Puzzles posed by a chilly northern winter

The northeastern United States copped a chilly hiding for the second year in a row this winter, as massive storms dumped record-breaking snowfalls in January, February and March. 

It may seem more akin to global cooling than global warming but one camp of climate scientists is convinced this is an early symptom of human-induced climate change. Another says the changes are entirely in line with natural weather fluctuations.

At the centre of the controversy is the northern polar jet stream, a 200-kilometre-wide fast-moving river of air in the upper atmosphere.

There are four major jet streams on Earth – one polar and one subtropical in each hemisphere – and a host of smaller streams scattered around. They circle the planet 11 kilometres above and have a huge effect on the weather below. Because they set the boundary between cold and hot air, changes in their path can mean the difference between a mild winter and a deep freeze.

The Arctic is warming twice as fast as the average of the rest of the planet

Over the past couple of northern winters the polar jet stream, which usually traces a circle west to east around the Arctic, has broken its banks and extended south across Russia and down the centre of North America. In the first three months of the year, the jet stream poured cold Arctic air, usually confined to a compact patch above the North Pole, down through the upper Midwest, dumping a heavy band of snow across New England. 

A number of natural factors can affect a jet stream’s speed and course, such as mountain ranges and changes in sea-surface temperature patterns. But Jennifer Francis, a climate scientist at Rutgers University in New Jersey, believes global warming is the main culprit causing recent changes. In 2012, Francis – with Steven Vavrus, a climate modeller from the University of Wisconsin – proposed Arctic warming contributes to the polar jet stream swinging further north and south. 

The disproportionate warming of the Arctic is beyond doubt: the National Oceanic and Atmospheric Administration’s (NOAA) 2014 Arctic Report Card found the Arctic is warming twice as fast as the average of the rest of the planet, a phenomenon known as “Arctic amplification”. Satellite data this March showed that the winter maximum extent of Arctic sea ice has dwindled to its lowest levels since records began in 1979. As of 19 March, the US National Snow and Ice Data Centre reported the Arctic sea ice cap peaked at 14.75million square kilometres – around 130,000 square kilometres below the previous record low in 2011. In the 10 days between February 25 and March 7, sea ice coverage receded by 175,000 square kilometres – around two and a half times the area of Tasmania.

With the warming Arctic, Francis and Vavrus hypothesised that the temperature gradient across the jet stream has flattened. In the pre-warming era, the jet stream was like a river confined to a steep canyon; but the flatter gradient made it more like a shallow stream that can break its banks to spread north and south.

Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, also believes a steadily warming Arctic played a role in recent winter extremes in North America, but by a slightly different means. 

He believes that as Arctic sea ice melts, it adds moisture to the polar atmosphere which ends up as more snowfall in Eurasia. The ensuing column of cold air over Siberia bumps the polar jet stream into a wavier path. 

Climate models show that the poles don’t affect the atmosphere anywhere near as much as the lower latitudes.

As evidence that this bump also affects the rest of the northern hemisphere, Cohen studied weather satellite data extending back to the 1960s and found that a heavier and more extensive Siberian snowfall in October heralded a severe northern hemisphere winter. “If there was nothing [in the hypothesis] then our forecasts would be garbage, but I think they’ve been very good,” he says – a sentiment echoed by Washington Post weather editor Jason Samenow in April 2013, who called Cohen’s winter forecasts “amazingly accurate”. 

But many climate researchers are unconvinced by the warming Arctic hypothesis. Meteorologist Cliff Mass, at the University of Washington, believes the snowy conditions are the result of natural variability: “We’ve had anomalies like this before.” And he’s not alone. In 2013 Colorado State University atmospheric scientist Elizabeth Barnes analysed the meanderings of the jet stream and Arctic temperatures from 1980 to 2011 but found no link between the two.

“Observed trends in mid-latitude weather patterns are complex and likely not simply understood in terms of Arctic amplification alone,” she wrote. 

Mass and Barnes’ views were backed in February 2014 when John Wallace from the University of Washington and four climate scientists from Princeton, Colorado State University, the National Centre for Atmospheric Research and University of Alaska Fairbanks published a letter in Science, pointing out that chilly bursts of cold Arctic air, more severe than the winter of 2013-14, occurred in the early 1960s, late 1970s and in 1983, “when the Arctic sea ice was thicker and more extensive than it is today” and well before the proposed shallowing of the bed of the polar jet stream.

Francis counters that her hypothesis is not about record-breaking cold spikes but that these cold spells will occur for extended periods down the track. “This winter and last winter were excellent examples of the very persistent patterns that we expect to happen more frequently in connection with a rapidly warming Arctic,” she says.

Yet another argument is that the tropics have far more influence on the weather than the Arctic. Mass says climate models show that the poles don’t affect the atmosphere anywhere near as much as the lower latitudes, which receive the lion’s share of solar energy and churn it through the upper atmosphere. “Think about the volume of the atmosphere at the tropics compared to the poles – the tropics are much bigger,” he says. Natural changes in the tropical atmosphere could push up through the polar jet stream, forcing it to dip and bulge in other parts.

Most climate scientists do agree that human-driven climate change will only make the weather more extreme in the long term, regardless of whether it’s happening now or not. 

“Ocean temperatures are warming but not evenly, tropical storms may become less frequent but stronger and may be able to survive farther north and later into the fall and the Arctic will continue to warm faster than elsewhere,” Francis says, adding that one challenge facing climate science is separating the influences of those factors. “But it’s been a very exciting winter, especially for a meteorologist!” 

As for next winter, Mass believes the chilly blasts will snap back up to the Arctic and leave North America alone come December 2015: the snowy weather is “not going to be here forever. It’ll probably be over next winter.” Time will tell whether Mass is right – or whether a third winter of heavy snowfall in a row lends weight to Francis’ argument.