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This winter’s weather has been weird across much of the Northern Hemisphere. Record storms in Europe; record drought in California; record heat in parts of the Arctic, including Alaska and parts of Scandinavia; but record freezes too, as polar air blew south over Canada and the U.S., causing near-record ice cover on the Great Lakes, sending the mercury as low as minus 50 degrees Celsius in Minnesota, and bringing sharp chills to Texas.

Everyone is blaming the jet stream, which drives most weather in mid-latitudes. That would be a significant development. For what happens to the jet stream in the coming decades looks likely to be the key link between the abstractions of climate change and real weather we all experience. So, is our recent strange weather a sign of things to come? Are we, as British opposition leader Ed Milliband put it this month while surveying a flooded nation, “sleepwalking to a climate crisis”?

The story gets tangled because trying to identify long-term trends amid the noise of daily weather is hard.

The U.K. Met Office, which keeps a global weather watch, said in a rush report put out in mid-February that we are experiencing a “hemispheric pattern of severe weather,” and that the events are linked. The most extreme days of the U.S. cold event, for instance, coincided with some of the most intense storms over the U.K. And physically the connection is through the polar jet stream, which the report said showed a “persistent pattern of perturbations” – in other words, it ran wild.

The polar jet stream is a narrow stream of fast wind circling the globe from west to east at the top of the troposphere from 7 to 12 kilometers up, and usually between 50 and 70 degrees north. It forms where cold, dense air from the Arctic meets warmer and less dense air from mid-latitudes. At the boundary, winds rush in to equalize the pressure difference. The earth’s rotation diverts these winds to travel eastward.

As the jet roars around the world, it drags weather systems with it. Most of Europe’s weather rides in under the jet stream from the Atlantic, and most of the western U.S.’s weather comes from the Pacific in a similar manner.