Long-abandoned 80-year-old aerial photographs found in a Danish basement document the unexpectedly rapid response ofGreenland glaciers to changes in average temperatures, researchers have found. The studies show that landlocked glaciers were melting faster in the 1930s than they are now, but that those extending into the ocean are melting faster now, a team headed by climatologist Jason E. Box of Ohio State University and graduate student Anders A. Bjork of the University of Copenhagen in Denmark reported in the journal Nature Geoscience. "We've confirmed that glaciers are very sensitive indicators of climate," Box said.
Most studies of such glaciers using satellite and aerial imagery have occurred since the 1970s, when the techniques became widely available. Very few earlier images have been available, limiting researchers' ability to analyze trends early in the century. Bjork has been searching for and compiling such early images. In the archives of the Arctic Institute in Copenhagen, he found flight journals from "some old planes" and a reference to the National Survey and Cadastre of Denmark. Coincidentally, archivists at the survey were contacting Bjork because they had found some glass photographic plates with glaciers on them in the basement there. The images were produced during Danish explorer Knud Rasmussen's seventh Thule Expedition in the early 1930s. The plates had been used to compile maps but were largely forgotten afterward.
The team digitized the images and used software to study differences in the southeast Greenland coastline where ice meets the ocean. They found two periods of significant glacier retreat -- one in 1933-1934 and a second from 2000 to 2010.
In the 1930s, fewer glaciers were melting than are melting today, and most of those that were melting terminated on land. The glaciers were retreating at an average of about 20 yards per year, with the fastest retreating about 375 yards per year. In the last decade, the melting glaciers were primarily those that terminated in the ocean. They were retreating at an average rate of about 50 yards per year, with the fastest melting rates close to 900 yards per year.
From 1943 to 1972, southeast Greenland cooled and the glaciers advanced, albeit slowly. The cooling was probably due to sulfur dioxide in the atmosphere, which reflects light from the sun back into space. Sulfur dioxide pollution results from volcanic emissions and industrial pollution. The amount of such pollution peaked shortly after the passage of the U.S. Clean Air Act in 1963.
In an editorial accompanying the report, climatologist Benjamin E. Smith of the University of Washington's Applied Physics Laboratory noted that unusually warm temperatures during the last decade have cause glaciers to retreat very rapidly, doubling the flow of glacial water into the ocean. If the output continues, he noted, that could have a significant effect on the ocean's level. He also noted that the slower melting rate of land-terminating glaciers during that period may result because the earlier warming shifted them to higher altitudes, which are less sensitive to climate change.
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