Antarctic ice tells conflicting story about climate change's role in big melt

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.

The BAS team, led by Nerilie Abram a researcher with the Australian National University and the BAS, analyzed portions of a 1,194-foot-long ice core from a glacier on James Ross Island.

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.

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