On February 28, an ice shelf the size of Connecticut State on the southwest Antarctic Peninsula began to collapse. The world watched via satellite as a 160-square mile portion of the floating ice mass fractured and strained and finally disintegrated into billions of ice shards, some the size of railroad cars, most a composite of slush.

Believed to be several hundred years old, the Wilkins ice shelf is the world’s second large ice shelf to meet its end by the before-unseen process of disintegration. The first was the Larsen B on the eastern side of the Antarctic Peninsula, which broke up in 2002.

Ice shelves extend from the coastline and are often formed by inland glaciers flowing toward the sea. As the shelf grows farther from the shore, it naturally weakens and calves icebergs. But the crumbling that scientists have recently witnessed is new and believed to be a result of rising global temperatures.

“The new phenomenon that’s been seen with the Larsen and now Wilkins is shattering of the ice sheet, where it doesn’t just calve off a glacier, but a bunch of little shards of ice bergs, that’s a new feature that hadn’t been seen before,” says National Snow and Ice Data Center scientist Walt Meier. 

Scientists have been working to try and understand the new scenario since the Larsen event. Ted Scambos of the National Snow and Ice Data Center has made progress in unraveling the process behind the shattering. He says it begins with an increase in melted ice, which is a result of a warmer atmosphere. The water pools on the surface of ice sheets, forming lakes that can be tens of feet deep and hundreds of feet wide. In some cases the water drains down into crevasses in the ice. If they’re deep enough, the heavier water acts as a wedge against the ice, prying and breaking it apart.

If this deepening and multiplying of crevasses coincides with other factors, such as a lack of sea ice to buffer the ice shelf from wind and waves, the shattering process may commence. Scambos and his colleagues are busy trying to identify the precursors of disintegration in order to better predict when they’ll occur.

“The main reason to care,” says Scambos, “is that when these ice shelves collapse like this, they retreat not just back to a stable line like normal ice shelves do, they retreat all the way to the coastline. When that happens, the glaciers that flow into these large plates of flowing ice are less obstructed, so [they] flow more rapidly.”

Once the ice shelf is gone, the unleashed glaciers dump more ice into the ocean. When that melts, sea level rises. Meier described a domino effect scenario in the Antarctic in which the largest ice shelves collapse and multiple glaciers begin to surge toward the sea. “It’s quite within the realm of possibility that in the coming centuries that the West Antarctic ice sheet would disintegrate and that would raise sea level about 20 feet,” Meier says.

Where speculation of this sort once seemed fantastical, it’s quickly becoming a reality. In recent weeks climate scientists, including the world-renowned head of the NASA Goddard Institute for Space Studies James Hansen, have said current carbon emission limits are too liberal. While Hansen and scientists collaborating on the IPCC reports released last year stated the world must prevent atmospheric concentrations of greenhouse gasses from exceeding 450 parts per million, in a new study they say, “Our current analysis suggests that humanity must aim for an even lower level of GHGs.”

The paper explains that, “’slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less.”

Scambos says this reassessment of modeling data and recasting of recommendations made by the IPCC applies directly to ice shelves. There was disagreement in the 80s and 90s, he explained, over whether the loss of ice shelves would increase glacier speed and sea level rise. When Larsen occurred, the debate was laid to rest. But as a result, the old models don’t incorporate disintegration events like Larsen and Wilkins when trying to determine how quickly and how much sea level will rise.

According to Scambos, “We should really get this going and aim for a reduction in the current level of greenhouse gases because we’re a long way away from seeing the final effects of our current concentration of greenhouse gases.”

See more articles from In Depth

Posted on Apr 11, 2008 at 8:00 AM
Permalink | Comments (0) | TrackBacks (0)