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The northern polar jet stream, a band of high-altitude winds that circles the Arctic and influences the climate of the northern hemisphere, is forecast to start noticeably shifting in the 2060s if greenhouse emissions remain high, leading to dramatic changes in temperature and rainfall, especially in Europe.

鈥淭his would have drastic consequences for society,鈥� says at the University of Arizona. 鈥淏ut the ultimate trajectory of the jet stream is still largely under our control.鈥�

Direct observations of this polar jet stream only began a few decades ago thanks to the proliferation of weather satellites, says Osman, so it hasn鈥檛 been clear how global warming is affecting it.

Now he and his team have worked out how the average position and intensity of the jet stream over the North Atlantic has changed over the past 1250 years by analysing Greenland ice cores. The position of the jet stream determines storm tracks over the North Atlantic, which in turn determines the temperature of Greenland and how much snow falls there.

The results show that the position and intensity of the jet stream naturally vary a lot. 鈥淚t seems to be fairly random,鈥� says Osman. Because there is so much variability, global warming hasn鈥檛 yet had a discernible effect. The situation is like being on a beach when the tide turns 鈥� if the waves are big, it takes time before the rising tide is obvious.

But climate models forecast that the jet stream will shift north as the world warms and the temperature gradient between the Arctic and lower latitudes weakens. Osman鈥檚 findings suggest that this effect will start to become obvious from around 2060. By 2100, the average position of the jet stream could be 1 to 3 degrees further north in high-emissions scenarios.

This would have a dramatic effect on Europe, with southern regions becoming even drier, and even more rain or snow falling over already wet parts of Scandinavia. In moderate emissions scenarios, the northward shift is halved.

The effects on the UK are harder to predict as they depend on the intensity rather than the position of the jet stream, says Osman.

The team鈥檚 reconstruction suggests that even natural variations in the jet stream can have major impacts, and could be the cause of some famines in Europe. There was an extreme northward shift in the jet stream in 1374 and 1375, for instance, a time when there was drought and famine around the Mediterranean.

Jet stream intensity was unusually low in 1740, when nearly half a million people in Ireland died in a famine after little rain fell for a year. And from the 9th to the 11th century, jet stream intensity was just 60 per cent of the average, leading to warmer temperatures in Greenland. It was during this time that the island was colonised by the Vikings.

Some researchers think that global warming is already making the jet stream wavier, with large loops that persist for days or weeks leading to unusual extremes of heat and cold, such as the cold snap in Texas earlier this year. Osman鈥檚 study looked at annual averages and so isn鈥檛 directly comparable to research examining much shorter periods, he says.

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