Slow flow.Satellite observations of glacial flow in Greenland reveal that many of the island's glaciers have slowed in recent years, a sign that ice
loss during the 21st century could be less dire than in the worst-case scenarios.


Whoa There! Some Greenland Glaciers Slowing Down

For lumbering hunks of ice, glaciers have a surprising amount of personality. A new study reveals that not all of Greenland's glaciers behave alike, with some slowing their advance seaward in recent years, whereas others have surged in their forward march. Sea levels will continue to rise during the 21st century, as many studies have predicted, but the new findings indicate that the impact of Greenland's shedding of icebergs may not be as dramatic as worst-case scenarios had predicted.

The study "is the most comprehensive analysis of ice dynamics that's ever been undertaken," says Jonathan Bamber, a glaciologist at University of Bristol in the United Kingdom who wasn't involved in the research.

Even though Greenland is a major contributor to rising sea levels—thanks to the icebergs it sheds and the meltwater running off the ice sheet that covers much of its surface—field studies of its glaciers are relatively limited. For the most part, broad-scale studies of Greenland's 200-plus glaciers have either focused on limited sectors of the ice sheet or they only chronicle a small period of time.

Now, Ian Joughin, a glaciologist at the University of Washington, Seattle, and colleagues have used a decade's worth of data from Canadian, German, and Japanese satellites to garner a more complete view of Greenland's glaciers. By tracking the movement of surface features on the flowing ice, the team was able to estimate the speeds for 178 of the island's largest glaciers for the winters of 2000 and 2005, and for 195 of them for the winters of 2006 through 2010. The overall picture of glacial behavior is one of complexity rather than consistency, says Joughin. "Previous studies have suggested that all glaciers have sped up, but that's not quite right," he notes.

In northwestern Greenland, for example, where most of the glaciers move relatively quickly and flow directly into the sea rather than ending on land, average speed jumped by 8% between 2000 and 2005 and rose another 18% from 2005 to 2010. Nevertheless, the researchers report online today in Science, the glaciers in this region showed no uniform pattern of acceleration. About one-third flowed at the same rate throughout the decade, one-fourth slowed during the interval, and about 15% slowed during the first half of the decade and then surged from 2005 to 2010.

Similarly, many of the individual glaciers in southeastern Greenland don't follow the region's overall trend. Although the average speeds for these glaciers increased by 28% over the decade, substantial accelerations by some glaciers were balanced by considerable slowing by others. About 43% of the glaciers in the region sped up between 2000 and 2005, but around 25% slowed down by more than 15% from 2005 to 2010.

Despite the complexity of glacial behavior revealed by the study, the trove of data may lead to better models of ice sheet flow, says Richard Alley, a glaciologist at Pennsylvania State University, University Park. "This study is really helpful," he notes. "This is a large enough suite of glaciers that we can really start to see what influences glacial behavior, and how."

Previous studies have suggested that ice loss from Greenland might increase sea level by as much as 46.7 centimeters by 2100, Joughin and his colleagues note. But results of the team's new analyses hint that Greenland's contribution to sea-level rise will likely measure less than 9.3 centimeters over that interval.

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