Alaskan Glaciers Advance and Retreat in Satellite Imagery

The 49th state is no slouch when it comes to ice: More than a tenth of the planet’s glacial area outside of the ice sheets is located in Alaska. But Alaskan glaciers, which anchor local ecosystems and enchant visitors, are changing, new research reveals. Maritime glaciers in Alaska have variably advanced and retreated since the 1980s, scientists found by analyzing hundreds of satellite images. This nuanced view of glacier evolution is a valuable resource for researchers and decision makers alike, the team suggests.

A View from Above

“Over the past 40, 50 years or so, since we’ve started getting satellite images, we’ve seen that glaciers are retreating.”

Kenai Fjords National Park in southcentral Alaska is home to nearly 40 glaciers. To better understand how some of the largest and most prominent of those ice masses have changed in recent decades, researchers turned to archival Landsat images taken from 1984 through 2021. They analyzed 19 glaciers captured by satellite imagery, which had a spatial resolution of 15– 30 meters.

For each glacier of interest, Taryn Black, a glaciologist at the University of Washington in Seattle, manually mapped its lower margins in suitably cloud-free images. “We ended up with a little over 600 glacier outlines,” said Black. In collaboration with Deb Kurtz, the physical science program manager at Kenai Fjords National Park, Black then measured the changes in the length and area of ​​each glacier over time using three different methods.

Overall, Black and Kurtz found that the glaciers in their sample decreased in area between 1984 and 2021. The cumulative ice loss—roughly 42 square kilometers—was distributed roughly evenly between the glaciers’ termini and their lateral margins. It’s not a surprise that most of these glaciers are retreating and narrowing, said Robert McNabb, a glaciologist at Ulster University in Northern Ireland not involved in the research. “Over the past 40, 50 years or so, since we’ve started getting satellite images, we’ve seen that glaciers are retreating.”

Good-News Glaciers

The researchers next divided their sample into three groups based on where each glacier terminated—on land, in a lake, or in a marine environment like a fjord. The third group, tidewater glaciers, were the only ones to reliably advance in the winter, Black and Kurtz showed. Researchers aren’t exactly sure why tidewater glaciers grow reliably each season, but it’s a known phenomenon, said Black. “That’s something that’s pretty commonly observed in tidewater glaciers.”

Bear Glacier in 1990 (left) and 2020 (right). Credit: (left) US National Park Service/M Tetreau and (right) US National Park Service/E Kulack

Only two of the glaciers in the researchers’ sample both advanced and grew in size between 1984 and 2021. One of them, the tidewater Holgate glacier, advanced by about half a kilometer. That finding was a surprise, said Black. “Based on reports from people who had been out there, the park thought that the glacier was retreating,” she said. “But our data showed that it’s been advancing since around 2014.”

One explanation for this behavior may be the so-called shoal that Holgate glacier has built up, the researchers propose. Glaciers commonly transport sediments and deposit them at their termini. “They’re basically pushing this pile of sediment and pebbles in front of them,” said McNabb. “They can kind of be like big bulldozers.”

All that material helps shield the glacier from relatively warm marine water. Kurtz observed Holgate’s shoal in 2020 when she visited the glacier.

The Benefits of an Earthquake

“It’s thick enough that it actually insulates the glacier.”

The only other glacier that advanced and grew larger was Paguna, a land-terminating glacier. It likely owes its success to a natural disaster: the magnitude 9.2 earthquake that struck Alaska’s Prince William Sound in 1964. That whopper of a quake—the second largest earthquake ever recorded—launched untold numbers of landslides, some of which blanketed the Paguna glacier in debris. . Like Holgate glacier’s shoal, this material blocks incoming solar radiation and helps prevent the glacier from melting, said Black. “It’s thick enough that it actually insulates the glacier.”

These results, which were published in August in the Journal of Glaciology, are just the first step in what will hopefully be long-term monitoring of Alaska’s glaciers, said Black. “It’s a detailed look at a particular region.” Future studies can build on these measurements to continue to track how these glaciers weather our changing climate, she said. “The data that we collected for this paper can serve as a baseline for other studies.”

—Katherine Korney (@KatherineKornei), Science Writer

Citation: Kornei, K. (2022), Alaskan glaciers advance and retreat in satellite imagery, Eos, 103, https://doi.org/10.1029/2022EO220439. Published on 12 September 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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