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 18.
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.
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 18.
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 18, 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 18, 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.
Sources: NASA
