U-Idaho prof and team: Thin, low Arctic clouds key to massive 2012 Greenland ice melt
Wednesday, April 3 2013
Clouds over the central Greenland Ice Sheet last July were “just right” for driving surface temperatures there above the melting point, according to a new study by University of Idaho geography professor Von P. Walden, doctoral student Christopher Cox and scientists at the National Oceanic Atmospheric Association (NOAA), the University of Wisconsin and the University of Colorado.
The study, to be published April 4 in the journal Nature, found that 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. This combination played a significant role in last summer's record-breaking melt.
“It’s kind of like the story of Goldilocks. If the sky had no clouds on July 11, it would have been too clear and cold to melt the surface. But if the clouds were too thick, it also would have been too cloudy and cold,” Walden said. “It is essential that we increase our understanding of the clouds that form over Greenland so that we can better predict future conditions.”
Walden is principal investigator for the National Science Foundation-supported ICECAPS project – short for the Integrated Characterization of Energy, Clouds, Atmospheric state and Precipitation at Summit Station – which has been studying the atmosphere high atop the ice sheet for nearly three years.
Scientists around the world are trying to understand how quickly Greenland is warming because ice melt there contributes to global rises of sea level rise. The Greenland Ice Sheet is second only to Antarctica in ice volume. In July, more than 97 percent of the Greenland Ice Sheet surface experienced some degree of melting, including at Summit Station. 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 led to the ice melt, the authors modeled the near-surface conditions. The model was based on observations from a suite of sophisticated atmospheric sensors operated as part of ICECAPS.
Jack McIver, U-Idaho’s vice president of research and economic development, said ICECAPS’ capturing of the melt event demonstrates the relevance of the university’s research on a global scale.
“As the lead institution on the ICECAPS project, the University of Idaho was at the right place at the right time,” McIver said. “Walden and his team have explained one major atmospheric event, and I expect that they’ll be there for the next. This work is a major contribution to understanding the long-term implications of climate change in Greenland.”
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.
“Clouds are still one of the greatest uncertainties in climate models that we use to predict the future. Even though the models are generally correct, we need better measurements to improve them,” Walden said. “We’re doing this to avoid future surprises, and we need to expand our knowledge of the details.”
To read more about Walden and his Arctic research, visit www.uidaho.edu/research/research-articles/atmosphere-on-top-of-the-world.
Other institutions’ news releases regarding this paper: University of Colorado at Boulder: http://cires.colorado.edu/news/press/2013/greenland-ice-melt.html
; University of Wisconsin-Madison: www.news.wisc.edu/21638
; Swiss Federal Research Institute: www.wsl.ch/medien/news/greenland_clouds_2013/index_EN
; National Oceanic Atmospheric Administration: http://researchmatters.noaa.gov/news/Pages/ArcticClouds.aspx
; National Science Foundation: www.nsf.gov/news/news_summ.jsp?cntn_id=127438
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