In a scant four days this month, the surface of Greenland's ice sheet melted to an extent not witnessed in 30 years of satellite observations, Nasa reported yesterday.
On average, about half of the surface of the ice sheet melts during the summer. But from July 8th to July 12th, the ice melt expanded from 40 per cent of the ice sheet to 97 per cent, according to scientists who analysed the data from satellites deployed by Nasa and India's space research institute.
"I started looking at the satellite imagery and saw something that was really unprecedented" since the advent of satellite imaging of the earth's frozen surface, or cryosphere, said Thomas Mote, a climate scientist at the University of Georgia who for 20 years has been studying ice changes on Greenland detected by satellite.
While scientists described it as an "extreme event" not previously recorded from space, they hastened to add that it was normal in a broader historical context.
Ice core samples taken from the summit of Greenland's ice sheet that shed light on 10,000 years of its history show that a similar large-scale melting event has happened roughly every 150 years, said Lora Koenig, a glaciologist with Nasa's Goddard Space Flight Center who has also studied the satellite imagery. Because the previous vast melt occurred in 1889, this year's is more or less on schedule, she said.
During the event, the surface ice on the sheet's summit was always within a degree or so of refreezing, Ms Koenig added. Around July 14th, the ice loss began to reverse, she said.
Nonetheless, the scientists said, the melt was significant because Greenland's ice sheet is unequivocally shrinking as a result of the warming of the world's oceans, and the event could help broaden their insights into climate change and earth systems.
"Even though this one event might be part of normal variation, it's still a fantastic experiment for us so we can try to understand how the ice sheets are going to change," Thomas Wagner, head of Nasa's cryosphere programme, said in an interview.
He said he and other scientists would continue to sort through ocean, weather and surface data from the melt to seek a deeper understanding of why it happened and what it means in the context of the global climate system and other events.Just last week, for example, a chunk of ice about two times as large as Manhattan broke free from the remote Petermann Glacier in northwestern Greenland, Mr Wagner noted.
Two years ago, the glacier lost a chunk of ice four times as big as Manhattan.
Normally, ice "calving" of this magnitude is seen only every 10 or 20 years, scientists pointed out last week.
Mr Mote said the unusual surface melt in Greenland's ice sheet this month coincided with a series of "ridges" of warm weather - areas of high pressure that range from the earth's surface up to the jet stream - that had been moving across Greenland since the end of May.
"Normally during the month of July, on any given day we'll see about a quarter of the ice sheet covered in melt," he added.
Mr Mote said that although he typically studied satellite images that were a few months old, he had taken a look at the current pictures after a colleague conducting research in Greenland notified him that there seemed to be an unusually high amount of melting.
New York Times