Record Ice Melt cause identified

When scientists saw melting across a whopping 97 percent of Greenland's icy surface last summer, they were quick to note that such an event is rare, but not unprecedented. The last time it happened was in 1889, so while manmade global warming is clearly involved it isn't necessarily the entire story.

A new report in Nature on Wednesday has now helped flesh out the explanation: data from Summit Station, at the frozen island's highest point, 10,551 feet above sea level, show that unusually warm temperatures in the region were enhanced by a blanket of low-level clouds that trapped extra heat from the Sun.

But while the events that led to the melt may have been unusual, said lead author Ralf Bennartz, an atmospheric scientist at the University of Wisconsin, in an interview, "I tend to believe we will see more of them toward the end of century."

If so, the consequences could be dire: combined with faster-flowing glaciers dumping more ice into the sea, episodes of surface melting could accelerate the disintegration of Greenland's 680,000 cubic miles of ice. If all of that ice entered the ocean, it would raise sea level by some 20 feet, inundating the world's coastal regions, displacing hundreds of millions of people and destroying trillions of dollars worth of property.

Even with enhanced melting, that's unlikely to happen for several hundred years — but climate scientists already expect ice and meltwater to drive sea level up by some 3 feet by 2100, enough to do enormous damage.

Bennartz and his colleagues can't say at this point whether last summer's cloudiness portends a trend. "The Arctic tends to be a cloudy place," said Julienne Strove, of the National Center for Atmospheric Research, an expert on Arctic ice, in an interview, "and last summer was cloudier in general as you had several storms enter the Arctic." In theory, clouds could be on the increase, as warmer temperatures force more water to evaporate from the oceans.

Whether that adds to global warming, however, depends on what sorts of clouds they are: high, wispy cirrus clouds, made of ice crystals, would enhance the greenhouse effect by trapping heat from the Sun. Thick cumulus clouds, made of water droplets, would shield the planet from some of the Sun's heat, keeping temperatures in check. The clouds that covered Greenland last July were intermediate: they were made of water droplets, but were thin enough to let some sunlight through.

"If those clouds had not been around there would not have been a melt event," Bennartz said. "If they had been thicker, there also would not have been a melt event like the one we saw. What we do not know is what role clouds are going to play in the future."

The instruments he and his colleagues used to study clouds have only been in place for three years, which is too short a time to detect any sort of trend. Warming temperatures alone, however, would likely increase melting over the rest of the century even without significant increases in cloud cover.

The new study raises the question of whether clouds played a role, not only in the melting of Greenland, but also in last summer's record melting of sea ice in the Arctic Ocean. The latter event didn't contribute to rising seas, because the ice that melted was already floating in the ocean. But it did expose a huge expanse of water to the Sun's heat, which in turn warms the surrounding air — and ultimately the entire planet — in a self-reinforcing cycle known as "Arctic Amplification."

Indeed, sea ice has been on a downward spiral ever since satellites first started to make observations in the 1970s, to the point where commercial shipping across the Arctic Ocean could soon be a real possibility.

Since nobody made a careful study of clouds over the sea ice, it would be hard to say for sure that they made the melting worse. In general, however, "last summer was cloudier than usual. So that certainly could have played a role," Stroeve said.

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