Alaska contains several thousand valley glaciers, and fewer than 20 of them are advancing, while many of the rest are retreating, according to a study by U.S. Geological Survey (USGS) scientist Bruce F. Molnia. Significant glacier retreat, thinning, stagnation, or a combination of these changes characterizes all eleven mountain ranges and three island areas that presently support glaciers there.

?The Earth recently emerged from a global climate event called the ?Little Ice Age,? during which Alaskan glaciers expanded significantly. The Little Ice Age began to wane in the late 19th century. In some areas of Alaska, glacier retreat started during the early 18th century, prior to the beginning of the industrial revolution,? says Molnia.

An example of glacial retreat that started as the Little Ice Age began to wind down is found at Glacier Bay, a popular destination for cruise ships. During the 1790s, when European explorers first sailed in the vicinity of Glacier Bay, they noted only a small coastline because a large glacier filled much of the basin of Glacier Bay. By the 1880s, continued glacier retreat resulted in a bay that extended nearly 40 miles.

Today Glacier Bay extends more than 60 miles and cruise ships travel there regularly. ?Ironically, the climate event that made cruising into Glacier Bay not only possible, but popular, could ultimately take away its top attraction as many tidewater glaciers now retreat out of the water,? says Molnia.

The retreating valley glaciers of Alaska are temperate, or ?warm? glaciers, composed of liquid water that coexists with glacier ice for all or part of the year. Temperate glaciers are scattered throughout the world, and nearly all show post-Little Ice Age reductions in volume. In many cases, these reductions are spectacular.

In contrast, polar or ?cold? glaciers, which have temperatures significantly below freezing, show only minor responses to changing climate. More than 96 percent of the glacier ice on Earth is polar.

Molnia has conducted aerial reconnaissance and field observations of Alaskan glaciers for more than 30 years. He has examined descriptions and maps of glaciers published during the past 200 years, photographs dating back more than 115 years, aerial photography dating back 75 years, and airborne and satellite data dating back more than 20 years.

Molina says, ?At the peak of the Little Ice Age, glaciers covered about 10 percent more area in Alaska than they do today. During the 20th century, most Alaskan glaciers receded and, in some areas, disappeared. But it is important to note that our data do not address whether or not any of these changes are human induced.?

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