Shrinking Snow and Ice Cover
The decreases in Earth's snow and ice cover over the last 30 years have, on average, exacerbated global warming more than models predict they should have, new research from the University of Michigan (U-M) shows.
To conduct this study, Mark Flanner, assistant professor in the Department of Atmospheric, Oceanic, and Space Sciences at U-M , analyzed satellite data showing snow and ice during the last three decades in the Northern Hemisphere-which holds the majority of the planet's frozen surface area. The research is published online in Nature Geoscience.
Snow and ice reflect light and heat from the Sun back into space, causing an atmospheric cooling effect. But as the planet warms, more ice melts and, in some cases, less snow falls, exposing additional ground and water that absorb more heat, which amplifies the effects of warmer temperatures. This change in reflectance contributes to what is called albedo feedback,, one of the main positive feedback mechanisms that adds fuel to the planet's warming trend.
"If the Earth were just a static rock, we could calculate precisely what the level of warming would be, given a perturbation to the system," Flanner says. "But because of these feedback mechanisms, we don't know exactly how the climate will respond to increases in atmospheric carbon dioxide."
"Our analysis of snow and sea ice changes over the last 30 years indicates that this cryospheric feedback is almost twice as strong as what models have simulated," Flanner says. "The implication is that Earth's climate may be more sensitive to increases in atmospheric carbon dioxide and other perturbations than models predict."
The cryosphere is the planet's layer of snow, sea ice, and permanent ice sheets. Since 1979, the average temperature in the Northern Hemisphere has risen by about 0.7[degrees]C, whereas the global average temperature has risen by about 0.45[degrees]C, Flanner says.
For every 1C rise in the Northern Hemisphere, Flanner and his colleagues found that an average of 0.6 fewer watts of solar radiation are reflected to space per square meter because of reduced snow and sea ice cover. In the 18 models taken into consideration by the International Panel on Climate Change, the average was 0.25 watts per square meter per degree Celsius over the same time period.
Flanner points out that the models typically calculate this feedback over 100 years-significantly longer than this study, which might account for some of the discrepancy. Satellite data only goes back 30 years.
To further put the results in context, each square meter of Earth absorbs an average of 240 watts of solar radiation. These new calculations show that the Northern Hemisphere's cryosphere is reflecting 0.45 watts less per square meter now than it did in 1979, due mostly to reduced spring snow cover and summer sea ice.
"The cryospheric albedo feedback is a relatively small player globally, but it's been a surprisingly strong feedback mechanism over the past 30 years," Flanner says. "A feedback of this magnitude would translate into roughly 15% more warming, given current understanding of other feedback mechanisms."
To avoid the worst effects of climate change, the scientific consensus is that the global average temperature should stay within 2[degrees]C of preindustrial levels. Scientists are still trying to quantify the extent to which the planet will warm as greenhouse gases accumulate in the atmosphere.
"People sometimes criticize models for being too sensitive to climate perturbations," Flanner says. "With respect to cryospheric changes, however, observations suggest the models are a bit sluggish." (University of Michigan) http://ns.umich.edu/htdocs/releases/story. php?id=8199
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