Is climate change behind US cold snap?

The arctic blast that brought record low temperatures across the US in the past few days has provided fuel for global-warming sceptics, even as scientists debate whether the extreme cold might, in fact, be a consequence of a warming climate.

"Of course, we can't say that this particular pattern is due to arctic warming, but it's very consistent with what we expect to see happen," said Jennifer Francis, an atmospheric scientist at Rutgers University in New Brunswick, New Jersey.

Arctic warming is leading to declines in sea ice and increased snowmelt on land. Because ice and snow are bright, they reflect sunlight back into space, said a latest report published in the journal Nature Thursday.

When they melt, more solar energy can be absorbed by the Arctic, the report added.

"One theory is that a warmer Arctic will reduce the temperature differences between the Arctic and warmer latitudes - leading to a weaker polar jet stream that would be more likely to wander off course from time to time," the Nature report added.

According to Judah Cohen, who heads seasonal forecasting for Atmosphere and Environmental Research, a private research firm in Lexington, Massachusetts, sea-ice melt is a precursor to another powerful driver - Siberian snowpack.

"Warming increases evaporation, and more water vapour in the air leads to additional snowfall later in the year."

"My research suggests that autumn snowpack in Siberia - which was well above normal this past year - is a good predictor of winter severity in places as far away as the United States," said Cohen.

The combination of melting sea ice and extra Siberian snow seems to ripple through the Arctic weather system, destabilising the polar jet stream and leading to colder winters on the northern continents, the report added.

At least 21 deaths were reported across the US since Sunday due to the extreme cold conditions owing to 'polar vortex' - the arctic blast that brought record low temperatures across the United States.

The mercury dipped as low as -52 Celsius (-61 Fahrenheit) in Montana and was in the -40 to -50 Celsius (-40 to -58 Fahrenheit) range in parts of several north American states early this week.

Yet the experts don't find climate change as the main culprit.

"If Arctic warming were to blame, the effect should be the strongest in the northern hemisphere's summer and fall," claimed Kevin Trenberth, a climate scientist at the National Centre for Atmospheric Research in Boulder, Colorado.

"It is unclear how this seasonal warming effect would persist into the dead of winter," Trenberth added.

Elizabeth Barnes, an atmospheric scientist at Colorado State University in Fort Collins, also did not find any connection between a warming Arctic and a meandering of the jet stream, the report in Nature concluded.