FROM: NASA
WASHINGTON -- For several days this month, Greenland's surface ice
cover melted over a larger area than at any time in more than 30
years of satellite observations. Nearly the entire ice cover of
Greenland, from its thin, low-lying coastal edges to its 2-mile-thick
center, experienced some degree of melting at its surface, according
to measurements from three independent satellites analyzed by NASA
and university scientists.
On average in the summer, about half of the surface of Greenland's ice
sheet naturally melts. At high elevations, most of that melt water
quickly refreezes in place. Near the coast, some of the melt water is
retained by the ice sheet and the rest is lost to the ocean. But this
year the extent of ice melting at or near the surface jumped
dramatically. According to satellite data, an estimated 97 percent of
the ice sheet surface thawed at some point in mid-July.
Researchers have not yet determined whether this extensive melt event
will affect the overall volume of ice loss this summer and contribute
to sea level rise.
"The Greenland ice sheet is a vast area with a varied history of
change. This event, combined with other natural but uncommon
phenomena, such as the large calving event last week on Petermann
Glacier, are part of a complex story," said Tom Wagner, NASA's
cryosphere program manager in Washington. "Satellite observations are
helping us understand how events like these may relate to one another
as well as to the broader climate system."
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 last week 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?"
Nghiem consulted with Dorothy Hall at NASA's Goddard Space Flight
Center in Greenbelt, Md. Hall studies the surface temperature of
Greenland using the Moderate-resolution Imaging Spectroradiometer
(MODIS) on NASA's Terra and Aqua satellites. She confirmed that MODIS
showed unusually high temperatures and that melt was extensive over
the ice sheet surface.
Thomas Mote, a climatologist at the University of Georgia, Athens,
Ga., and Marco Tedesco of City University of New York also confirmed
the melt seen by Oceansat-2 and MODIS with passive-microwave
satellite data from the Special Sensor Microwave Imager/Sounder on a
U.S. Air Force meteorological satellite.
The melting spread quickly. Melt maps derived from the three
satellites showed that on July 8, about 40 percent of the ice sheet's
surface had melted. By July 12, 97 percent had melted.
This extreme melt event coincided with an unusually strong ridge of
warm air, or a heat dome, over Greenland. 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
Mote. This 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.
Even the area around Summit Station in central Greenland, which at 2
miles above sea level is near the highest point of the ice sheet,
showed signs of melting. Such pronounced melting at Summit and across
the ice sheet has not occurred since 1889, according to ice cores
analyzed by Kaitlin Keegan at Dartmouth College in Hanover, N.H. A
National Oceanic and Atmospheric Administration weather station at
Summit confirmed 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."
Nghiem's finding while analyzing Oceansat-2 data was the kind of
benefit that NASA and ISRO had hoped to stimulate when they signed an
agreement in March 2012 to cooperate on Oceansat-2 by sharing data.
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