In Canada it was dubbed 'Snowmageddon', while the Brits called it 'The Big Freeze'. Across Europe and the eastern United States temperatures plummeted for weeks on end during the winter of 2009/2010 and heavy snowfall caused major disruption. And then it happened all over again during the winter of 2010/2011, causing many to question whether global warming was happening at all.

Now research suggests that colder winters have become more frequent over the past two decades, and were likely a product of global warming.

Judah Cohen, from Atmospheric and Environmental Research (AER) in Massachusetts, US, and his colleagues studied northern hemisphere temperature, humidity, snow cover, and sea ice cover data from the last twenty years. They show that as northern hemisphere summer temperatures have risen, September sea ice cover on the Arctic Ocean has shrunk. At the same time atmospheric moisture levels in the Arctic have risen, October snowfall across Eurasia has increased, and winters have become colder (Environmental Research Letters).

"Possibly the increase in open water over the Arctic Ocean means that more moisture can be evaporated into the atmosphere, leading to increased precipitation events," Cohen told environmentalresearchweb. In addition a warmer atmosphere is capable of holding more moisture.

Previous work has shown that increased autumn snowfall across Eurasia triggers a negative Arctic Oscillation pattern, leading to colder winter temperatures across the eastern US, Europe and eastern Asia. Meanwhile, north-eastern Canada, the Mediterranean and North Africa all experience warmer than average winters.

The new data suggests that the probability of a negative Arctic Oscillation establishing has been increasing since the 1980s, and Cohen sees no reason why increasing autumn snow cover will cease to favour a negative winter Arctic Oscillation in the coming decades. However, if summer and autumn temperatures continue to rise then the extra atmospheric moisture may fall as autumn rain, rather than snow.

Of course natural variability will continue to play a role, and there will still be mild winters too. The current winter may be a case in point. "So far the negative Arctic Oscillation pattern has failed to set up, and it appears to be the opposite pattern to the previous two winters," said Cohen.

By comparing the real data with climate model predictions, Cohen and his colleagues show that the models generally failed to predict cooler winters, and instead indicated a trend for warmer winters. "The models do not simulate well the link between autumn Eurasian snow and winter temperatures," said Jason Furtado, co-author on the paper.

Now Cohen and his colleagues are hoping to incorporate this negative feedback pattern into climate models, to improve future predictions.