In a four-day period this July, the Greenland ice sheet melted at a faster rate than satellite data has ever recorded and at higher elevations than we've seen in our lifetimes. So what caused this extraordinary melt?
Since May, a series of "ridges" consisting of high pressure and warm air have moved across Greenland, at times hovering over the island, said Thomas Mote, a climatologist and professor of geography at the University of Georgia. These ridges are generally carried by the jet stream, but occasionally, he said, the warm air "gets pinched off, and you're left with a warm pool sitting there, and it doesn't have anything to push it out of the way." And that causes direct sunlight and unusually high temperatures. Which means melting... and more melting.
On July 8, 40 percent of the ice sheet had thawed. By July 12, the number had shot up to 97 percent. Any single day in July might see a quarter of the ice sheet experience melt, and about half of the sheet usually melts over the full month, Mote said. This year, 90 percent of the ice sheet melted on July 11 alone.
And the island is melting from the top. Central Greenland's Summit Station, which sits nearly at the ice sheet's highest point at two miles above sea level, showed some signs of melting. That hasn't happened since 1889, according to NASA and Kaitlin Keegan at Dartmouth College.
This comes a week after a giant ice chunk roughly twice the size of Manhattan tore loose from Greenland's Petermann Glacier.
This is the sixth summer in a row in Greenland that's experienced this strong, high pressure system carrying warm air from the southwest over Greenland, said Jason Box, a climate scientist at Ohio State University. That's combined, he said, with clear sky that maximizes the solar sunlight falling on the ice sheet and little snow fall.
"You have a one, two, three punch," Box said. "Warm air from the south, reduced surface reflectivity and fewer summer snow fall events to brighten up the surface."
And there's a fourth punch too. The temperature of the permafrost at higher elevations has been rising, which means it takes increasingly less energy to drive those temperatures to the melting point. Scientists call that "cold content" or "thermal erosion."
So how alarmed should we be?
"I don't think anyone can look at a four-day period and say it's alarming in itself," Mote said. But, he added, it is associated with a warm summer in Greenland and melt at lower elevations accompanied by thinning at the margins of the ice sheet. If these extreme melting bursts become a pattern, it will reinforce these other changes.
Box said that ice sheets can be a good indicator of climate change.
"Ice is nature's thermometer," he said. "It does tell us something. What we are witnessing - and it's already underway - is the de-glaciation of this ice sheet." Changes won't happen overnight, he said, but implications can include shifts in global ocean circulation and sea-level rise that could start having more effects closer to home.
"This is a relatively simplified system, and when it does something like this, you know that there's something going on," Box said.
Mike Melia contributed to this post.
