New study highlights need to get a better handle on the role of clouds in changing climate
By Summit Voice
FRISCO — Widespread melting of the Greenland ice sheet last summer were triggered by an influx of unusually warm air from North America, but the melting was intensified by a layer of clouds that were “just right” for driving surface temperatures there above the melting point.
The thin, low-lying clouds allowed the sun’s energy to pass through and warm the surface of the ice, while at the same time trapping heat near the surface of the ice cap, according to a new study published this week in Nature that took a close look at Greenland’s summer heat wave.
“Thicker cloud conditions would not have led to the same amount of surface warming,” said Matthew Shupe, research meteorologist with NOAA’s Cooperative Institute for Research in Environmental Sciences at the University of Colorado and the NOAA Earth System Research Laboratory. “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.” said Shupe, part of a team that included scientists from NOAA and the Universities of Wisconsin, Idaho and Colorado.
The study is part of a global effort to understand how quickly Greenland is warming because ice melt there contributes to sea level rise globally. In July 2012, more than 97 percent of the Greenland Ice Sheet surface experienced some degree of melting, including at the National Science Foundation’s Summit Station, high atop the ice sheet. According to ice core records, the last time the surface at Summit experienced any degree of melting was in 1889, but it is not known whether this extended across the entire ice sheet.
To investigate whether clouds contributed to, or counteracted, the surface warming that melted the ice, the authors modeled the near-surface conditions with data from atmospheric sensors operated as part of an ongoing research project at the Greenland summit.
“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,” said said David Turner, research meteorologist with the NOAA National Severe Storms Laboratory and one of the lead investigators.
Clouds can cool the surface 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.
In certain conditions, these 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. Thicker clouds would have reflected away more solar radiation; thinner ones couldn’t have trapped as much heat, and in either of those cases, there would have been less 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,” said 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.”
Better understanding of clouds also 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,” said Ralf Bennartz, lead author for the study and professor at the University of Wisconsin-Madison. “Above all, this study highlights the importance of continuous and detailed ground-based observations over the Greenland Ice Sheet and elsewhere. Only such detailed observations will lead to a better understanding of the processes that drive Arctic climate.”