Antarctica is shrinking
Gravity survey shows overall loss in ice.
First Greenland, and now Antarctica. Research shows that both of these massive ice sheets are getting smaller.
"In my mind, there is no doubt that Antarctica is losing mass," said Isabella Velicogna of the University of Colorado at Boulder, and lead author of a paper published in Science1, the latest study in an avalanche of research on the topic.
Just two weeks ago, Science also published an article stating that Greenland is melting more quickly than it is growing (see ' Glacial pace picks up').
Previous work on both of these areas has shown that the edges of the ice sheets are melting while their centres are growing thanks to increased snowfall. It has been unclear until recently whether the sum of these two effects is a growing or shrinking mass of ice.
The big melt
Antarctica is twice as large as Australia and harbours 90% of Earth's ice. It is divided by the Transantarctic Mountain Range into two main ice sheets: the West Antarctic ice sheet, a large peninsula that stretches from the South Pole towards the southern tip of South America, and the East Antarctic ice sheet that forms the rest of the continent.
There is evidence that West Antarctica is melting. Two sections of the Larsen ice shelf, in the West Antarctic ice sheet, broke apart in 1995 and 2002, which has sped up the melting of glaciers that were behind it.
University of Colorado at Boulder.
To get the full picture, Velicogna and her colleagues used a special pair of NASA satellites known as the Gravity Recovery and Climate Experiment (GRACE). These orbit in tandem several hundred metres from Earth and provide monthly estimates of Earth's global gravity field and mass distribution. If the lead satellite passes over a structure with a large mass that causes it to speed up, this pulls it away from the trailing satellite. So the changing distance between the two satellites helps to determine the mass of underlying objects.
By looking at data from Antarctica from April 2002 to August 2005, Velicogna calculates that Antarctica is losing something between 72 and 232 cubic kilometres of ice per year. This is equivalent to an annual rise in the ocean of 0.4 millimetres.
The main reason for the relatively large uncertainty in how much ice is being lost is that with GRACE data it is hard to distinguish a change in mass due to extra snowfall from a change in mass due to shifts in the crust beneath, says Velicogna.
The Antarctic is moving slowly upwards as it rebounds from the melting of glaciers that lay above it during the last ice age. As it shifts, the mass of the Earth's crust is redistributed. This has to be modelled and removed from the equation before the GRACE data can be used to show changes in ice mass.
"They're getting the important answer that Antarctica is losing ice mass," says Jay Zwally, a glaciologist from the Goddard Space Flight Center in Greenbelt, Maryland. "But the numbers need more work done on them."
Zwally and his colleagues recently used satellite data showing surface altitudes to work out the overall mass loss of Antarctica; they concluded it was losing a smaller 19-43 gigatonnes of ice per year, contributing 0.08 millimetres to sea-level rise2.
Velicogna remains optimistic that these kinds of studies will help to determine exactly how fast the ice is melting, and why. "Combining this with other data will give us the opportunity to understand the process that is causing mass loss," she says.
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- Velicogna I.& Wahr J. . Science express. Doi: 10.1126/science.1123785
- Zwally J., et al. J. Glaciology, 51. 509 - 527 (2005). http://www.ingentaconnect.com/content/igsoc/jog/2005/00000051/00000175/art00001
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