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Rare constellation of clouds and warm air allowed record-breaking 2012 ice melt in Greenland
Thin, low-lying clouds over the central Greenland Ice Sheet last July allowed the sun’s energy to warm the surface of the ice, while at the same time trapping infrared radiation near the surface which led to additional warming. This combination played a significant role in last summer's record-breaking melt, according to a new study by scientists at NOAA, the Universities of Wisconsin, Idaho and Colorado as well as at the Swiss Federal Research Institute WSL. The results of the study are published now in Nature.
Matthew Shupe, research meteorologist at the University of Colorado and at NOAA (National Oceanic and Atmospheric Administration), says that "thicker cloud conditions would not have led to the same amount of surface warming." Thinner clouds, however, could not have trapped as much infrared radiation, and in either of those cases, there would have been less surface warming. "To understand the region’s future, you’ll need to understand its clouds. Our finding has implications for the fate of ice throughout the Arctic," Shupe says.
Prof. Konrad Steffen (Swiss Federal Research Institute WSL and ETH Zürich) says: "We verified the atmospheric model that predicted surface melt with low-laying liquid water clouds using the Baseline Surface Radiation Network (BSRN) instruments at the Summit Station in Greenland." Steffen has maintained these measurements in Greenland for several years. "Long-term monitoring of environmental parameters such as solar and thermal radiation is crucial for the understanding and interpretation of the changing climate in the Arctic", Steffen says.
Scientists around the world are trying to understand how quickly Greenland is warming because ice melt there contributes to global sea-level rise. The Greenland Ice Sheet is second only to Antarctica in ice volume. In July 2012, more than 97 percent of the Greenland Ice Sheet surface experienced some degree of melting, including at the US National Science Foundation’s Summit Station, high atop the ice sheet. According to ice core records, the last time the surface at this site experienced any degree of melting was in 1889, but it is not known whether this extended across the entire ice sheet.
Combination of warm air intrusion and thin clouds
"The July 2012 ice melt was triggered by an influx of unusually warm air sweeping in from North America, but that was only one factor," says David Turner, research meteorologist at NOAA and one of the lead investigators. "In our paper, we show that low-lying clouds containing a low amount of condensed water were instrumental in pushing surface air temperatures up above freezing and causing the surface ice to melt", Turner says.
Clouds can cool the surface of the ice sheet by reflecting solar energy back into space, and can warm it by radiating heat energy back down to the surface. The balance of those two processes depends on many factors, including wind speed, turbulence, humidity and cloud “thickness,” or liquid water content. Under certain conditions, clouds can be thin enough to allow some solar radiation to pass through, while still “trapping” infrared radiation at ground level. That is exactly what happened last July: the clouds were just right for maximum surface warming.
The researchers also found these thin, low-lying liquid clouds occur 30 to 50 percent of the time in summer, both over Greenland and across the Arctic. Current climate models tend to underestimate their occurrence in the Arctic, which limits those models’ ability to predict how clouds and their warming or cooling effects may respond to climate change.
"The cloud properties and atmospheric processes observed with the Summit Station instrument array provide a unique dataset to answer the large range of scientific questions we want to address," says Turner. "Clouds play a big role in the surface mass and energy budgets over the Greenland Ice Sheet. Melting of the world’s major ice sheets can significantly impact human and environmental conditions via its contribution to sea-level rise", he adds.
Better understanding of clouds improves climate models
"Our results may help to explain some of the difficulties that current global climate models have in simulating the Arctic surface energy budget, including the contributions of clouds," says Ralf Bennartz, lead author for the study and professor at the University of Wisconsin-Madison. "Above all, this study highlights the importance of continuous ground-based observations over the Greenland Ice Sheet. Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate."
Bennartz, R., Shupe, M. D., Turner, D.D., Walden, V. P., Steffen, K., Cox, C. J., Kulie, M. S., Miller, N. B., C. Pettersen (2013): July 2012 Greenland melt extent enhanced by low-level liquid clouds. Nature, online on 04. April 2013.