Data from satellites monitoring the Greenland ice sheet show that 97 percent of the ice sheet surface has begun to thaw.

On 8 July, Modis satellites that study the surface temperature of the ice showed that about 40 percent of the ice sheet had melted at or near the surface—slightly below the average of 50 percent at this time of year.

Just a few days later, however, that proportion had risen dramatically to 97 percent, covering almost the entirety of the massive island, from the low-lying, sparsely covered edges to the three-kilometer-thick (1.86mi) interior.

Son Nghiem of NASA's Jet Propulsion Laboratory first noticed the event while analyzing radar data from the Indian Space Research Organisation's Oceansat-2 satellite. "This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?" he said.

So he went to NASA to check. He got in touch with Dorothy Hall at NASA's Goddard Space Flight Center, who uses spectroradiometers on the space agency's Terra and Aqua satellites. She was able to use this independent data to confirm Nghiem's fears—that Greenland was experiencing an extensive melt.

The pair triple-checked the data with passive-microwave satellite data from a US Air Force meteorological satellite, operated by Thomas Mote and Marco Tedesco from the University of Georgia and City University of New York. All three data sets showed the same thing.

The culprit appears to be an unusually strong ridge of warm air, which NASA describes as a "heat dome", over Greenland—one of a series that's dominated weather in Greenland since the end of May 2012. "Each successive ridge has been stronger than the previous one," said Mote.

By 16 July, the ridge had begun to dissipate, but not before Summit Station in the center of Greenland, which sits three kilometers above sea level in the center of the continent, close to its highest point, had shown signs of melting. Ice core data shows that melting of this magnitude hasn't been seen since 1889.

While no one event can be blamed on climate change, the melt is consistent with rising global temperatures. "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 Goddard glaciologist and a member of the research team analyzing the satellite data.

"But if we continue to observe melting events like this in upcoming years, it will be worrisome."