Measurements from three independent satellites show that for several days in July, Greenland’s surface ice cover melted over a larger area than at any time in more than 30 years of satellite observations, according to NASA and university scientists.

On average, about half of the surface of Greenland’s ice sheet melts during the summer. But according to data from NASA’s Terra and Aqua satellites and the Indian Space Research Organisation’s Oceansat-2 satellite, an estimated 97 percent of the ice sheet surface thawed at some point in mid-July, NASA said in a July 24 press release.

Son Nghiem, a researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., was analyzing Oceansat-2 radar data when he noticed that most of Greenland appeared to have undergone surface melting July 12.

“This was so extraordinary that at first I questioned the result: Was this real or was it due to a data error?” he said in a statement.

Researchers at NASA’s Goddard Space Flight Center in Greenbelt, Md., using the Moderate Resolution Imaging Spectroradiometers on Terra and Aqua confirmed that Greenland experienced unusually high temperatures and extensive melting. Passive-microwave satellite data from a U.S. Air Force meteorological satellite lent further credence to the observations.

NASA said the melting spread quickly between July 8 and July 12 and coincided with an unusually strong ride of warm air, or heat dome, over Greenland. By July 16, the heat dome had begun to dissipate, but not before Summit Station — a central region near the highest point of Greenland’s ice sheet — showed signs of melting.

“Ice cores from Summit show that melting events of this type occur about once every 150 years. With the last one happening in 1889, this event is right on time,” Goddard glaciologist Lora Koenig said. “But if we continue to observe melting events like this in upcoming years, it will be worrisome.”