[Vision2020] NASA Goddard Space Flight Center: 7-24-12: Unprecedented Greenland Ice Sheet Surface Melt

[Ted Moffett]

http://www.nasa.gov/topics/earth/features/greenland-melt.html

*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. In
the image, the areas classified as “probable melt” (light pink) correspond
to those sites where at least one satellite detected surface melting. The
areas classified as “melt” (dark pink) correspond to sites where two or
three satellites detected surface melting. The satellites are measuring
different physical properties at different scales and are passing over
Greenland at different times. As a whole, they provide a picture of an
extreme melt event about which scientists are very confident. Credit:
Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth
Observatory
› Hi-res of left image<http://www.nasa.gov/images/content/670401main_melt-1.jpg>
› Hi-res of right
image<http://www.nasa.gov/images/content/670402main_melt-2.jpg>
*

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 two-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.



Maria-José Viñas
NASA's Earth Science News Team
Goddard Space Flight Center <http://www.nasa.gov/goddard>, Greenbelt, Md.
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Vision2020 Post: Ted Moffett
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