May 7, 2012
Greenland's glaciers prove hard to pin down
Greenland's outlet glaciers behave differently depending on their geographical location and whether they terminate in the ocean, on an ice shelf or on land. On average, they increased in speed by about 30% between 2000 and 2010 but there is a large amount of intraregional variability, according to a new study of satellite data.
"Despite some consistency in regional trends, the data show a marked degree of overall variability," wrote the researchers, from the University of Washington and Ohio State University, US, in Science. "Substantial acceleration (28%) in the southeast and on Jakobshavn Isbrae (32%) from 2000 to 2005 garnered much attention and raised concern about the climate sensitivity of the Greenland ice sheet, particularly because these changes were not included in IPCC sea-level rise predictions."
But the scientists say later studies found that acceleration was not sustained on the southeast's largest glaciers, although it continued on Jakobshavn. Now they believe their extended record indicates that the patterns are truly region-wide. "Early acceleration in the southeast decreased, with little change from 2005 to 2010, whereas the northwest overall maintained relatively steady acceleration throughout the decade. As a result, 2000 to 2010 acceleration in the northwest (28%) is comparable to that in the southeast (34%)."
Marine-terminating glaciers were typically slower in east Greenland (mean speed 1040 m/year) than the southeast (mean speed 2830 m/year) or northwest (1630 m/year). The eastern glaciers showed only negligible change from 2000 to 2010, although a number showed surge behaviour.
In the northwest, fast-flow marine-terminating glaciers sped up on average by 8% from 2000 to 2005 and by 18% from 2005 to 2010. But one third of the glaciers showed no trend and a quarter slowed over the decade. In the southeast many (43%) of the glaciers increased in speed during the first half of the decade but most did not maintain this rate of acceleration and some subsequently slowed. A quarter of glaciers in the region slowed by more than 15% from 2005 to 2010.
Greenland's largest land-terminating glaciers lie mainly along the southwest coast, with a few in the northeast. They tend to flow at a peak velocity of 10–100 m/year. Of those that showed a notable trend, 70% slowed from 2005 to 2010, the researchers found.
Glaciers terminating on ice shelves had a mean velocity of 300–1670 m/year, making them faster than the land-terminating glaciers but slower than other marine-terminating glaciers (average speed 1890 m/year). Most of the ice-shelf-terminating glaciers showed a negligible change in speed for 2000 to 2010.
But what is causing the different behaviour? In the northwest many glaciers are embedded into the surrounding ice sheet; the team believes this may limit rapid thinning. Southeast glaciers, on the other hand, tend to flow through long fjords. This could enable rapid thinning as a glacier speeds up, potentially creating faster and larger fluctuations in speed. Ocean warming as well as local factors such as fjord, glacier and bed geometry, local climate, small-scale ocean-water flow and terminus sea-ice conditions are also likely to play a role in creating differences.
Four years ago scientists estimated that Greenland's melting ice would cause sea-level rise between 9.3 and 46.7 cm by 2100. To do this, they considered a doubling or ten-fold acceleration of glacier velocity from 2000 to 2010 followed by stabilization. But the findings from this new study indicate that even if glacier acceleration continues, Greenland's contribution to the global sea level is only likely to approach the lower of these two figures.
"There is the caveat that this 10-year time series is too short to really understand long-term behaviour," said Ian Howat of Ohio State University. "So there still may be future events – tipping points – that could cause large increases in glacier speed to continue. Or perhaps some of the big glaciers in the north of Greenland that have not yet exhibited any changes may begin to speed up, which would greatly increase the rate of sea-level rise."
The team used synthetic aperture radar data from the Canadian Space Agency's RADARSAT-1, German TerraSAR-X and Japanese Advanced Land Observation Satellite (ALOS) to produce velocity maps for the winter of 2000/2001 and for each winter from 2005/06 to 2010/2011.
The results are reported in Science.
About the author
Liz Kalaugher is editor of environmentalresearchweb.