Loss in last 40 years was over-estimated, but rate of melting has doubled in last two decades
By Bob Berwyn
SUMMIT COUNTY — Alaskan glaciers may not be melting quite as fast as previously thought, but the pace at which it’s melting has doubled in the last 20 years, a team of scientists reported this week.
Researchers from Northern Arizona University, the University of British Columbia, the University of Northern British Columbia Université de Toulouse studied 75 percent of Alaskan glaciers intensively, comparing optical data from satellites with topographic maps dating back to the 1950s.
They concluded that glacier melt in Alaska between 1962 and 2006 contributed about one-third less to sea-level rise than previously estimated.
Northern Arizona University publicized the results of the research in a press release this week, and a paper on the project was published in the February issue of Nature Geoscience.
Click here to see an photo essay of melting Alaska glaciers published by NPR.
Previous studies overestimated the rate of melting because of thick deposits of rock debris that offer protection from solar radiation and, thus, melting. They also did not account for the thinner ice along the edges of glaciers that resulted in less ice melt.
Erik Schiefer, the Northern Arizona University geographer who co-authored the paper on the research, said melting glaciers in Alaska originally were thought to contribute about .0067 inches to sea-level rise per year. The team’s new calculations put that number closer to .0047 inches per year. The numbers sound small, but as Schiefer said, “It adds up over the decades.”
“We’re also talking about a small proportion of ice on the planet. When massive ice sheets (such as in the Antarctic and Greenland) are added in, you’re looking at significantly greater rates of sea-level rise.”
Schiefer said the team plans to use the same methodologies from the Alaskan study in other glacial regions to determine if further recalibrations of ice melt are in order.
The team’s techniques used satellite imagery that spans vast areas of ice cover. Previous methods estimated melt for a smaller subset of individual glaciers.
The most comprehensive technique previously available used planes that flew along the centerlines of selected glaciers to measure ice surface elevations. These elevations were then compared to those mapped in the 1950s and 1960s.
From this, researchers inferred elevation changes and then extrapolated this to other glaciers.Two factors led to the original overestimation of ice loss with this method, Schiefer said. One is the impact of thick deposits of rock debris that offer protection from solar radiation and, thus, melting. The other was not accounting for the thinner ice along the edges of glaciers that also resulted in less ice melt.
While the team determined a lower rate of glacial melt during a greater than 40-year span, Schiefer said other studies have demonstrated the rate of ice loss has more than doubled in just the last two decades.
“With current projections of climate change, we expect that acceleration to continue,” Schiefer said. This substantial increase in ice loss since the 1990s is now pushing up the rise in sea level to between .0098 inches and .0118 inches per year—more than double the average rate for the last 40 years.
Working on the Alaskan glacial melt revision with Schiefer and Berthier were Garry Clarke of the University of British Columbia, Brian Menounos of the University of Northern British Columbia and Frédérique Rémy of the Université de Toulouse.