Why did two Antarctic ice shelves fail? Scientists say they know it now.

The rapid collapse of two ice shelves in the Antarctic Peninsula during the last quarter of a century was probably caused by the arrival of huge plumes of warm, humid air that created extreme conditions and destabilized them. the ice, researchers said Thursday.

The disintegration of the Larsen A platform in 1995 and the Larsen B platform in 2002 was preceded by the arrival on land of these plumes, called atmospheric rivers, from the Pacific Ocean. They generated extremely warm temperatures over several days that caused the surface melting of the ice that caused the fracture and reduced the sea ice cover, allowing ocean waves to flex the ice shelves and weaken them further.

“We identify atmospheric rivers as a mechanism that can create extreme conditions on the ice shelves of the Antarctic Peninsula and potentially lead to their destabilization,” said Jonathan Wille, a climatologist and meteorologist at the Université Grenoble Alpes in France and lead author of ‘a study. study describing the research in the journal Communications Earth and Environment.

Although there have been no collapses on the peninsula since 2002, Dr. Wille and colleagues found that atmospheric rivers also triggered 13 of the 21 major iceberg events between 2000 and 2020.

Dr. Wille said that the largest Larsen C platform, which is still mostly intact and, at about 17,000 square miles, is the fourth largest ice platform in Antarctica, could suffer the same fate as A and B.

“The only reason the merger hasn’t been significant so far is because it’s further south compared to the others, so it’s colder,” he said. But as the world continues to warm, atmospheric rivers are expected to become more intense. “The Larsen C will now be at risk from the same processes,” he said.

Kyle R. Clem, a researcher at Victoria University of Wellington in New Zealand who did not participate in the study, said the work also showed that other parts of Antarctica are not warming as fast as the peninsula. they could also be susceptible, as the mechanism documented by the researchers depends more on the warming where the atmospheric river originates.

“The amount of heat and moisture carried by atmospheric rivers is greater than it would be without global warming,” said Dr. Clem. “So the mass of air that crashes into Antarctica is much, much warmer. And it’s these episodes of extreme events that lead to the collapse of the ice shelf.”

“You can get it anywhere in Antarctica,” he said.

Shelves are floating tongues of ice that serve to hold most of the ice that covers Antarctica at depths of up to almost 3 miles. When a platform collapses, the flow of this land ice into the ocean accelerates, increasing the rate of sea level rise.

Although the ice sheet of the Antarctic Peninsula is relatively small (if it all melted, the seas would rise by less than a foot), the collapse of ice shelves elsewhere on the continent could lead to an increase in the level. of the sea much larger over the centuries.

Last month, a small ice shelf collapsed in East Antarctica, which is considered the most stable part of the continent. In the previous days a river of intense atmosphere arrived in the region. It caused record high temperatures, but researchers are still unsure of the role it played, if any, in the disintegration of the shelf.

Atmospheric rivers occur when a large stationary area of ​​high-pressure air encounters a system of low-pressure storms. A narrow stream of moist air flows from the confluence of the two.

In a typical summer in the southern hemisphere, the peninsula goes from one to five of these events, the researchers said. They looked only at those that contained the highest volume of water vapor.

If a river is strong enough, it can cause several days of surface melting of the ice shelf. As the melting water flows into the cracks it freezes again, expanding and expanding the cracks. Finally, this repeated hydrofracturing, as the process is called, can cause the ice shelf to disintegrate.

The atmospheric river can also stimulate the process by melting sea ice, or if its associated winds move sea ice away from the platform. This allows ocean waves to move the ice shelf, further stressing it.

Some large ice shelves in West Antarctica are thinning as a result of melting bottom by warm ocean water. Catherine Walker, a glaciologist at the Woods Hole Oceanographic Institution in Massachusetts who did not participate in the study, said that regardless of long-term warming and slimming trends, “this article raises the important point that weather events are very can push an ice platform beyond its turning point. “

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