U. MICHIGAN (US) — Decreases in the Earth’s snow and ice cover have exacerbated global warming more than previously thought, according to a new study.
Researchers 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. Details are published online in the journal 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,” says Mark Flanner, assistant professor of atmospheric, oceanic, and space sciences at University of Michigan.
“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. 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 says.
For every 1 degree Celsius rise in the Northern Hemisphere, Flanner 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 Intergovernmental Panel on Climate Change, the average was 0.25 watts per square meter per degree Celsius over the same time period.
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. The 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 says. “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 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 says. “With respect to cryospheric changes, however, observations suggest the models are a bit sluggish.”
Researchers from Oregon State University and the U.S. Army Corps of Engineers contributed to the study, which was funded by the National Science Foundation.
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