Monday, August 8, 2016

River of Ice: Witnessing the 'Catastrophic Retreat' of Alaska's Columbia Glacier

Water & Watersheds

By John Shepard

A harrowing ride down a steep mountainside in the brutal white embrace of an avalanche introduced me some years ago to a miraculous quality of water. It can flow—no, too tame; make that rage—down-slope very much like a wild and tumbling river, even though it has taken shape as a solid. Recently I had another extraordinary experience of solid water's fluidity at the cliff-like terminus of the Columbia Glacier in Alaska's Prince William Sound. This time I was ready with a camera to record an unforgettable moment of what glaciologists call the "catastrophic" retreat of a magnificent river of ice.

A Victim of Climate Change?

In a state with more than 100,000 glaciers, the Columbia is part of a special subset. It is one of Alaska's tidewater glaciers—that is, its terminus reaches the sea; at least for the time being. As recently as 1980, the glacier's snout was parked where it was when first surveyed by British explorers in 1794, and it likely was there long before that. Some 20 km. (12 mi.) down Columbia Fjord from where the glacier ends today, it was nestled against its terminal moraine, a submerged ridge of rock and gravel stretching across the fjord that was bulldozed into place by the glacier.

Since 1980, the Columbia has been retreating at a break-neck pace, for a glacier; nearly faster than any other on earth. Encountering this geologic and hydrologic phenom, I initially assumed that the remarkable rate of the Columbia's retreat was caused by our rapidly changing climate. While climate change is a factor, this much-studied glacier has revealed that more is going on.

Cycles of Retreat and Advance

Alaska's tidewater glaciers follow common patterns of advance and retreat. They originate in massive ice fields that cap Alaska's coastal mountain ranges where heavy amounts of snow accumulate at elevations of 3,000 meters (10,000 feet) and higher. Under this weight, the glaciers flow slowly downhill toward and into the sea at rates measured in meters per year. When they reach seawater, they will continue to advance, grinding and carving deep fjords, as long as the accumulation of snow at higher elevations outpaces the melting and calving of icebergs at sea level. Tip the balance of snowfall and calving more evenly, and a tidewater glacier will stabilize, its terminus remaining in place for hundreds of years. But when the scales tip the other way, sometimes "catastrophically" (i.e., at a tremendous rate), a glacier will retreat.

Since 1980 the Columbia has retreated at a rate that peaked in 2001 at 30 meters (98-feet) per day, on average. It has thinned as well, having lost about half of its thickness and volume. By about 2030 it will have retreated another 15 kilometers (9 mi.)—far enough to finally get its nose out of salt water. The Columbia is then expected to reverse course and start slowly advancing again, at a pace that may be influenced by the prevailing climate.

When early glaciologists encountered Alaska's tidewater glaciers, this kind of behavior didn't jibe with scientific explanations that had been developed by studying Europe's alpine glaciers, which cling to high elevations and remain distant strangers to the sea. Especially puzzling was the fact that one tidewater glacier could be advancing while its near neighbor was retreating. New theories have been developed by scientists investigating the Columbia as it has frantically shed an amazing abundance of icebergs of all shapes and sizes into Prince William Sound.

Click to enlarge. (Creative Commons)
The illustration at right helps illuminate a dynamic that accounts for the Columbia's dramatic retreat. Prior to 1980, the glacier's stable position against its terminal moraine kept sea water at bay. Possibly with a nudge from a warming climate, the terminus retreated slightly from the moraine. This diminished support for the terminus and admitted warmer sea water beneath the glacier, which initiated the hasty retreat. The pace accelerated even more as the glacier retreated through deep areas of the fjord where part of the glacier's terminus was afloat. At times, the calving has been so active that the parade of icebergs emerging from Columbia Fjord posed threats to oil tankers crossing Prince William Sound to access the Alaska pipeline in Valdez.

The video below, in addition to documenting my exploration of the Columbia's retreat aboard the research and ecotourism vessel Auklet, includes commentary by captain Dave Janka, providing further background about the forces at work in the cyclic retreat and advance of the region's extraordinary tidewater glaciers.

Columbia Glacier, Alaska, by Adam Voiland. NASA Earth Observatory
Calving Glaciers with Emphasis on Columbia Glacier, Megan Kennedy, College of Woster

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