An Antarctic ice shelf that is twice the size of Hawaii is at “imminent risk” of collapse and needs to be monitored carefully, a new study finds.

The ice shelf—Larsen C—is located in roughly the same geography as the Larsen A and B ice shelves, which disintegrated in 1995 and 2002, respectively. Larsen C covers 19,300 square miles and is the largest shelf on the Antarctic Peninsula. If it melts, it could significantly raise global sea levels, said Paul Holland, the lead author of the study and a scientist with the British Antarctic Survey.

“If [Larsen C] collapses, this will cause several centimeters of sea-level rise, potentially within a few decades,” he said via email. “We therefore need to understand and predict this risk.”

The study was published yesterday in the journal The Cryosphere, and joins a steady drumbeat of bad news from Antarctica. A comprehensive study last month from the continent found that some shelves have thinned by as much as 18 percent in two decades.

Ice shelf collapse is worrisome because the shelves are fingers of ice that stick out into the ocean and act as plugs trapping Antarctic glaciers on land. In their absence, continental glaciers accelerate their slide into the sea.

For instance, glaciers that were behind the Larsen B are flowing eight times as quickly as they were before the ice shelf collapsed in 2002, studies have found (ClimateWire, March 27).

Melting from the top and bottom
Scientists have recently focused on Larsen C, which is also thinning. Holland and his colleagues think they know why. The ocean below Larsen C has warmed in recent years up to minus 2 degrees Celsius. That is warm enough to melt ice, Holland said.

The ice has been thinning from below by 28 centimeters per year for the past 15 years, the study finds. To get this data, the scientists used airplanes to shoot radar waves at the shelf and measure its ice content. They also used satellite data.

Larsen C is also melting from the top at a rate of 4 centimeters per year, the study finds. The top melt is caused by a combination of decreased snowfall and occasional warmer temperatures. The Antarctic Peninsula has warmed by 2.5 degrees C in the past 50 years.

For the shelf to disintegrate due to the thinning alone would take centuries. Rather, as the thinning progresses, the shelf could reach tipping points, the study finds.

One tipping point would occur if the ice shelf’s front retreats past a critical zone, destabilizing the shelf. A crack in the shelf that may hasten this event already exists.

Another possibility is that the shelf may detach from the Bawden Ice Rise, which is “grounded” ice that attaches the ice shelf at its northern end to the continent. This, too, would cause the ice shelf to collapse rapidly, Holland said.

The scientists said they would like to test their results further, but running experiments in Antarctica can be prohibitively expensive.

“It would require a dedicated set of ocean, ice and atmosphere measurements lasting over 10 years ... this would require a very large amount of funding ... tens or hundreds of millions of dollars,” he said.

“What is more achievable is to keep repeating these radar surveys, and there are projects underway to look more into the atmosphere and ocean that are causing the changes we see,” he added.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500