ANN ARBOR, Mich.---The decreases in Earth's snow and ice cover over the past 30 years have exacerbated global warming more than models predict they should have, on average, new research from the University of Michigan shows.
To conduct this study, Mark Flanner, assistant professor in the Department of Atmospheric, Oceanic and Space Sciences, analyzed satellite data showing snow and ice during the past three decades in the Northern Hemisphere, which holds the majority of the planet's frozen surface area. The research is newly published online in Nature Geoscience.
Snow and ice reflect the sun's light and heat back to 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, amplifying the effects of warmer temperatures. This change in reflectance contributes to what's called "albedo feedback," one of the main positive feedback mechanisms adding fuel to the planet's warming trend. The strongest positive feedback is from atmospheric water vapor, and cloud changes may also enhance warming.
"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. But because of these feedback mechanisms we don't know exactly how the climate will respond to increases in atmospheric carbon dioxide," Flanner said.
"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. 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.
In the Northern Hemisphere since 1979, the average temperature rose by about 0.7 degrees Celsius, whereas the global average temperature rose by about 0.45 degrees, Flanner said.
For every 1 degree Celsius rise in the Northern Hemisphere, Flanner and his colleagues calculated an average of 0.6 fewer watts of solar radiation 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 could 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 cryosphere is reflecting .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 said. "A feedback of this magnitude would translate into roughly 15 percent 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 Celsius, or 3.6 degrees Fahrenheit, of pre-industrial 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 said. "With respect to cryospheric changes, however, observations suggest the models are a bit sluggish."
The paper is called "Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008." This research is funded by the National Science Foundation.
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