CLIMATE change is supposed to unfold slowly, over decades. But that is not true up in the great white north, as those attending this year’s meeting of the American Association for the Advancement of Science were reminded in the session on climate change in the Arctic. Temperatures there are 2°C higher than their long-term average (around twice the increase in the rest of the world), and the upper layers of parts of the Arctic Ocean are hotter than they have been for at least 2,000 years. Summer sea ice has been vanishing faster than even the gloomiest researchers thought likely, with some now predicting the first completely ice-free summer as soon as the 2020s.

The Arctic is not, though, isolated from the rest of the world; rapid changes there could have knock-on effects elsewhere. Whether or not that is happening was a question addressed by Jennifer Francis, an atmospheric scientist at Rutgers University. It is a topical subject. Along with much of the rest of America, Chicago endured a fierce and prolonged cold snap in January, in which temperatures fell to -27°C, the lowest since 1884. Meanwhile, Brits at the conference were fleeing a country that had been soaked by the heaviest winter rains in two and a half centuries, and battered by a seemingly endless succession of Atlantic storms and gales.

Campaigners in both countries have been quick to blame climate change for the rotten weather. But things are rarely so straightforward in climatology. The best Dr Francis could offer was a theory as to why a warmer Arctic might be expected to lead to wilder weather in mid-latitudes, and some tentative but suggestive evidence that this is already happening.

Her idea rests on the jet stream, a powerful, persistent, high-altitude “river of air” which flows around the world from west to east, affecting the weather as it goes. The jet stream is driven in part by the temperature difference between cold Arctic air and the warmer air of middle latitudes. Because the Arctic is warming more rapidly than the rest of the planet, that difference is shrinking. This ought to produce a less potent jet stream. And a less potent jet stream is a more wayward one.

In the same way that a mountain torrent runs straight down steep slopes, while a slower river meanders across its plain, a weak, slow jet stream should swing north and south more, taking its weather with it. That meandering would also mean any weather fronts it propelled moved more slowly than normal, leading to more persistent weather patterns. And it would increase the likelihood of eddies splitting from the jet stream and sitting in the sky blocking atmospheric movement. This would further increase the persistence of weather patterns, and cause heatwaves, cold snaps and droughts.

That, at least, is the theory. And climate models (mostly) agree with it. But models are not evidence, and so Dr Francis has gone out looking for some. She showed her audience several suggestive charts which measured how the difference between the thickness of the atmosphere at the poles and at mid latitudes (a proxy for temperature differences, since warm air takes up more room than cool air) has fallen in recent years. That correlates nicely with another set of data showing wind speeds in the jet stream, with the bits of the world that have the smallest temperature gradient showing the biggest drop in speeds. The places with the smallest temperature gradient also seem to be where the jet stream meanders most.

A set of observations that mostly match what a sensible-sounding theory predicts may seem a strong case. But not everyone is convinced. One problem is that the changes in atmospheric thickness and wind speed Dr Francis observed have become apparent only since the mid-1990s, which gives fewer than 20 years of data to work with. Another difficulty concerns the blocking patterns. Dr Francis’s theory suggests there will be more of them in future, as the world warms. But climate models unanimously disagree, saying that global warming will produce fewer of them. Someone has to be wrong.

Trying to apply the theory to this year's batch of wild weather causes even more headaches. America’s cold snap was indeed produced partly by a wayward jet stream. But Dr Francis herself points out that Britain’s gales and rain are a different story, and that this year’s North Atlantic jet stream has in fact been stronger than normal. That illustrates one of the chief frustrations of climate science. The weather is naturally changeable, which means many years of data are needed to extract a clear trend from among all the noise. Were the American freeze and the British floods caused by a weaker jet stream? No one can say for sure. But if Dr Francis and her colleagues are right, the odds on a repeat performance will get shorter every year.