Since the early 1990s, glaciers draining Antarctica's vast ice sheets have dumped ice into the ocean at an an eye-popping rate.
Now, two new studies of ice cores from different parts of the continent are yielding important clues as to why the loss rates have been so high.
On the Antarctic Peninsula, global warming appears to be taking a direct toll. Glaciers are melting mainly from the top down. The peninsula is losing land ice in the summer at a rate unmatched in the past 1,000 years.
For the West Antarctic Ice Sheet, one of two vast continental sheets, losses also have been relatively large. There, however, floating ice shelves that form the seaward end of glaciers are melting from the bottom up. Today's losses are comparable to those that have occurred a few other times over the past 2,000 years. The authors say that for now, the evidence points to the extended reach of naturally shifting climate patterns in the tropical Pacific as driving the losses.
At first blush, the two might appear to be at loggerheads. Instead, researchers suggest, the two highlight how, as on other continents, the intensity of global warming's impact at the bottom of the world depends on location, location, location. And both point to the challenge researchers still face in forecasting the future of the continent's ice chest in a warming climate.
Each in its own way "provides guidance on projecting the future of sea-level rise," notes Richard Alley, a glaciologist at Penn State University in University Park, Pa., who was not a participant in either study.
Researchers have a keen interest in trying to understand and project ice losses in Antarctica, as well as on Greenland, with global warming. Previous studies have shown that since 1992, the loss of ice from polar caps is raising sea levels by an average of about 0.59 millimeters a year.
The West Antarctic Ice Sheet alone could boost sea levels an average of 10 feet if it melts – an increase that would occur over hundreds to thousands of years, notes Eric Steig, a researcher at the University of Washington who led one of the two research efforts.
Between 1992 and 2011, the peninsula lost ice at rate of 20 billion tons a year, according to a study published last November in the journal Science. The West Antarctic Ice Sheet lost 65 billion tons a year, and the East Antarctic Ice Sheet – the continent's largest – lost 14 billion tons a year, although the uncertainties in that number are so large the loss could just as well have been nothing.
The Antarctic Peninsula is an extended arm of land that last shook hands with the southern tip of South America roughly 235 million years ago when the two continents drifted apart. It's mountainous and extends into the Southern Ocean to some 250 miles above the Antarctic Circle.
"In some ways, it's a climate oddity," writes Robert Mulvaney, a researcher with the British Antarctic Survey (BAS) and a member of the team formally reporting its results on the region's ice Sunday in the journal Nature Geoscience, in an e-mail. Relatively warm westerly winds, laden with with ocean moisture, blow across the peninsula. So it tends to be warmer than the mainland and experiences higher snowfall rates.
Even so, the buildup of the loss of ozone in the stratosphere and the buildup of greenhouse gases – both from human industrial activity – have affected circulation patterns over the region in ways that have left the peninsula as one of the fastest-warming regions on the planet.
They found that from the coldest point in their record, between 1410 and 1460, melt rates are about 10 times higher today than they were then. But most the most intense melting has occurred since the middle of the last century. And it's been occurring at the surface, providing water that can lubricate the bottom of land glaciers and filling crevasses to act as ice-breaking wedges when the trapped water refreezes.
The results show that "the Antarctic Peninsula has warmed to a level where even small increases in temperature can now lead to a big increase in summer ice melt," said Dr. Abram in a prepared statement.
West Antarctic Ice Sheet
Meanwhile, Dr. Steig's team analyzed an ice core from high on the West Antarctic Ice Sheet, some 350 miles inland from the Amundsen Sea – comparable to the distance between Los Angeles; and Flagstaff, Ariz. The core covers 2,000 years of climate history. In addition, the team used data from other cores taken in the region and found that its core does a good job of representing conditions in the region as a whole.
In general, the climate on the ice sheet is significantly different, notes Steig. It lacks the warmer temperatures that accompany an oceanfront view. And the surface at the drill sites is around 6,000 feet above sea level. When the sheet loses ice, it's at the ocean end of glaciers that typically become grounded on the sea floor. But changing wind patterns can bring to the surface warm water that usually stays deeper in the ocean. This warm water melts the shelves from underneath until they no longer are grounded. The ungrounded portions break free as vast icebergs, leaving the glacier to deliver more interior ice to the coast, where the process repeats.
As the BAS team found, Steig and colleagues noted warming during the 1990s, accelerating the ice loss. But the team also found that comparable conditions in the 1830s and 1940s, as well as further back. That suggests a loss of ice comparable to the rates seen today, he and his colleagues say.
The '90s, '40s, and 1830s were characterized by strong El Niños – conditions in the tropical Pacific in which waters in the eastern tropical Pacific are warmer than normal, while the waters in the western Pacific are cooler than normal. This alters atmospheric circulation patterns. While the effects are strongest in the tropics, they appear at higher latitudes as well.
Thus, at least for now, it would appear that natural climate swings are playing a greater role in the loss of ice from West Antarctica than global warming, the researchers suggest.
If conditions are largely governed by conditions in the tropical Pacific, as they appear to be, an ability to project the ice sheet's future in a warmer world depends on researcher's ability to figure out whether El Niño conditions will predominate in the future, or La Niña conditions – El Niño's opposite. This study also was published Sunday in Nature Geoscience.