Greenland's surface ice cover last month melted over a larger area than ever detected in more than 30 years of satellite observations, NASA has said.
According to measurements from three separate satellites analysed by NASA and university scientists, an estimated 97% of the ice sheet surface thawed at some point in mid-July, the agency said in a statement.
"This was so extraordinary that at first I questioned the result: was this real or was it due to data error?," said NASA's Son Nghiem.
97% of the ice sheet surface had melted
The expert recalled noticing that most of Greenland appeared to have undergone surface melting on July 12, while analysing data from the Indian Space Research Organisation's Oceansat-2 satellite.
Results from other satellites confirmed the findings. Melt maps drawn up showed that on July 8 about 40% of the ice sheet's surface had melted, rising to 97% four days later.
The news came just days after NASA satellite imagery showed that a massive iceberg twice the size of Manhattan had broken off a glacier in Greenland.
Half usually melts
"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 the summer, on average about half of the surface of Greenland's ice sheet melts naturally, NASA said. Normally, most of that melt water quickly refreezes at high elevations, while in coastal regions some of it is retained by the ice sheet while the rest flows into the ocean.
"But this year the extent of the ice melting at or near the surface jumped dramatically," NASA added.
Melting of this type occurs every 150 years
Researchers have yet to determine whether the melt, which coincided with an unusually strong ridge of warm air over Greenland, will contribute to a rise in sea level.
NASA said that even the area near the highest point of the ice sheet, located 3km above sea level, showed signs of melting.
According to glaciologist Lora Koenig, who was part of the team analysing the data, melting incidents of this type occur every 150 years on average.
"With the last one happening in 1889, this event is right on time," Koenig said. "But if we continue to observe melting events like this in upcoming years, it will be worrisome."