Greenland ice sheet experiences massive melt in mid-July

The surface of Greenland’s ice sheet melted over a larger area during a four-day period in mid-July than at any time in the last 30 years, NASA reported Tuesday.

Between July 8 and July 12, the ice melt expanded from from 40 percent to 97 percent of the island, according to satellite measurements analyzed by NASA and university scientists.

In a typical summer, about half of the ice sheet naturally thaws before much of it refreezes. But much of the ice that melts near the coast slips into the ocean.

The ice sheet is losing more ice than it gains — about 150 gigatons each year — which is causing the sea level to rise three to four millimeters per year. NASA scientist Jay Zwally said it would rise about 16 inches over the next 100 years if it continued at its current rate.

Researchers have not yet determined whether the July melt would affect the overall volume of ice loss this summer and contribute to greater sea-level rise.

NASA scientist Dorothy Hall said ice core records showed that ice melts like this had occurred in the past. But the one that happened in mid-July, the largest one in more than 30 years of satellite observations, could be a sign of what’s to come.

“If this type of thing starts to happen frequently, then we will really think that it’s related to global warming,” Hall said.

The melt coincided with an unusually strong ridge of warm air — an area of high pressure between the earth’s surface and the jet stream — over Greenland. The ridge was one of a series that has dominated the country’s weather since the end of May.