Why did two ice shelves fail in Antarctica? Scientists say they know now.

Rapid collapses of ice shelves on the Antarctic Peninsula over the past quarter century were likely caused by the arrival of huge plumes of warm, moisture-laden air, creating harsh conditions and destabilizing the ice, researchers said Thursday.

The disintegration of the Larsen A shelf in 1995 and the Larsen B shelf in 2002 was preceded by the fall of these plumes, called atmospheric rivers, from the Pacific Ocean. They generated extremely warm temperatures over several days that caused surface ice to melt, cracking and reducing the sea ice cover, allowing ocean swells to bend and weaken the ice shelves further.

said Jonathan Wylie, a climate scientist and meteorologist at Grenoble Alpes University in France and lead author of a study book describing the research in the journal Communications Earth and Environment.

Although there have been no avalanches on the peninsula since 2002, Dr. Wylie and colleagues found that rivers in the atmosphere also caused 13 of 21 major iceberg-forming events from 2000 to 2020.

Dr. Wylie said the larger, mostly still intact Larsen C shelf, the fourth largest in Antarctica, at 17,000 square miles, may eventually suffer the same fate as A and B.

“The only reason the melt is so unremarkable is that it is farther south compared to the others, and therefore it’s much cooler,” he said. But as the world continues to warm, the rivers of the atmosphere are expected to become more dense. “Larsen C will now be in danger from the same operations,” he said.

The work also showed that other parts of Antarctica that aren’t warming as quickly as the peninsula could be vulnerable to eventual infection as well, said Kyle R. Klem, a researcher at Victoria University in Wellington, New Zealand who was not involved in the study. The mechanism the researchers documented depends more on the warming where the atmospheric river originates.

“The amount of heat and moisture that rivers are carrying into the atmosphere is higher than it would be without global warming,” said Dr. Clem. “So the air mass that hits Antarctica is much warmer. It is these extreme events that lead to the collapse of the ice shelf.”

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

The shelves are floating ice tongues that hold back most of the ice covering Antarctica to depths of nearly 3 miles. When the shelf collapses, the flow of this land ice into the ocean accelerates, increasing the rate of sea level rise.

While the Antarctic Peninsula’s ice sheet 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 even greater sea level rise over centuries.

Last month, a small ice shelf collapsed in East Antarctica, which is considered the most stable part of the continent. In earlier days, a highly atmospheric river had reached the area. This resulted in record high temperatures, but the researchers are not yet sure how much, if any, role they played in the shelf disintegration.

Atmospheric rivers occur when a large fixed area of ​​high pressure air meets a low pressure storm system. A narrow stream of moist air flows from the meeting of the two.

In a typical summer in the Southern Hemisphere, the peninsula experiences one to five of these events, the researchers said. They only looked at those with the largest volume of water vapor.

If the river is dense enough, it can melt the surface of the ice shelf for several days. When melt water flows into the cracks it freezes again, causing the cracks to widen and widen. Eventually such repeated water fracturing, as the process is called, can cause the ice shelf to disintegrate.

An atmospheric river can also stimulate the process by melting sea ice, or if accompanying winds push sea ice away from the shelf. This allows ocean waves to shake the ice shelf, increasing pressure on it.

Some large ice shelves in West Antarctica are weakening as a result of melting from under warm ocean waters. Regardless of long-term trends in warming and mitigation, “this paper makes the important point that very short weather events can drive the ice shelf past the tipping point,” said Catherine Walker, a glaciologist at Woods Hole Oceanographic Institution in Massachusetts, who was not involved in the study. “.

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