Nearing the Limit of Retreat

Nearing the Limit of Retreat

Long recognized as one of the world’s most rapidly retreating glaciers, the Columbia Glacier in southern Alaska has been slowing down in recent years. “The total loss of ice is down substantially,” said Shad O’Neel. “But there is still impressive retreat.”

O’Neel, a glaciologist at the USGS Alaska Science Center, has kept a watchful eye on Columbia Glacier for years. Since the 1980s, the glacier has lost more than half of its total thickness and volume. Its front has retreated more than 20 kilometers (12 miles) north in Columbia Bay, separating around 2011 into the West Branch (Post Glacier) and the Main Branch. (You can view the retreat from 1986 to present in our World of Change feature.)

The images above, acquired with the Operational Land Imager on Landsat 8, show the glacier’s position on June 21, 2019. The first image shows the glacier in natural color; the second is false-color to help differentiate between snow and ice (bright cyan) and other components of the landscape such as open water (dark blue). Vegetation is green and exposed bedrock is brown, while rocky debris on the glacier’s surface is gray.

By the time these images were acquired, the West Branch had retreated so far that it had divided into several independent glaciers. O’Neel thinks the branch could be at the limit of its retreat. “I haven’t confirmed that yet from a site visit,” O’Neel said, “but it is unlikely that much, if any, of the glacier bed is below sea level anymore.”

O’Neel was visiting the glacier on August 27, 2019, when he snapped this photograph of the glacier’s West Branch. The area was shrouded in smoke from wildfires that were burning nearby.

Meanwhile, the Main Branch has thinned and resumed its retreat, shedding icebergs from its front and retreating again in summer 2019. Ample ice has been lost by volume—from the glacier’s front and surface—but it still has plenty of room to retreat.

Scientists think the Main Branch could eventually pull back to Divider Mountain. (The mountain’s edge is just visible in the center of the Main Branch along the top-right edge of the satellite images.) The Main Branch could retreat even further if the shape of the fjord and land surface below the glacier allow it.

Analyzing Columbia Glacier’s retreat from beginning to end could help scientists understand what’s in store for the many other tidewater glaciers across southern Alaska. Changes happening along the way can be informative too.

“Although we don’t usually think of single years as being important to glaciers, the 2019 summer has been so anomalous that it may be driving substantial change at many of Alaska’s glaciers,” O’Neel said.

NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Story by Kathryn Hansen.