A set of 80-year-old photographs discovered in a basement archive reveals the remarkable sensitivity of Greenland's glaciers to climate change, according to a new study that one scientist called "glaciological research with a splash of Indiana Jones."
The research, published online May 27 in the journal Nature Geoscience, reveals a pattern of stop-and-go melting along Greenland's southeastern coast. Aerial photographs dating back to 1931 show a period of glacier retreat between 1933 and 1943, followed by a cool period of advancing ice until 1972. More recently, most of those gains have been lost as temperatures creep upward.
"From these images, we see that the midcentury cooling stabilized the glaciers," said Jason Box, a geographer at the Byrd Polar Research Center at Ohio State University. "That suggests that if we want to stabilize today's accelerating ice loss, we need to see a little cooling of our own." [Images: Greenland's Gorgeous Ice]
Early retreat
The long-lost photographs were taken during an expedition led by Danish explorer Knud Rasmussen and include aerial photos of land, sea and ice in southeastern Greenland. After expedition researchers created a map from the photographs, the glass-plate images were tucked away at the National Survey and Cadastre of Denmark and forgotten.
National Survey researchers were cleaning out the basement of their archives when they ran across the glass plates. They contacted Anders A. Bjørk, a doctoral fellow at the Natural History Museum of Denmark. For Bjørk, the find was a gold mine. Satellites have kept an orbiting eye on Greenland's ice since the 1970s, but measurements from before then are rare. That makes it tough to determine the ice's sensitivity to temperatures.
Bjørk, Box and their colleagues digitized the photographs and used software to compare them with images taken by the U.S. military in the World War II era and to modern satellite and aerial photographs. They found the 1933-43 ice retreat followed an unusually warm period in Arctic history. From about 1919 to 1932, temperatures in the region rose by about 3.6 degrees Fahrenheit (2 degrees Celsius) per decade — about a half-degree Celsius cooler than today's Arctic temperature, but still a useful parallel.
Between 1933 and 1943, glaciers retreated by about 33 to 164 feet (10-50 meters) per year, the photos revealed. Glaciers that terminated on land retreated just as fast as glaciers that fed the sea. In the current period of ice loss that began in the 2000s, ocean-abutting glaciers are melting much more quickly than land-bound glaciers. It could be that the 1930s ice loss pushed glaciers back to higher elevations and stripped them of surface area, making them less vulnerable to warming temperatures.
Today, average ice loss in southeastern Greenland is 164 feet (50 meters) of retreat each year, higher than the 1930s rates. Several fast-melting glaciers, including one losing 2,910 feet (887 meters) of ice each year, are driving up the average.
A cool period
Between these periods of melt, things were looking up for Greenland's glaciers. During the 1943-72 cooling period, 60 percent of southeastern Greenland's glaciers advanced, and 12 percent stayed stationary.
The cooling was likely due in part to natural atmospheric cycles and in part to sulfur dioxide pollution. Sulfur dioxide, an industrial pollutant that is a main cause of acid rain, wrecks havoc on human health, but it also reflects sunlight away from the Earth. Atmospheric sulfur dioxide levels decreased following the Clean Air Act of 1963.
The glaciers' response to heat and cold was faster than previous studies had suggested, the researchers found, suggesting a high degree of sensitivity to air and ocean temperatures.
The ice losses of the last decade or so largely has wiped out the gains of the midcentury cool period. The current loss of ocean-terminating glaciers is a problem because it is the major contributor to sea-level rise, according to Benjamin Smith, a University of Washington researcher. Smith, who was not involved in the study, wrote an accompanying editorial in Nature Geoscience. He compares the study launched by the long-lost photos to an Indiana Jones quest.
Although recent melting has outpaced the 1930s melting, the patterns of melt are similar, Smith says.
"This indicates that the retreat in the 2000s was a typical response of the ice sheet to warmer air and ocean temperatures, and that future warming events can be expected to have similar consequences," he wrote.
Recent images reveal that Greenland's glaciers are moving 30 percent faster than they were a decade ago.
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