Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory

About 40 percent of Greenland's ice sheet thawed at or near the surface on July 8. Four days later, the melt had dramatically accelerated and an estimated 97 percent of the ice sheet surface had thawed.

Three satellites found that 97 percent of Greenland -- the land mass second only to Antarctica for its volume of ice -- underwent a thaw never before seen in 33 years of satellite tracking, NASA reported Tuesday.

Satellite experts at first didn't trust their readings, especially since they showed an incredible acceleration. Over four days, Greenland's ice sheet -- which covers 683,000 square miles -- went from 40 percent in thaw to nearly entirely in thaw.

"This was so extraordinary that at first I questioned the result: Was this real or was it due to a data error?" Son Nghiem of NASA's Jet Propulsion Lab in Pasadena, Calif., said in NASA's statement about the findings.

Scientists on the ground in Greenland had been reporting an unusually warm summer thaw, including damage at a snow airfield and strong runoff threatening a bridge, Tom Wagner, who manages NASA's ice research programs, told NBC News.

Ice cores from Greenland's highest region do reveal that such island-wide thaws have happened every 150 years or so, at least over the last few thousand years, but the fear now is that it might occur much more frequently due to warming sea and air temperatures.

"We can't lose sight of the fact that Greenland's ice sheet is losing 150 gigatons of ice a year," Wagner said. That translates into raising sea levels by one-one hundredth of an inch. Additionally, the danger of greater warming and greater melt persists. 

"If we continue to observe melting events like this in upcoming years, it will be worrisome," said Lora Koenig, a NASA glaciologist who helped analyze the satellite data.

The director of the top ice research center in the U.S. said the discovery fits into "the larger picture of a strongly warming Arctic."

"Arctic sea ice extent this summer is so far tracking at very low, near record levels, and the ice cover is unusually diffuse," Mark Serreze of the National Snow and Ice Data Center told NBC News.

On top of that, he said, the seasonal melt that followed the 2012 winter "started unusually early over most of the Arctic Ocean."

The center's latest report, issued Tuesday, noted that" Arctic sea ice continued to track at levels far below average through the middle of July, with open water in the Kara and Barents seas reaching as far north as typically seen during September."

Thomas Mote, a University of Georgia climatologist who looked at the satellite data, said the melt followed an unusual series of warm air ridges over Greenland since late May, with the strongest coinciding with the rapid thaw in mid-July.

Each successive ridge, Mote told NBC News, was "stronger than the previous one" and it looks like the pattern has finally broken down. 

The ridges happened just as a cyclical weather phase known as the North Atlantic Oscillation shifted. "Together, they produced near perfect conditions for this event," Mote added.

Related: Huge Greenland iceberg breaks off glacier

Because they hold so much ice on land, Greenland and Antarctica have the potential to raise sea levels significantly if warming continues or worsens. 

Sea levels have already risen by about 8 inches in the last century, partly due to some ice melt but also thermal expansion caused by warming seas.

The U.N. climate panel estimates sea level could rise between 7 inches and nearly two feet this century -- the latter a scenario that could prove catastrophic for many coastal areas around the globe.

NASA said researchers had not yet determined whether this summer's Greenland thaw would be significant enough to raise sea levels.

Greenland has enough ice to raise sea levels by 23 feet if it all melted off.

A recent study found that it could take a long-term increase in global temperatures of just 2-3 degrees Fahrenheit to completely melt Greenland's ice sheet in 2,000 years.