Experts get shrinking feel about Alaska glaciers

Low elevation a factor
Southeast Alaska’s glaciers are very sensitive to climate change because of their large surface areas at low elevations. In Juneau, the winters have been getting warmer and rainier — 6.8 degrees warmer compared to 50 years ago, according to Laurie Craig, a naturalist for the Tongass National Forest.

Those warmer temperatures can disrupt a glacier’s surface mass balance, the balance achieved between the melting period of summer and accumulation period of winter.

For many Alaska glaciers at lower elevations, warmer temperatures are causing the equilibrium line that separates the accumulation zone from the melting zone to rise. Yakutat Glacier, for example, has lost nearly all of its accumulation zone.

“This icefield will likely disappear completely under current conditions,” the authors of the new study wrote.

While climate change causes equilibrium shifts and thinning, it isn’t the only reason Alaska’s tidewater glaciers are retreating from lakes and the sea. The retreat may be triggered by warmer temperatures, but then the dynamic cycle of a tidewater glacier takes over.

The speed of the glacier increases, drawing down the ice from above at a faster rate and increasing calving below. In Southeast Alaska, the ice loss at their terminus can cause tidewater glaciers to retreat more than half a mile a year — and that loss can’t be directly attributed to climate change, the scientists say.

“Once initiated, these calving losses are largely independent of climate change and can be an order of magnitude greater than ice losses driven solely by climate change,” they wrote.

A few get bigger
Then there are the anomalies. Five percent of the glaciers studied, such as the Taku in the Juneau Ice Field, are expanding and thickening.

Many of these glaciers extend higher in elevation, giving them a larger zone where snow can accumulate.

Glacier dynamics have the opposite effect with these glaciers. Their accumulation zones are expanding and their melting zones are shrinking. The result is a different kind of imbalance, one that causes the glaciers to advance.

Motyka said scientists will have a better understanding of what has happened to the glaciers since the 2000 space shuttle data once new photos taken this summer are analyzed. With the last analysis showing glaciers melting at twice the rate previously thought, he said he expects more of the same.

“Presumably, things have accelerated,” he said.

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