Rising sea levels are bad news for residents of coastal communities and island nations. Precise predictions of future conditions in these areas are vital for policy formation, but scientists are not yet certain exactly why this phenomenon is occurring. To address the problem, researchers from around the world are working to quantify the contributions of various melting ice masses, but so far data have been skimpy, leaving conclusions drawn from them fraught with uncertainty. Now a new study conducted in Alaska presents a significantly more comprehensive analysis of the contribution of mountain glacial melting to rising seas. The findings, published today in the journal Science, are ominous: most glaciers have been thawing at an increasingly fast pace since the 1950s, and the Alaskan group has itself caused a significant increase in sea levels.
In the past, work on mountain glaciers has looked at only a few small and easily reachable icy entities, making extrapolation to global sea levels somewhat sketchy. Anthony Arendt of the University of Alaska and his colleagues improved upon that approach by applying a new and more thorough technique. Strapping a laser altimeter--a device used to measure vertical height--to the belly of an airplane, the team flew over 67 glaciers covering 90,000 square kilometers in the Alaskan mountains. The group compared the glacial heights they computed to those marked in U.S. Geological Survey maps from the 1950s to the present day. "Most glaciers have thinned several hundred feet at low elevations in the last 40 years, and about 60 feet at higher elevations," team member Keith Echelmeyer reports. Additionally, glaciers thawed twice as quickly during the past five to seven years as they did from the 1950s to the mid-1990s.
Using these results, the team calculated that from the 1950s to the mid-1990s, Alaskan glacier melt caused sea levels to swell one-tenth of a millimeter a year. And from the mid-1990s to the present day, the same glaciers raised levels twice as quickly. Previous projections of sea level increase did not anticipate such a large contribution from the Alaskan glaciers. "These recent losses are nearly double the estimated annual loss from the entire Greenland Ice Sheet during the same time period and are much higher than previously published loss estimates for Alaska glaciers," the team notes.
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In the same issue of the journal Science, other scientists reported on research from the opposite end of the world, observing that water around the south pole has become less salty, owing to the melting of the West Antarctic ice sheet. These findings, along with those from Alaska, point to global warming as the culprit, but additional work over a longer period of time is needed before scientists can be certain of that.