On a hot summer's day, clouds are welcome. They offer shade and a merciful break from the sun. But if you're an ice sheet in Greenland, clouds can give you a meltdown instead. Clouds don't always cool things off. In fact, they can make things even hotter, and that's exactly what happened in the middle of July, 2012 on the Greenland ice sheet. There's no doubt that unusually warm weather was primarily responsible for Greenland's most extensive melt since 1889, but thin, low-level clouds were instrumental accomplices. Some clouds are thin enough to let most of the sun's energy through, but they remain just thick enough to keep some ground heat from escaping. In effect, these clouds provide extra warmth at Earth's surface, and that's bad news for ice.
Image courtesy of NASA.
Scientists first noticed how clouds affected Greenland's ice sheet during the July, 2012 heat wave when they compared air temperatures from cloudy and clear days. When these thin clouds were present on July 11 and 12, temperatures rose above freezing at the Summit weather station. The following three days were cloud-free, but the thermometer never rose above freezing. Scientists used a sophisticated radiative transfer model to confirm their suspicions, and they discovered that these thin clouds gave 100 more watts of heat to every square meter of earth beneath them. That's like shining your brightest light bulb over every three-foot by three-foot section of your living room floor. This heat, of course, melts ice rather quickly, and these clouds hover above the Arctic about 30 to 50 percent of the time.
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