A new 30-year analysis has found that the northern hemisphere's loss of reflectivity due to snow and sea ice decline is more than double current estimates.
This 'albedo decline' means that the Earth's cryosphere - that part of the Earth's surface that's covered in ice and snow - is reflecting much less of the sun's energy back into space than was believed, affecting current climate change models.
"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.
"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, amplifying global warming. And while scientists have known for some time about this amplification effect, Shell says almost all the climate models her team examined underestimated the impact.
The team compared northern hemisphere cryosphere changes between 1979 and 2008 in 18 different climate models to changes in actual snow, ice and reflectivity measurements of the same period. They determined that mean radiative forcing – or the amount of energy reflected into the atmosphere – ranged from 4.6 to 2.2 watts per meter squared.
During the 30-year study period, cryosphere cooling declined by 0.45 watts per meter squared. The authors attribute that decline equally to loss of snow and sea ice.
"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."