for National Geographic News
Alaska's glaciers are retreating, reports glacial geologist Bruce Molnia. Significant glacier retreat, thinning, stagnation, or a combination of these changes characterizes all 11 mountain ranges and three island areas that presently support glaciers.
Alaska is home to around 2,000 valley glaciers, including nearly 700 that are named. Fewer than 20 are advancing, according to a major study presented by Molnia, a U.S. Geological Survey scientist, at the annual fall meeting of the American Geophysical Union held December 10 to 14 in San Francisco.
What this means in terms of global warming and human impact on global climate is complex.
"We are certainly experiencing a climatic change that's having a pronounced effect in some areas of the world. But we don't know what component of the change is natural versus what's human induced," said Molnia. "The Earth is not warming uniformly. In Alaska there's no question it's warming, and has been for at least the last five decades. But whether this is a function of natural climate processes or driven by human activities, we just can't say."
Little Ice Age
Scientists disagree about the precise dates, but the world endured a 500-year cold snap from around 1350 to 1850 called the Little Ice Age. Although the global climate event affected different areas at different times, in general it was marked by temperatures several degrees cooler than present. Alaskan glaciersand glaciers worldwideadvanced considerably during the Little Ice Age.
"At the peak of the Little Ice Age, glaciers covered about 10 percent more area in Alaska than they do today," said Molnia. "During the 20th century, most Alaskan glaciers receded and, in some areas, disappeared."
To put this in perspective, when European explorers first sailed along the Alaska coastline in the 1790s, they noted only a small embayment along the coastline. By the 1880s the glacier that filled what became known as Glacier Bay had retreated, leaving a bay that extended nearly 40 miles (64 kilometers). The glacier has continued to retreat and today Glacier Bay extends more than 60 miles (96 kilometers) into the Alaskan coastline.
What complicates the human-induced global warming question is the fact that some of the glaciers in Alaska began their retreat more than 250 years ago, before the human population expanded and the industrial revolution. Some of Alaska's glaciers began their retreat only in the last 25 years.
"The popular perception of global warming is that the entire Earth is warming everywhere. The record doesn't show that," said Molnia. Alaska's temperature changes are far more dramatic than in other regions of the world, and the retreat of Alaska's glaciers is quite significant, concludes Molnia. But there are many regions that show very little temperature change or none at all, and not all the glaciers in the world are melting.
Temperate and Polar Glaciers
The glaciers in Alaska, Chile, Indonesia, New Zealand, and Iran are all temperate glaciers. Temperate or "warm" glaciers are always very close to melting. They tend to lose mass in the summer heat, but gain it from winter precipitation. Nearly all of the world's temperate glaciers are retreating, thinning, or stagnating.
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