Satellite images of Earth help us observe locations that can be difficult to reach in person. Glaciers are sensitive to changes in regional and global climate, so scientists want to monitor them regularly. While some scientists study glaciers in the field, the Landsat satellites allow many others to monitor glacial change from the comfort of their office.
Bear Glacier is an outlet glacier of the Harding Icefield in Kenai Fjords National Park, Alaska. Glaciers form when fallen snow compresses into an ice mass over many years; the process usually takes centuries. The ice then flows to lower elevations. Besides showing visible retreat over the past few decades, Bear Glacier has also thinned about 2.5 feet (0.75 meters) per year from the early 1950s to the 1990s.
Since Bear Glacier and many other remote glaciers are largely inaccessible, satellite images provide important insights into how they change over time.
(Black stripes run through some of the images because of the Scan Line Corrector failure on Landsat 7 in May 2003.)
A stable glacier advances a little in the winter and retreats the same amount in the summer. Bear Glacier likely did this and gradually built up a terminal moraine . (A moraine is a buildup of glacial material, and a terminal moraine is one that is built up at the end of the glacier.)
During the last 100 years in Alaska, the annual average temperature has increased by about twice the global annual average temperature change. A temperature increase like this can change the regular pattern of glacial advance and retreat.
Between 1950 and the 1990s, Bear Glacier retreated 1.55 kilometers (1 mile). Small icebergs were calving into a lagoon that had developed. By 2004, the glacier’s floating terminus calved, causing an additional 2 kilometers (1.25 miles) of retreat. Between 2000 and 2007, the terminus retreated another 3.5 kilometers (2 miles). Large icebergs now float in the lagoon, visible in the Landsat images as light blue spots in the water.
USGS uses repeat photography of glaciers to quantify changes in glaciers over time. Repeat photography is a technique in which a historical photograph and a modern photograph, both having the same field of view, are compared.
For example, oblique aerial photographs of Bear Glacier were taken facing north and show glacial retreat along with icebergs floating in Bear Glacier Lagoon.
Photographs taken from the ground look north and were from the same location on the eastern part of the terminus of Bear Glacier. In the 96 years between photographs, Bear Glacier has retreated more than 3 kilometers (1.9 miles) and thinned by as much as 200 meters (656 feet). Only a very small part of Bear Glacier is visible from this location today, and the terminus is obscured by the trees.
The USGS has more repeat photography of other Alaskan glaciers.
Before 1950, the entire basin was covered by Bear Glacier and ended at a terminal moraine. By 1961, a small lake had formed, referred to as Bear Glacier Lagoon. By 1984, the size of the lagoon had doubled. It has continued to grow quickly as the glacier has retreated over time. People can now go kayaking among the icebergs on Bear Glacier Lagoon.
In August 2014, water from Bear Glacier Lagoon breached the moraine that separates the lagoon from Resurrection Bay. Lake levels dropped by 1–2 feet (0.3–0.6 meters). These glacier lake outburst floods occur regularly. The breach is visible in the extreme close-up in the 2014 Landsat image, in which different infrared wavelengths were used to make the breach more visible.
Glaciers move slowly, but Bear Glacier seems to have racing stripes. Glaciers pick up dirt and debris from the rocks they pass. They deposit that material in accumulations called moraines. A lateral moraine is the material on the sides of a glacier.
Two glaciers flowing together form a medial moraine in the middle where they join and show up as those dark stripes. When a glacier has a medial moraine, it’s made up of more than one ice flow.