Estimated icecap loss halved

Estimates of the rate of ice loss from Greenland and West Antarctica, one of the most worrying questions in the global warming debate, should be halved, according to Dutch and US scientists.

In the last two years, several teams have estimated Greenland is shedding roughly 230 gigatons of ice, or 230 billion tons, per year and West Antarctica around 132 gigatons annually.

But, according to the new study, published in the Sept issue of the journal Nature Geoscience,(subscription required) the ice estimates fail to correct for a phenomenon known as glacial isostatic adjustment. This has led to a serious over-estimation of the rate of ice melt in the Arctic and Greenland.

Measuring a disappearing ice cap is actually quite difficult to do, as the areas in question are remote, hostile environments and the exact depth of ice is often unknown. This has caused a lot of argument among climate scientists regarding how much ice is melting and running into the sea, as this affects predictions of sea-level rise and other aspects of climate modeling. (Floating sea ice, like that which makes up most of the Arctic cap apart from Greenland, is less of an issue as its melting doesn't affect the sea level.)

Glacial Isostatic adjustment is the term for the rebounding of Earth's crust following the last Ice Age. Glaciers that were kilometers thick smothered Antarctica and most of the northern hemisphere for tens of thousands of years, compressing the elastic crust beneath it with their titanic weight.

'A good analogy is that it's like a mattress after someone has been sleeping on it all night,' said researcher Bert Vermeersen of Delft Technical University, in the Netherlands. The weight of the sleeper creates a hollow as the material compress downwards and outwards. When the person gets up, the mattress starts to recover.

Satellite data from NASA's Gravity Recovery and Climate Experiment (GRACE) has been used for preliminary ice rate estimates. GRACE consists of two spacecraft flying in tandem to measure Earth's gravitational field very precisely. These measurements enable a better understanding of ocean surface currents and ocean heat transport. The mission is able to measure changes in sea-floor pressure and show how the mass of the oceans change. It will also measures and monitors ice sheets and changes in the storage of water and snow on the continents.

The estimates from GRACE were until now uncorrected for the phenomenon of "rebound", where the Earth's crust rises as ice is removed. No one, however, was sure how much impact the "rebound" effect would have on the estimate. It turns out that it has quite a bit of impact.

Now a team of researchers based at NASA's Jet Propulsion Laboratory and in the Netherlands say they have the answer.

"The corrections for deformations of the Earth’s crust have a considerable effect on the amount of ice that is estimated to be melting each year," explains Dr Bert Vermeersen. "We have concluded that the Greenland and West Antarctica ice caps are melting at approximately half the speed originally predicted."

Vermeersen and his colleagues' calculations show that as little as 500 gigatons of ice or even less could have melted from Greenland during 2003-2009, translating into less than 2mm of sea-level rise. In the case of Greenland, it could be that the current estimates are triple what they should be.

Source:
The Straits Times, "Estimated icecap loss halved", accessed September 8, 2010
The Register, "Greenland ice loss rates 'one-third' of what was thought", accessed September 8, 2010