Most of the Earth’s frozen areas, known as the cryosphere, are highly reflective, bouncing sunlight back into the atmosphere and helping to keep the Earth cooler than it would otherwise be. As the Earth warms and snow and ice melts, the reflectivity decreases, creating a feedback loop that leads to further warming.

In the present study, a team led by Mark Flanner of the University of Michigan compared model estimates of changes in the Northern Hemisphere’s cryosphere between 1979 and 2008 with changes in actual snow, ice and reflectivity measurements over the same period. They found that during the 30-year period, cryosphere cooling declined by 0.45 watts per square metre. They also calculated that for every degree of warming, an average of 0.6 fewer watts of solar radiation are reflected to space per square metre because of reduced snow and sea ice cover. In the 18 models used by the International Panel on Climate Change, the average was 0.25 watts per square metre per degree Celsius over the same time period. 

‘The cryospheric albedo feedback is a relatively small player globally, but it has been a surprisingly strong feedback mechanism over the past 30 years,’ Flanner said. ‘A feedback of this magnitude would translate into roughly 15 per cent more warming, given current understanding of other feedback mechanisms.’

March 2011