"Global warming is now making our winters colder!"
Only the craziest of eco-doom mongerers would be foolish enough to spout such nonsense, surely? The threat from a warming planet may take many guises, but making us colder – that's plainly on the 'wing-nut' side of things, isn't it. But in fact just such a claim is now being made, not by the deep-green fringe of the environmental movement – but by respected climate scientists from Columbia University, Georgia Tech and China's Institute of Atmospheric Physics. And the tale that their research lays out, in the most recent edition of the PNAS, is anything but a madman's babblings.
Patterns all-a-flux on the climate's patchwork quilt
Instead, their paper coolly describes how the patchwork-quilt climate of the northern hemisphere is quickly unravelling, as the world warms. And for some – including parts of North America, Europe and Asia – the new pattern has plenty of colder winters woven into the fabric. That's something that no-one living there can fail to have noticed, in recent years. Deep snows and freezing temperatures have made themselves unwelcome guests, repeatedly, over the last four winters. But the culprit for all that cold may turn out to be the very thermostat dials being turned up, to keep us snug – all thanks to the global warming being bought on by higher greenhouse gas emissions.
The story begins 30,000 feet up, with the wandering band of winds – the polar jet stream – whistling around the Arctic regions, at up to 250 mph. This high, narrow wind-belt in fact marks the boundary between the cold airs at the North pole, and the warmer moist airs of the temperate zones. The jet stream keeps all that cold where it should be, up north – and the stronger the jet stream is, the more tightly those frigid airs are locked in at the pole. It's when the jet stream weakens and slows, that the cold air has a chance to spill down below the Arctic Circle, bringing harsher winters in its wake.
The sea-ice, the cliff and the snow
The driver for the jet stream, and the wall of winds separating the cold north from the warmer south, arises precisely from that stark contrast in temperatures. The cold dense air at the pole sinks to form high pressure; while the warmer air surrounding it rises, making a ring of low pressure systems. And the frozen sea-ice over the Arctic ocean acts as an anchor, providing a deep pool of cold that is one of the drivers for a strong jet stream.
But something has changed of late, and changed dramatically. The Arctic's cap of sea-ice, which was slowly receding thanks to global warming, fell off a cliff in 2007. Sea-ice volumes for the last 5 years have been half, or less, what they were before 2007 – as can be seen in the plummeting graph above. With less sea-ice after each summer's melt, those waters up north are taking longer to freeze over in the autumn.
That's where the team of researchers –looking for reasons for the last few winters of record snowfalls – have turned, in the search for an explanation. ′For the past four winters, for much of the northern US, east Asia and Europe, we had this persistent above-normal snow cover,' paper co-author Dr Jiping Liu, of Georgia Tech, explained. In order see if there was a connection between less sea-ice for the Arctic, and more snow for us, the team looked afresh at the mounds of sea-ice and snow data from 1979 to 2010. They also ran computer simulations that modeled the climate, with simulated sea-ice being reduced, as has unfolded in the real-world.
Spinning top ready to topple
The conclusions drawn from these two different approaches were the same, according to Dr Liu. More open water at the start of winter means more heat is passed back into the air, when the sea freezes. That heat makes the winter Arctic air mass warmer, and so softens the contrast in air temperatures. That in turn makes the jet stream more weak and wobbly – a bit like a spinning top losing speed, and on the verge of toppling over.
′We think the recent snowy winters could be caused by the retreating Arctic ice altering atmospheric circulation patterns by weakening westerly winds, increasing the amplitude of the jet stream and increasing the amount of moisture in the atmosphere,' he said. ′These pattern changes enhance blocking patterns that favor more frequent movement of cold air masses to middle and lower latitudes, leading to increased heavy snowfall in Europe and the Northeast and Midwest regions of the United States.′
So while the world gets relentlessly warmer – on average, and over a number of years – some of us are getting slaps of rude winter cold, much more often we used to. It's almost as if the planet is pressing out its own desperate S.O.S to the world's big polluters. And the madness, maybe, is not in the planet's contradictory message, but in the perpetrators – addicted to fossil-fueled growth – who continue to ignore it, at their own peril.