Greenland’s surface ice cover experienced a broader thaw during a three-day period this month than in nearly four decades of satellite record-keeping, according to three independent satellite measurements analyzed by NASA and university scientists.

About half of the surface of Greenland’s ice sheet melts on average each summer. But between July 11 and 13, roughly 97 percent of the the sheet — from its coastal edges to its 2-mile-thick center — experienced some thawing.

The unusual amount of melt — coming on the heels of the Petermann glacier’s loss of ice last week — has highlighted the extent to which warming temperatures are affecting the Arctic. There has been an unusually strong ridge of warm air, or a heat dome, over Greenland.

Researchers said it is too early to connect the new readings with broader climate change. The recent glacier calving, for example, was linked to warmer ocean temperatures rather than to the recent spike in air temperatures.

If satellites document the same degree of melting in August and next summer, said Dorothy Hall, a senior scientist at NASA’s Goddard Space Flight Center, “then we’re going to start to think it is related to global warming, but at this point we can’t say.”

The extent of surface melt over Greenland’s ice sheet on July 8, left, and July 12, right. 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. (AP/AP/NASA)

Dartmouth College professor Kaitlin Keegan, who has sampled ice cores taken from Summit Station in central Greenland, which is near the highest point of the ice sheet, said ice core samples indicate such pronounced melting at Summit and across the ice sheet has not occurred since 1889.

“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,” said Lora Koenig, who is also a Goddard glaciologist and belongs to the research team analyzing the satellite data. “But if we continue to observe melting events like this in upcoming years, it will be worrisome.”

Ohio State University geography professor Jason Box, who has worked extensively in Greenland, has warned in scientific publications that melting on the Greenland ice sheet has reduced its ability to reflect the sun’s rays, also known as albedo, which in turn can accelerate future melting. With this decreased reflectivity, he wrote in a paper recently accepted for publication, “it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.”

Hall said that while the lower albedo has contributed to melting, “whether it’s going to cause complete melting, I wouldn’t want to discuss that,” because it’s impossible to predict.