The visitors center must have seemed like a can't-miss idea when it opened in 1984 and quickly became a top tourist attraction.
People flocked to the site to take in the dramatic view of the crystalline blue Portage Glacier that the center's floor-to-ceiling windows offered.
How things have changed. Now, tourists watch a film about the famed glacier, complete with scenes of gigantic chunks breaking off into the water. Then the curtains are pulled back from the viewing windows to reveal - a lake. Portage Glacier has retreated out of sight behind a ridge a mile off, melting away to leave behind a lake dotted with icebergs.
It didn't come as a surprise that Portage Glacier, like others, went into reverse gear; what was startling was its pell-mell pace.
''It's starting to knock on my consciousness that (global warming) is real, '' said visitors center interpreter Charlu Choate. ''The glacier retreated around the corner out of our view about 26 years before geologists predicted.''
Portage Glacier's disappearance is a stark example of the quickening retreat of glaciers outside the polar regions, what a growing number of scientists believe is an early indicator of the effects of global warming. While the Earth's average temperature has increased by 1 degree Fahrenheit this century, temperatures in northerly locations such as Alaska, northwestern Canada, and Siberia have risen as much as 5 degrees because the Earth's air and water circulation patterns diffuse increased heat to the coldest regions.
Mountain glaciers - they account for most of the world's glaciers outside of Greenland and Antarctica - appear to have been dramatically affected by the higher temperatures, including the record hot years of the past decade. On average, says the Switzerland-based World Glacier Monitoring Service, glaciers thinned about 12 feet from 1980-95 and more since then, but still a relatively small percentage of the glaciers' total bulk.
However, the fact that glaciers are melting alone doesn't prove that the higher temperatures were caused by human actions, and glaciologists are quick to point out that a cooling trend could help fatten them up again. After all, the planet just ended a long cooling trend called ''The Little Ice Age'' a couple of centuries ago.
In southern Alaska, where 29,000 square miles of land are still ice-covered, some retreats have been spectacular. Columbia Glacier has retreated 8 miles since 1982, releasing icebergs all over Prince William Sound. (The Exxon Valdez tore itself open on Bligh Reef in 1989 partly because the crew was trying to avoid an iceberg from Columbia Glacier.)
Glaciologists at the U.S. Geological Survey, though, are more concerned about the less spectacular but record-breaking retreat of two glaciers in south-central Alaska thought to be especially sensitive to air temperature. Wolverine Glacier in southern Alaska thinned an unprecedented 24 feet from 1989 through 1995, while Gulkana Glacier near Fairbanks thinned by 15 feet, even though neither is exposed to standing water, which speeds the melting process.
The dwindling mountain glaciers have attracted relatively little attention, perhaps because they are tiny compared to the massive ice sheets on Greenland and Antarctica that have shrunk little and, together, account for 95 percent of the Earth's glaciers.
But Dennis Trabant of the U.S. Geological Survey says some regions, such as the Pacific Northwest, will feel the consequences. They will have less water to drink, to generate hydro-electric power or even to provide spawning grounds for salmon if the glaciers melt. Mountain glaciers, he noted, are a key water source in some regions, particularly in summer.
Warmer air has an even more direct effect on the permafrost, the frozen ground on which Alaskans have built houses, roads and other structures. Already, the effect of the softening permafrost can be seen in the state's ''drunken forests'' - where trees tilt chaotically as the soil yields. And in northern Canada, mudslides have become frequent because of the soft ground.
If the mountain glaciers are going, they are not going quietly. Matanuska Glacier, about 80 miles northeast of Anchorage, burps and bloops, fizzles and gurgles, as rivulets of water race down the ice, carving caves and chasms as it melts.
Glaciologists point out that the current glacial retreat must be seen in the context of long time scales. At the height of the last Ice Age, 17,000 years ago, 30 percent of the world was covered in ice, compared with 10 percent today, and when it thawed, the sea level rose 330 feet. That makes the current melt seem modest.
Moreover, glaciologists stress that the current warming trend is still a blip on the planetary screen that could reverse itself as quickly as it came.
Further complicating predictions of the glaciers' fate: Glaciers are not governed solely by air temperature. The amount of snow, the glacier's fuel, also is important, as is whether the glacier's ''face,'' or lower end, is in water, which transfers heat to the ice 20 times more efficiently than air.
Portage Glacier, for example, probably started melting because of the warming climate, but the deep meltwater lake it created has since become the driving force of its accelerating retreat.
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