Almost the entire surface layer of ice over Greenland melted in the space of four days this month — faster than at any time in the satellite era — in an event that has stunned and alarmed NASA scientists.
The rapid melting occurred over 97 percent of the surface of Greenland, deepening fears about the pace and future consequences of climate change.
In a statement on NASA’s Web site, scientists said the satellite data were so striking they thought at first there had to be a mistake.
“This was so extraordinary that at first I questioned the result: Was this real or was it due to a data error?” Son Nghiem of NASA’s Jet Propulsion Laboratory in California said.
The data came from three satellites.
He consulted several colleagues, who confirmed his findings. Dorothy Hall, who studies the surface temperature of Greenland at Goddard Space Flight Center in Maryland, confirmed the area experienced unusually high temperatures in the middle of this month, and there was widespread melting over the surface of the ice sheet.
Climatologists Thomas Mote, at Georgia University, and Marco Tedesco, of the City University of New York, also confirmed the melt.
However, scientists are still coming to grips with the images.
“I think it’s fair to say this is unprecedented,” Goddard glaciologist Jay Zwally said.
The images released by NASA show a rapid thaw between July 8 and July 12, when measurements from three satellites showed a swift expansion of the area of melting ice, from about 40 percent of the ice sheet surface to 97 percent.
Scientists attributed the sudden melt to a heat dome, or a burst of unusually warm air, which hovered over Greenland from July 8 to July 16. The country had returned to more typical summer conditions by last Saturday or Sunday, Mote said. He added the event, while exceptional, should be viewed alongside other compelling evidence of climate change, including on the ground in Greenland.
“What we are seeing at the highest elevations may be a sort of sign of what is going on across the ice sheet,” he said. “At lower elevations on the ice sheet, we are seeing earlier melting, melting later in the season and more frequent melting over the last 30 years and that is consistent with what you would expect with a warming climate.”
Jason Box, a glaciologist at Ohio State University, had predicted a big melt year for this year, because of earlier melt and a decline in summer snow flurries.
He said the heat dome was not necessarily a one-off.
“This is the seventh summer in a row with this pattern of warm air being lifted onto the ice sheet. What is surprising is how persistent this circulation anomaly is,” he said.
He added surfaces at higher elevation, now refrozen, could be more prone to future melting, because of changes in the structure of the snow crystals. He expected melting to continue at lower elevations.
About half of Greenland’s surface ice sheet melts during a typical summer, but Zwally said he and other scientists had recorded an acceleration of that melting process over the last few decades. This year his team had to rebuild their camp, at Swiss Station, when the snow and ice supports melted.
He has never seen such a rapid melt over his three decades of almost yearly trips to the Greenland ice sheet. He was most surprised to see indications of melting even around the area of Summit Station, about 3,000m above sea level.