A couple of particularly cloudy days and unseasonably warm temperatures likely caused the "astonishing" Greenland ice melt of 2012, in which 97 percent of the island's ice sheet melted over the course of four days, atmospheric scientists said Wednesday.
Atmospheric scientists say that a layer of low-lying thin clouds that allowed sunlight to pass through but trapped the Earth's thermoradiation between the clouds and the ice sheet caused the rapid melt. The event is unlikely to occur regularly—ice core records show a similar event occurs about once every 150 years.
Temperatures during those few days reached above freezing, which is rare for the area, even in the summer. The temperatures would have been enough to melt the low altitude ice on the outskirts of Greenland, but inland ice at higher altitudes would have been expected to stay frozen had there not been cloud cover according to the study, published Wednesday in the journal "Nature."
"It was the perfect combination of conditions that made the event happen," says Ralf Bennartz, an atmospheric physicist at the University of Wisconsin-Madison's Space Science and Engineering Center and one of the authors of the study. "One of the major factors in the Greenland-wide ice melt was the very warm temperatures sitting over Greenland, but it needed the clouds, which enhanced the surface temperature as well."
Bennartz says that while the overall Greenland ice sheet has been slowly melting over the past several decades, much of the July melt has since refrozen over the winter. He says that while his study is good for determining why the ice melted last July, it's difficult to predict when a similar melt will happen again.
"For global climate models, clouds are about the hardest thing to predict right now and they do have a big impact," he says. "They're a major uncertainty factor in our knowledge about climate change."