (1,200 km2) Conger ice shelf collapsed in East Antarctica. It was the first such collapse that scientists have documented in the region.

“It does make us a bit concerned because we thought the ice shelves in East Antarctica were stable, and now maybe now this is not the case,” Wille says.

The climatologist says Antarctica may be entering a new period of local warming. “We seem to be entering a new phase of high temperatures,” he says. This time, however, those regional temperature variations are occurring after twenty years of intensifying global climate change.

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“It's something we need to pay attention to because now we're back into place where these atmospheric rivers could have bigger consequences,” he says.

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