Antarctic Melting as Deep Ocean Heat Rises

Global warming is sneaky. For more than a century it has been hiding large amounts of excess heat in the world's deep seas. Now that heat is coming to the surface again in one of the worst possible places: Antarctica. This trend means that Earth is already locked into long term melting and sea level rise.

New analyses of the heat content of the waters off Western Antarctic Peninsula are now showing a clear and exponential increase in warming waters undermining the sea ice, raising air temperatures, melting glaciers (melting iceberg at right) and wiping out entire penguin colonies.

"In the area I work there is the highest increase in temperatures of anywhere on Earth," said physical oceanographer Doug Martinson of the Lamont-Doherty Earth Observatory. Martinson has been collecting ocean water heat content data for more than 18 years at Palmer Island, on the western side of the Antarctic Peninsula.

"Eighty-seven percent of the alpine glaciers are in retreat," said Martinson of the Western Antarctic Peninsula. "Some of the Adele penguin colonies have already gone extinct."

Martinson and his colleagues looked not only at their very detailed and mapped water heat data from the last two decades, but compared them with sketchier data from the past and deep ocean heat content measurements worldwide. All show the same rising trend that is being seen in Antarctica.

"When I saw that my jaw just dropped," said Martinson. The most dramatic rise has happened since 1960, he said.

What the rising water heat means, he said, is that even if humanity got organized and soon stopped emitting greenhouse gases, there is already too much heat in the oceans to stop a lot of impacts -- like the melting of a huge amount of Antarctic ice.

"There's the potential that we're locked into long term sea level rise for a long time," Martinson told Discovery News. Martinson presented his latest ocean heat results on Monday at the meeting of the American Geophysical Union in San Francisco.

As for how fast the ice will melt and in what locations, that depends largely on whether the upwelling warm water comes in contact with the thick ice shelf that crowds the coast and holds the block the glaciers from reaching the sea.

That, in turn, depends on the winds which drive away the surface waters and make it possible for the deeper waters to rise to the surface, said senior researcher Robert Bindschadler of NASA's Goddard Earth Science and Technology Center and the University of Maryland-Baltimore County.

"It can destroy the ice shelf if that heat can get to it," said Bindschadler, who at the same meeting presented his work from the melting Pine Island Ice Shelf (right) in Antarctica.

Now that the upwelling deep sea water is the clear cause of the melting ice shelf, rather than summer melt water, as had been thought in the past, it's a question of how winds will change in a warming world and whether they will drive more warm water into the ice shelves.

"So we have thrown the problem back over the fence to the climate modelers," said Bindschadler.

Source:
Discovery News, "Antarctic Melting as Deep Ocean Heat Rises", accessed December 16, 2010

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