Earthshots: Satellite Images of Environmental Change

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» Columbia Glacier, Alaska, USA

Some landscape changes around the world happen at glacial pace—very slowly—and have not been captured in the satellite era. However, glacial pace would not describe the changes to Alaska’s Columbia Glacier over the past few decades.

Columbia Glacier is a large tidewater glacier that flows south out of the Chugach Mountains in Alaska to Prince William Sound. Since 1980, its terminus has retreated 20 kilometers to make it one of the most rapidly changing glaciers in the world.

When it was surveyed by British explorers in 1794, Columbia’s terminus was at the northern edge of Heather Island. It stayed there until 1980, when its current rapid retreat began. A 300-meter deep fjord now replaces the portion of the valley once occupied by the glacier.

This series of Landsat images shows the rapid retreat, including an acceleration of the retreat in the early 1990s, followed by slowdowns in 1994–1997 and 2000–2006. It has picked up the pace of retreat again since 2006. Columbia has also become narrower, as shown by the expansion of bedrock areas in the images.

The false-color images use shortwave-infrared, near-infrared, and green wavelengths to highlight these changes, where snow and ice appear cyan, vegetation is green, open water is dark, and exposed bedrock is brown.


Why this matters
Land-based ice flowing into the ocean from tidewater glaciers is a leading cause of sea level rise. A 2015 study found that Columbia Glacier sent about 4 gigatons (billions of tons) of ice into the ocean every year from 1994 to 2013. An increasing rate of iceberg calving can also pose a danger to ships in shipping lanes.

Map of the featured area.

Landsat images show chunks of ice floating on the surface of the water as Columbia Glacier retreats. These icebergs have broken off, or calved, from the front edge of the glacier. This conglomerate of floating ice and chunks of icebergs is known as an ice mélange, which can slow the rate that glaciers slip into the sea.

The ice mélange looks less defined in the images than the glacier itself, which usually has some smooth-looking longitudinal lines. The mélange appears more mottled, and in some images there is very little of it at all (like in 2006). So don’t confuse ice mélange with the end of the glacier.

Columbia’s terminal moraine is still evident long after the end of the glacier retreated far upstream and serves as a marker for the maximum advance of the glacier. A terminal moraine is the accumulated mass of sediment, rocks, and debris that the glacier deposits at the terminus. The moraine at this historical terminus prevents the mélange from drifting beyond it.

View an animation showing more Landsat images of Columbia Glacier.

Air temperature increases can bring about long-term loss of ice mass from a tidewater glacier. An increase of less than 2 degrees Celsius over the mean annual air temperature is all it takes to trigger glacial retreat. In fact, Alaska has seen an increase in mean annual air temperature of 1.7 degrees Celsius over the past 60 years. But once a glacier has begun retreating, temperature alone does not have as much influence on its behavior. The topography of the valley the glacier is in affects it much more.

For example, the pace of Columbia’s retreat has been uneven. Its retreat slowed between 2000 and 2006 because it reached a narrow spot in its valley. This spot between Great Nunatak Peak and Kadin Peak constricted the glacier’s movement. Known as a pinning point, this topographic constriction is a place where the glacier’s trough becomes either too narrow or too shallow to continue moving at the previous rate. A pinning point can be a location of enhanced stability for the glacier.

If a glacier retreats from a pinning point, it will retreat until it reaches another pinning point upstream. A glacier that remains at a pinning point for a longer time may build another moraine.

Calving is also affected by the topography of the glacier’s valley, and calving rates increase in deeper water. Researchers found that as Columbia continues to retreat, it will reach water that is shallow enough to provide a stable position within a few years and remain stable through 2100. In this shallower water, iceberg production will be slowed.


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