(CBS News) Satellite imagery has revealed what NASA calls an "unprecedented" melting event over Greenland. The far northern Atlantic island, covered year-round in a thick sheet of ice, has experienced melting in over 97 percent of its ice sheet. Over a period of just four days, Greenland's surface ice area melted over a larger area more than at any time in over 30 years of satellite observation.
Summertime in Greenland usually sees some thawing of the massive ice sheet that covers the majority of the island. On average half of the ice sheet - from the thin coastal edges to its two-mile-thick center - melts during the summer months. But according to data compiled by three independent satellites and studied by researchers at NASA, 97 percent of the ice sheet thawed at some point in mid-July.
The incredible speed of the melt caught researchers off guard. Satellite maps taken on July 8 showed 40 percent of Greenland ice had melted. By July 12, that number had jumped to 97 percent.
"This was so extraordinary that at first I questioned the result," said Son Nghiem, a satellite data analyst at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Was this real or was it due to a data error?"
Independent confirmation from three different satellites would seem to rule out an error. Researchers from NASA, the National Oceanic and Atmospheric Administration, and several universities confirmed the incredible melt.
NOAA has a weather station at Summit Station in central Greenland. The station, which at two miles above sea level is near the highest point of the ice sheet, reported temperatures hovering a degree or two above freezing for several hours July 11-12. Even this central location recorded some melting.
But while the incredible speed of the melting may be a cause for concern, the melting ice itself may not be.
"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, a NASA glaciologist and a member of the research team analyzing the data. "But if we continue to observe melting events like this in upcoming years, it will be worrisome."