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Thursday 31 January 2019

Polar paradox

A fridge door has been left open at the top of the world

By Giles Whittell

If the forecasters are right it will be colder today in Chicago than in most of Antarctica. Blizzards will bury central North America, and most of the Alps, and snow has even come to much of Britain. President Trump is wondering, “What the hell happened to global warming?”

The answer seems to be: this. This brutal freeze-up at latitudes where many of us live is what is happening to global warming, or at least as a result of it. It’s still very much with us and the science behind its paradoxical effect on our winter weather is not even all that complicated.

getty images

Falling snow in New York City.

In 2012, reacting to prodigious snowfalls in and around Washington DC two years earlier, a team of atmospheric physicists from Atlanta, Beijing and New York published a paper in the Proceedings of the National Academy of Sciences. Their study, which covered the period from 1979 to 2010, showed a strong correlation between heavy northern hemisphere snow and shrinking Arctic sea ice.

The team offered two explanations: increasing amounts of water vapour (turning eventually to snow) over an Arctic Ocean that has become darker and warmer than it used to be; and “changes in atmospheric circulation” that have not just coincided with the shrinking of the sea ice, but are linked to it.

In colder times a strong low pressure system would sit over the deep-frozen Arctic for most of the winter. It would pull prevailing anti-clockwise winds towards the top of the world and keep a pool of cold air known as the polar vortex centred on the North Pole.

Nowadays, warmer-than-usual air over the Arctic means higher-than-usual atmospheric pressure, pushing the polar vortex southwards and sometimes causing it to meander and lose its way.

Even in a warming climate, the air in the polar vortex is cold by non-polar standards. This year its meandering may have been exaggerated by an unseasonal injection of warm air from North Africa into the stratosphere high over the polar ice cap, breaking the vortex in two. That is why half of it is now sitting over the Midwest.

getty images

A commuter in sub-zero temperatures, Chicago

Pedestrians are being told not to breathe too deeply or talk too much if they have to go outside. It will be of little consolation to them, but the last four years are, overall, the hottest on record. Much of Australia is now having to cope with temperatures of more than 45 degrees.

Over the Arctic what has happened is that global warming has wedged the fridge door open. That means a big chill now, and a big defrost to come.

Further reading

Impact of declining Arctic sea ice on winter snowfall
(Proceedings of the National Academy of Sciences, February 2012)

Weakening of the stratospheric polar vortex by Arctic sea ice loss
(Nature Communications, September 2014)