WASHINGTON -- For several days this month, Greenland's surface icecover melted over a larger area than at any time in more than 30years of satellite observations. Nearly the entire ice cover ofGreenland, from its thin, low-lying coastal edges to its 2-mile-thickcenter, experienced some degree of melting at its surface, accordingto measurements from three independent satellites analyzed by NASAand university scientists.
On average in the summer, about half of the surface of Greenland's icesheet naturally melts. At high elevations, most of that melt waterquickly refreezes in place. Near the coast, some of the melt water isretained by the ice sheet and the rest is lost to the ocean. But thisyear the extent of ice melting at or near the surface jumpeddramatically. According to satellite data, an estimated 97 percent ofthe ice sheet surface thawed at some point in mid-July.
Researchers have not yet determined whether this extensive melt eventwill affect the overall volume of ice loss this summer and contributeto sea level rise.
"The Greenland ice sheet is a vast area with a varied history ofchange. This event, combined with other natural but uncommonphenomena, such as the large calving event last week on PetermannGlacier, are part of a complex story," said Tom Wagner, NASA'scryosphere program manager in Washington. "Satellite observations arehelping us understand how events like these may relate to one anotheras 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 ResearchOrganisation's (ISRO) Oceansat-2 satellite last week when he noticedthat most of Greenland appeared to have undergone surface melting onJuly 12. Nghiem said, "This was so extraordinary that at first Iquestioned the result: was this real or was it due to a data error?"
Extent of surface melt over Greenlands 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.
(Photo Credit: : Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory)
Nghiem consulted with Dorothy Hall at NASA's Goddard Space FlightCenter in Greenbelt, Md. Hall studies the surface temperature ofGreenland using the Moderate-resolution Imaging Spectroradiometer(MODIS) on NASA's Terra and Aqua satellites. She confirmed that MODISshowed unusually high temperatures and that melt was extensive overthe ice sheet surface.
Thomas Mote, a climatologist at the University of Georgia, Athens,Ga., and Marco Tedesco of City University of New York also confirmedthe melt seen by Oceansat-2 and MODIS with passive-microwavesatellite data from the Special Sensor Microwave Imager/Sounder on aU.S. Air Force meteorological satellite.
The melting spread quickly. Melt maps derived from the threesatellites showed that on July 8, about 40 percent of the ice sheet'ssurface had melted. By July 12, 97 percent had melted.
This extreme melt event coincided with an unusually strong ridge ofwarm air, or a heat dome, over Greenland. The ridge was one of aseries that has dominated Greenland's weather since the end of May."Each successive ridge has been stronger than the previous one," saidMote. This latest heat dome started to move over Greenland on July 8,and then parked itself over the ice sheet about three days later. ByJuly 16, it had begun to dissipate.
Even the area around Summit Station in central Greenland, which at 2miles above sea level is near the highest point of the ice sheet,showed signs of melting. Such pronounced melting at Summit and acrossthe ice sheet has not occurred since 1889, according to ice coresanalyzed by Kaitlin Keegan at Dartmouth College in Hanover, N.H. ANational Oceanic and Atmospheric Administration weather station atSummit confirmed air temperatures hovered above or within a degree offreezing for several hours July 11-12.
"Ice cores from Summit show that melting events of this type occurabout once every 150 years on average. With the last one happening in1889, this event is right on time," says Lora Koenig, a Goddard glaciologist and a member of the research team analyzing thesatellite data. "But if we continue to observe melting events likethis in upcoming years, it will be worrisome."
Nghiem's finding while analyzing Oceansat-2 data was the kind ofbenefit that NASA and ISRO had hoped to stimulate when they signed anagreement in March 2012 to cooperate on Oceansat-2 by sharing data.
Source: NASA/Goddard Space Flight Center