Research provides timely views of warming's impact on Alaska glaciers

Research provides timely views of warming's impact on Alaska glaciers

Alaska's glaciers respond to climate change by melting for three additional weeks with every 1 degree Celsius increase in the average summer temperature, data from satellite-mounted radars show.

A single degree Celsius equates to 1.8 degrees Fahrenheit.

Work by scientists at Carnegie Mellon University and the University of Alaska Fairbanks also shows that synthetic aperture radar, or SAR, can consistently and automatically monitor glaciers and their snowlines year-round. Those are usually only gauged at the end of the melt season using optical instruments.

SAR data is also more reliable than traditional surface-based optical instruments.

The findings were published Feb. 4 in Nature.

The lead author is recent Ph.D. graduate Albin Wells of Carnegie Mellon University. Co-authors are assistant professor David Rounce of Carnegie Mellon and Mark Fahnestock of the UAF Geophysical Institute. Rounce previously was a Geophysical Institute postdoctoral fellow and research associate.

The scientists used the radar data to track the number of glacier "melt days." A single melt day can be one 24-hour period in which an entire glacier is melting, or it can consist of multiple days where portions of the glacier melt and eventually reach the glacier's total surface area equivalent.

An increase in melt days over time signals a longer melt season and accelerates a glacier's net loss of ice.

Using European Sentinel-1 radar satellite data, the researchers tracked changes throughout each melt season at nearly all Alaska glaciers larger than about half a square mile from mid-2016 through 2024.

Synthetic aperture radar works by sending microwave pulses toward the ground from a moving aircraft or satellite and combining the returning echoes to create detailed images, even through clouds and in darkness.

Sentinel-1 passes over the same location every 12 days, covering more than 3,000 Alaska glaciers.

The team also found that short-term heat waves caused Alaska's glaciers to lose up to 28% more of their protective snow cover than in typical years. That percentage is at the scale of individual mountain ranges rather than applying uniformly to each glacier within a mountain range.

"Our ability to quantify these changes is really important," Wells said. "Melt extents and snowlines are proxies for glacier mass balance."

Glacier mass balance is the difference between how much snow and ice a glacier gains and how much it loses over time.

"These correlations with temperature begin to give a sense for how much melt or snowline retreat we can anticipate under future, warmer climates across the region," Wells said.

The snowline marks the division between a glacier's accumulation zone, where snow builds up and adds mass, and its ablation zone, where melting removes snow and ice.

Glaciologists typically use optical equipment to assess snowlines at the end of the melt season, usually in late summer or early fall.

"In optical data, the snowline can be really hard to observe," Fahnestock said. "If you're a day late taking your picture, it might have snowed on the entire glacier, and you can't see where the bare glacier ice is down below and where the snow and firn is above."

Firn is partially compacted granular snow that forms the surface part of the upper end of a glacier and can eventually become ice.

Fahnestock noted that optical instruments can be affected by variable lighting conditions, shading, clouds and whether firn is clean or dirty.

SAR overcomes that and can also provide regular in-season snowline updates.

"What Albin has done is operationalize the tracking of surface conditions on the glaciers in a way that can be applied anywhere," Fahnestock said.

The research paid close attention to a June 23-July 10, 2019, Alaska heat wave that encompassed all glaciated regions of Alaska except the Brooks Range.

Temperatures rose to 20 to 30 degrees above average at many locations for nearly two weeks. Several days set all-time records, including 90 degrees Fahrenheit at Ted Stevens Anchorage International Airport. Anchorage's typical summer highs are in the mid-60s.

The excessive heat caused glacier snowlines to retreat nearly 350 feet in elevation, according to the researchers. Snowlines do not retreat that high in a typical year until about two months later.

The change lengthened exposure of bare ice and firn, leading to increased mass loss.

The authors write that this underscores "the sensitivity of glaciers to short-term climatic variability."

Wells said the research also revealed consistent differences in the number of melt days between glaciers on the coastal side of mountain ranges and those farther inland. The pattern suggests the glaciers operate differently even though many are losing ice at broadly similar rates.

"This is an important finding," Wells said, "because it corroborates prior knowledge that glaciers in Alaska on the coastal side of mountains have more melt in summer and more accumulation in winter than those on the continental side of the ranges."

ADDITIONAL CONTACTS: Albin Wells, [email protected]; Mark Fahnestock, [email protected]

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