What’s driving the collapse of Antarctic ice shelves?

New CSU study eyes regional climate cycle as one factor

Disintegration of ice shelves in East Antarctica could result in a spiraling increase in sea levels. PHOTO BY BOB BERWYN.
Disintegration of ice shelves in East Antarctica has been linked with a regional climate pattern and could result in a spiraling increase in sea levels. @bberwyn photo.

Staff Report

This year’s rapid loss of sea ice around Antarctica may make the floating ice shelves along the coast of the continent even more vulnerable to collapse, and a  Hemisphere climate cycle known as the Southern Annular Mode is probably a factor in the equation.

Just during the past 50 years, more than 28,000 square kilometers of ice shelves have crumbled along the Antarctic Peninsula, and one of the last remaining large ice shelves looks set to shed another piece about as large as Delaware. A new study led by Colorado State University scientists offers some new information on climate processes in the region.

The findings show that the Southern Annular Mode affects storm activity and the extent of sea ice surrounding the Antarctic Peninsula. Sea ice may protect ice shelves from the impacts of ocean storms by weakening wave intensity before it reaches the coastline. Scientists have long thought that shifts in the Southern Annular Mode, which describes a large-scale pattern of atmospheric variability for the Southern Hemisphere similar to El Nino in the tropics, may produce conditions that can lead to the collapse of ice shelves.

To study those effects, the researchers looked at long-term variations in seismic signals, called microseisms, generated by ocean waves in the region. The data was collected over 23 years at Palmer Station on the Antarctic Peninsula and East Falkland Island near South America.

“We were able to show that storm and ocean wave activity in the Drake Passage, the ocean basin between the Antarctic Peninsula and South America, increases during positive phases of the Southern Annular Mode,” said geoscientist and lead author Robert Anthony. “We were also able to verify that sea ice cover does indeed impede ocean swell from reaching the coastline by showing which regions of sea ice impact the intensity of microseisms.

The findings suggest that the positive phase of the Southern Annular Mode may contribute to ice shelf weakening and potential collapse events by:

  • increasing air temperatures on the Antarctic Peninsula, which can enhance surface melting of ice shelves,
  • driving off sea ice, which enables ocean waves to directly impact ice shelves, and
  • generating stronger wave events.

Researchers had previously speculated on a link between ice shelf collapse and the Southern Annular Mode, based mainly on elevated air temperatures. But the CSU team now suspects that the reduction of sea ice and strong wave events in the Drake Passage could also play a role in rapid collapse events, such as the dramatic collapse of the Larsen A ice shelf in 1995 and, perhaps, the ongoing fracturing of the Larsen C ice shelf.

The team’s next steps include looking more closely at specific ocean swell events and sea ice conditions during known ice shelf collapses and large iceberg calving events.

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