Saturday, January 22, 2011

Albedo Feedback: Climate Models under estimate loss of reflectivity in the Arctic


Loss of reflectivity in the Arctic is double the estimate used in current state-of-art climate models according to new analysis of the Northern Hemisphere's "albedo feedback" over a 30-year period.

The albedo feedback is the change in the reflectivity of surfaces in a constant reinforcing feedback loop. In the Arctic snow and ice is highly reflective. Global warming has reduced sea ice exposing darker ocean and increased the size of dark melt pools on the Greenland ice sheet, which then increases the warming effect on ice in a positive feedback.

"The cryosphere isn't cooling the Earth as much as it did 30 years ago, and climate model simulations do not reproduce this recent effect," said Karen Shell, an Oregon State University atmospheric scientist and one of the authors of the study. "Though we don't necessarily attribute this to global warming, it is interesting to note that none of the climate models used for the 2007 International Panel on Climate Change report showed a decrease of this magnitude."


"Instead of being reflected back into the atmosphere, the energy of the sun is absorbed by the Earth, which amplifies the warming," Shell said. "Scientists have known for some time that there is this amplification effect, but almost all of the climate models we examined underestimated the impact - and they contained a pretty broad range of scenarios."

Mark Flanner, assistant professor in the Department of Atmospheric, Oceanic and Space Sciences University of Michigan, also involved in the research, 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.

Flanner said "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,"

"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." Flanner comented.

The research has been published in Nature Geoscience - Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008. The abstract says in part:

"We find that cyrospheric cooling declined by 0.45 W m-2 from 1979 to 2008, with nearly equal contributions from changes in land snow cover and sea ice. On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m-2 K-1, substantially larger than comparable estimates obtained from 18 climate models."


"Some of the decline may be natural climate variability," Shell said. "Thirty years isn't a long enough time period to attribute this entirely to 'forcing,' or anthropogenic influence. But the loss of cooling is significant. The rate of energy being absorbed by the Earth through cryosphere decline - instead of being reflected back to the atmosphere - is almost 30 percent of the rate of extra energy absorption due to carbon dioxide increase between pre-industrial values and today."

The reserach comes hard on the heels that Greenland had a record melt during 2010.

While climate models have sometimes suffered from criticism for being too sensitive, this study shows current models have underestimated the feedback effect. "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."

"While the current group of models underestimates these Northern Hemisphere cryosphere changes, new models will be released this year that will have better representations of snow and ice," Shell said. "This study will help climate modelers improve the new generation of models to better predict the rate of cryosphere and albedo decline in the future.


Sources:
* Adapted from Media release - Oregon State University, Jan 18, 2011 - New study: Loss of reflectivity in the Arctic double estimate of climate models
* Adapted from Media Release - University of Michigan, Jan 18, 2011 - Shrinking snow and ice cover intensify global warming
* Photo - "Detail of a supraglacial lake from the above picture. Note the streams feeding the lake and the darker area at the bottom. This could be cryoconite, a dark mixture of soot, sediments and organic matter." from greenland.cryocity.org/