Shrinking ice means Greenland is rising fast
- 18:06 02 November 2007
- NewScientist.com news service
- Catherine Brahic
And the large country's new-found buoyancy is a symptom of Greenland's shrinking ice cap, they add.
"The Earth is elastic and if you put a load on top of it, then the surface will move down; if you remove the load, then the surface will start rising again," explains Shfaqat Khan of the Danish National Space Center in Copenhagen.
In the case of Greenland, the "load" is its ice cap, he says.
Such uplift is not an unknown phenomenon. Relic "raised beaches" are relatively common in some areas, where the loss of ice after the last Ice Age caused the land to rise, leaving beaches often metres above the water.
Khan and his team detected the country's uplift using measurements from GPS stations located on the bedrock, underneath the ice.
Khan and his colleagues have been monitoring data from these stations since 2001 and have found that the southeastern tip of the country is definitely rising upwards. They have also found that the rate of rise has dramatically accelerated in recent years.
Sudden acceleration"Before 2004, the uplift was about 0.5 cm to 1 cm per year," Khan told New Scientist. Since then, however, the land has been rising four times faster. "This means that since 2004, Greenland has been losing four times more ice than before," he says.
These figures roughly correspond to other measurements of how much ice is being lost by the ice sheet.
In 2006, a team led by Eric Rignot from NASA's Jet Propulsion Laboratory in Pasadena, California, US, published findings suggesting that there had been a sudden acceleration in the rate at which Greenland was losing ice during 2004.
Khan says they are unsure what caused the acceleration and cautions that it is impossible to say if this speedy loss will be maintained in the long-term.
"It could be that more melt water is flowing into crevasses, which is making the glaciers flow into the ocean faster," he says. Research done at NASA has shown that warmer temperatures due to global warming are melting ice at the surface of Greenland's glaciers.
Vertical riversThe warm water is boring holes through the glaciers, creating vertical rivers whose water lubricates the bottom of the glaciers once it reaches the bedrock. This process makes glaciers speed faster towards the sea, where they break off and eventually melt.
Calculations by Khan and his team land suggest the uplift is mostly due to glaciers flowing out to the sea and breaking off. They calculated that some ice is also lost through melting, however.
This is consistent with a review of the polar meltdown which was published in March 2007.
At the time, Duncan Wingham of University College London in the UK had told New Scientist: "It has become very clear over the past 5 years that these sheets are not losing most of their mass through melting. They are losing it because the ice is flowing into the ocean faster than the snow is replacing it."
Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL031468)
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