Thursday, July 26, 2012

Greenland Ice Sheet experiences unprecedented surface melt

Surface melt on the Greenland ice sheet reached up to 97% of the ice sheet area by July 12, an unprecedented level of melting, according to readings gathered and cross-referenced from three different satellites by several scientists.

Satellite measurements showed that about 40 percent of the ice sheet had undergone thawing at or near the surface on July 8, but over the subsequent 4 days the melting had dramatically accelerated to cover an estimated 97 percent of the ice sheet surface by July 12. This was the largest area seen thawing ever measured on the Greenland ice sheet by satellite measurements.



Image Caption (NASA): Extent of surface melt over Greenland's ice sheet on July 8 (left) and July 12 (right). Measurements from three satellites showed that on July 8, about 40 percent of the ice sheet had undergone thawing at or near the surface. In just a few days, the melting had dramatically accelerated and an estimated 97 percent of the ice sheet surface had thawed by July 12. Image courtesy of NASA

The extreme melting was caused by an unusually strong ridge of warm air, or a heat dome, over Greenland. According to NASA, the ridge was one of a series that has dominated Greenland's weather since the end of May. "Each successive ridge has been stronger than the previous one," said Thomas Mote, a climatologist at the University of Georgia, Athens, Ga. The latest heat dome started to move over Greenland on July 8, and then parked itself over the ice sheet about three days later. By July 16, it had begun to dissipate.

Initial analysis last week of the extent of the extreme melting was interpreted as a possible data error. Son Nghiem of NASA's Jet Propulsion Laboratory in Pasadena, Calif., was analyzing radar data from the Indian Space Research Organisation's (ISRO) Oceansat-2 satellite when he noticed that most of Greenland appeared to have undergone surface melting on July 12. Nghiem said, "This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?"

But Nghiem consulted with several other Arctic scientists using data from 3 different satellites and confirmed that unusually high temperatures were experienced and that melt was extensive over the ice sheet surface.

Even around the weather station, called Summit Station, located at nearly 3km altitude in central Greenland, there were signs of surface melting, a highly unusual event. Ice cores from Summit Station show that the last time surface melting ocurred here was in 1889. Records show that air temperatures hovered above or within a degree of freezing for several hours July 11-12.

"Ice cores from Summit show that melting events of this type occur about once every 150 years on average. With the last one happening in 1889, this event is right on time," says Lora Koenig, a Goddard glaciologist and a member of the research team analyzing the satellite data. "But if we continue to observe melting events like this in upcoming years, it will be worrisome."

Was it only ten days ago I posted that the Arctic warming at more than double the global warming average? In 2011 melting was well above average with near record mass loss. There was also Record summer melting in Greenland in 2010.

As long ago as 2004 I reported on a study by British climatologist, Jonathan Gregory that "the Greenland ice sheet is likely to be eliminated unless much more substantial reductions in [carbon dioxide] emissions are made than those envisaged" so far by scientists or politicians. Once the process was started it will "probably be irreversible this side of a new ice age" he said.

More significantly I reported in March 2012 that the likely Global Warming threshold for Greenland Ice Sheet collapse reduced to 1.6 degrees C. But 1.6 degrees was only a best estimate from a range that started from 0.8 to 3.2 degrees - in other words, we could be seeing the tipping point now, at the current global average temperature increase of 0.8 degrees C above pre-industrial levels.

By the time we work out we have crossed the tipping point, it will be far too late to reverse the disintegration of the Greenland Ice sheet, which will result in raising sea levels some 7.2 metres over several hundred to several thousand years.

Even if we have reached that tipping point, strong climate mitigation action to reduce carbon pollution may slow the rate of disintegration and sea level rise allowing more time for human and environmental adaptation.

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