Nearly all of Greenlandâ€™s ice cover at least temporarily melted at the surface during an unusually warm stretch in mid-July - a level of melting not seen there in 123 years, NASA said.
In an average summer, melting happens on about half of the surface of Greenlandâ€™s ice sheet, which covers most of the land and is an average 1 mile thick.
But an unusually strong ridge of relatively warm air - hovering just above freezing for several hours at the highest elevation - rapidly accelerated melting this month, and satellites showed that an estimated 97% of the surface had melted at some point by July 12, NASA said.
While some of that melt water freezes in place, some of it is lost to rivers and the ocean â€“ and mid-Julyâ€™s melting caused river flooding that threatened a number of bridges, said Tom Wagner, NASAâ€™s cryosphere program manager in Washington. (The flooding has been captured on a number of YouTube videos, including this one.)
Where this falls in the larger context of Greenlandâ€™s changing ice cap - scientists say it is shrinking and causing ocean levels to rise, with warming ocean waters causing ice on the periphery to be lost through melting and rapid flow - is a complicated question, NASA says.
Ice core samples show that the surface melting seen this July happens once in about every 150 years, and the last such melt happened in 1889, NASA said.