Aerial photographs dating back to the second world war have shown that glaciers in Greenland are retreating twice as fast this century as they were in the 20th century.
Greenland is home to 20,000 peripheral glaciers located in mountain valleys and plateaus separate from its massive ice sheet, many of which are melting even faster than the sheet itself. Glaciers worldwide are now contributing about one-fifth of sea level rise, although the Greenland and Antarctic ice sheets will eventually surpass them.
Laura Larocca of the University Corporation for Atmospheric Research in the US and her colleagues located the front edge of 821 of land-terminating glaciers in aerial photographs taken between 1943 and 1987. The oldest were taken by US Navy planes mapping Greenland during the second world war, when both the Americans and the Nazis were trying to set up bases there.
When possible, the researchers also identified the moraine, a small ridge of rocks and sediment indicating the maximum extent of the glacier during the Little Ice Age, a cool period from around the 14th century to the 19th century. By comparing this to satellite imagery of the ice fronts, they found that the glaciers on average receded 7.7 metres a year in 1890-1999, compared with 14.8 metres a year in the past two decades. That means rising temperatures are outweighing the increased snowfall expected in some areas.
“The accelerated retreat of these glaciers is ubiquitous. It’s happening all over Greenland, even though Greenland’s climate zones are diverse,” Larocca says.
Glaciers react more quickly to climate warming than the ice sheet. While they also retreated after the end of the Little Ice Age, they are now moving back at an even higher rate in all parts of Greenland, except the especially cold north-east.
“These are kind of like a little early warning system for what we’re doing to the ice sheet as well, which is where there’s 20 feet of global sea level rise,” says team member Yarrow Axford of Northwestern University in Illinois.
The study is a major step toward figuring out exactly how much Greenland’s glaciers are contributing to global sea level rise, since scientists haven’t had a good estimate of their ice loss before the satellite era began in 1979, says William Colgan of the Geological Survey of Denmark and Greenland. “But of course they’re just looking at area changes in this study, not volume changes, so there’s still one more dimension of thickness to go,” he says.