Study examines feedback mechanism that may be hastening Greenland ice-sheet melt
environmentalresearchweb.org, July 27, 2009
The Greenland ice sheet and the surrounding Arctic sea ice have experienced record levels of melting in recent years. But are the two linked? When sea ice melts does it encourage the ice sheet to melt too? A new study suggests that the answer to this question may be yes.
Satellite measurements show that the area of the Greenland ice sheet that experiences melting has increased by around 16% over the last 30 years. Meanwhile, Arctic sea-ice extent shrunk to a record minimum in the summer of 2007; 39% less than the long-term average. The years of 2008 and 2005 were also extreme; on average the summer sea-ice extent has been decreasing at more than 10% per decade for the last 30 years.
Climate models indicate that melting of the Greenland ice sheet is likely to increase global sea level by around half a metre over the next 100 years or so. But there is a large degree of uncertainty in this estimate, as there may be feedback mechanisms that could speed or slow the melting process.
Asa Rennermalm from the University of California in Los Angeles, and her colleagues have been investigating one potential feedback mechanism that may be hastening ice-sheet melt. Using satellite data gathered over the last 30 years, they looked at the way that the Greenland ice sheet and surrounding sea ice have changed in size over time.
They found a strong covariance between sea-ice extent and Greenland ice-sheet melt, particularly in the late summer of each year. The smaller the sea ice extent, the greater the rate of melting of the ice sheet. "They appear to work in concert," Rennermalm told environmentalresearchweb.
Not all regions of the ice sheet showed this covariance, but where it did occur – in the west and the southwest – it was very strong.
"We think that the presence or absence of sea ice may be influencing the surface climate over the ocean," said Rennermalm. Sea ice tends to cool and dry the air above it, whereas open-ocean is associated with warmer and wetter air. "With a favourable wind direction the lack of ice could act as an agent for bringing warm air to the ice sheet, increasing the rate of melting," she explained.
Right now all eyes are on the Jakobshavn ice-stream in Western Greenland, which lies just north of the current area of maximum melting – the Kangerlussuaq region. Models suggest that sea-ice retreat is going to march northwards. If so, the Jakobshavn ice-stream should be the next region to be hit. "It is a sweet spot for detecting the link," said Rennermalm.
The findings are published in Environmental Research Letters.