The 2008 ice melt continues. Researchers at Ohio State University’s Byrd Polar Research Center have announced that satellite photos reveal that an 11 square mile piece of ice (about half the size of Manhattan) broke off Greenland’s Petermann Glacier in late July of this year. The breakup actually took place just a few days before the collapse of 7 square miles of the Ward Hunt ice shelf off Ellesmere Island west of Greenland that I reported on previously.
In the July 25th MODIS satellite image below you can clearly see that a large chunk has broken away from the glacier’s upper left-hand corner.
Even more worrisome, according to Ohio State University geographer, Jason Box, is the presence of a crack farther up the glacier’s tongue.
“If the Petermann glacier breaks up back to the upstream rift, the loss would be as much as 60 square miles (160 square kilometers),” Box said, representing a loss of one-third of the massive ice field.
Since the tongue is an ice shelf (meaning that the ice is already floating), the breakup will not directly contribute to sea-level rise, but as research on the Larsen B ice shelf in the Antarctic has shown, the demise of ice shelves, which often serve as breaks on the ice fields behind them, can accelerate the speed of ice field movement. In the case of the Larsen B the speed increased by between 3 and 8 times.
Meanwhile, the margin of the Jakobshavn Glacier on Greenland’s southwest coast has retreated to a point never before seen in 150 years of observations and perhaps where it has not been in 4,000 to 6,000 years.
The Northern branch of the Jakobshavn broke up in the past several weeks and the glacier has lost at least three square miles (10 square kilometers) since the end of the last melt season.
The Jakobshavn Glacier dominates the approximately 130 glaciers flowing out of Greenland’s inland into the sea. It alone is responsible for producing at least one-tenth of the icebergs calving off into the sea from the entire island of Greenland, making it the island’s most productive glacier.
The retreat of the Jakobshavn’s margin has already sped up the glaciers flow. Between 1997 and 2003 the glacier doubled its speed. with significant consequences for sea-level rise.
Jakobshavn Isbrae is Greenland’s largest outlet glacier, draining 6.5 percent of Greenland’s ice sheet area. The ice stream’s speed-up and near-doubling of ice flow from land into the ocean has increased the rate of sea level rise by about .06 millimeters (about .002 inches) per year, or roughly 4 percent of the 20th century rate of sea level increase.
Also, the rapid movement of ice from land into the sea provides key evidence of newly discovered relationships between ice sheets, sea level rise and climate warming.
The researchers found the glacier’s sudden speed-up also coincides with very rapid thinning, indicating loss of ice of up to 15 meters (49 feet) in thickness per year after 1997.
Research presented at last year’s meeting of the American Geophysical Union by Box and graduate student Adam Herrington, showed that the glacier behaved in a similar fashion in the 1920’s during another period of rising temperatures. However the situation now is more worrisome because of global warming:
The fact that recent changes to Greenland’s ice sheet mirror its behavior nearly 70 years ago is increasing researchers’ confidence and alarm as to what the future holds. Recent warming around the frozen island actually lags behind the global average warming pattern by about 1-2 degrees C but if it fell into synch with global temperatures in a few years, the massive ice sheet might pass its “threshold of viability” – a tipping point where the loss of ice couldn’t be stopped.
“Once you pass that threshold,” Box said, “the current science suggests that it would become an irreversible process. And we simply don’t know how fast that might happen, how fast the ice might disappear.”
A complete melting of the Greenland ice sheet, although not considered an immediate threat, (in reality, as mentioned above, scientists are uncertain about what the rate of melt might be if a tipping point were reached) would raise the level of the world’s oceans by over 20 feet.
Warmer temperatures are increasing the number of summer days when portions of the surface of the Greenland Ice Sheet melt. Along the margins of the ice sheet, up to 20 additional days of melting occurred in 2005 compared to the average since 1988. (NASA map by Robert Simmon and Marit Jentoft-Nilsen, based on data from Marco Tedesco, GSFC.)
For a fascinating account of how scientists have determined that the Greenland ice sheet as a whole is losing mass see this Earth Observatory article here.
Crossposted at Daily Kos