|
After 2 centuries of shrinking, Alaska glaciers got thicker this year By Craig Medred, Space & Earth science / Earth Sciences
Two hundred years of glacial shrinkage in Alaska, and then came the winter and summer of 2007-2008. Unusually large amounts of winter snow were followed by unusually chill temperatures in June, July and August.
"In mid-June, I was surprised to see snow still at sea level in Prince William Sound," said U.S. Geological Survey glaciologist Bruce Molnia. "On the Juneau Icefield, there was still 20 feet of new snow on the surface of the Taku Glacier in late July. At Bering Glacier, a landslide I am studying, located at about 1,500 feet elevation, did not become snow free until early August. "In general, the weather this summer was the worst I have seen in at least 20 years." Never before in the history of a research project dating back to 1946 had the Juneau Icefield witnessed the kind of snow buildup that came this year. It was similar on a lot of other glaciers too. "It's been a long time on most glaciers where they've actually had positive mass balance," Molnia said. That's the way a scientist says the glaciers got thicker in the middle. Mass balance is the difference between how much snow falls every winter and how much snow fades away each summer. For most Alaska glaciers, the summer snow loss has for decades exceeded the winter snowfall. The result has put the state's glaciers on a long-term diet. Every year they lose the snow of the previous winter plus some of the snow from years before. And so they steadily shrink. Since Alaska's glacial maximum back in the 1700s, Molnia said, "I figure that we've lost about 15 percent of the total area." What might be the most notable long-term shrinkage has occurred at Glacier Bay, now the site of a national park in Southeast Alaska. When the first Russian explorers arrived in Alaska in the 1740s, there was no Glacier Bay. There was simply a wall of ice across the north side of Icy Strait. That ice retreated to form a bay and what is now known as the Muir Glacier. And from the 1800s until now, the Muir Glacier just kept retreating and retreating and retreating. It is now back 57 miles from the entrance to the bay, said Tom Vandenberg, chief interpretative ranger at Glacier Bay. That's farther than the distance from glacier-free Anchorage to Girdwood, where seven glaciers overhang the valley surrounding the state's largest ski area. The glaciers there, like the Muir and hundreds of other Alaska glaciers, have been part of the long retreat. Overall, Molnia figures Alaska has lost 10,000 to 12,000 square kilometers of ice in the past two centuries, enough to cover an area nearly the size of Connecticut. Molnia has just completed a major study of Alaska glaciers using satellite images and aerial photographs to catalog shrinkage. The 550-page "Glaciers of Alaska" will provide a benchmark for tracking what happens to the state's glaciers in the future. Climate change has led to speculation they might all disappear. Molnia isn't sure what to expect. As far as glaciers go, he said, Alaska's glaciers are volatile. They live life on the edge. "What we're talking about to (change) most of Alaska's glaciers is a small temperature change; just a small fraction-of-a-degree change makes a big difference. It's the mean annual temperature that's the big thing. "All it takes is a warm summer to have a really dramatic effect on the melting." Or a cool summer to shift that mass balance the other way. One cool summer that leaves 20 feet of new snow still sitting atop glaciers come the start of the next winter is no big deal, Molnia said. Ten summers like that? Well, that might mark the start of something like the Little Ice Age. During the Little Ice Age – roughly the 16th century to the 19th – Muir Glacier filled Glacier Bay and the people of Europe struggled to survive because of difficult conditions for agriculture. Some of them fled for America in the first wave of white immigration. The Pilgrims established the Plymouth Colony in December 1620. By spring, a bitterly cold winter had played a key role in helping kill half of them. Hindered by a chilly climate, the white colonization of North America through the 1600s and 1700s was slow. As the climate warmed from 1800 to 1900, the United States tripled in size. The windy and cold city of Chicago grew from an outpost of fewer than 4,000 in 1800 to a thriving city of more than 1.5 million at the end of that century. The difference in temperature between the Little Ice Age and these heady days of American expansion? About three or four degrees, Molnia said. The difference in temperature between this summer in Anchorage – the third coldest on record – and the norm? About three degrees, according to the National Weather Service. Does it mean anything? Nobody knows. Climate is constantly shifting. And even if the past year was a signal of a changing future, Molnia said, it would still take decades to make itself noticeable in Alaska's glaciers. Rivers of ice flow slowly. Hundreds of feet of snow would have to accumulate at higher elevations to create enough pressure to stall the current glacial retreat and start a new advance. Even if the glaciers started growing today, Molnia said, it might take up to 100 years for them to start steadily rolling back down into the valleys they've abandoned. "It's different time scales," he said. "We're just starting to understand." As strange it might seem, Alaska's glaciers could appear to be shrinking for some time while secretly growing. Molnia said there are a few glaciers in the state now where constant snow accumulations at higher elevations are causing them to thicken even as their lower reaches follow the pattern of retreat fueled by the global warming of recent decades. _ © 2008, Anchorage Daily News (Anchorage, Alaska).
Visit the Anchorage Daily News online at http://www.adn.com/
Distributed by McClatchy-Tribune Information Services. http://www.physorg.com/news145187972.html
|
