In 1374, famines hit what is today Spain, Italy and France. The guilty? The North Atlantic Jet Stream, a rapid river of air flowing from North America to Europe, had moved north. The jet stream carries moisture laden storm clouds, and without them southern Europe has remained dry and crops are dead.
Research published this week in the Proceedings of the National Academy of Sciences reconstructed the history of the jet stream’s trajectory in the North Atlantic from the years 700 to 2000. The results suggest that while climate change has not yet altered the location of the crucial climate system, high emissions could push Europe into a world more like 1374.
Fluctuations in the jet stream have been implicated in extreme weather conditions over the past year, from flooding in Europe this spring to the heat wave that hit the Pacific Northwest, although the role of climate in these movements is always an open question.
“The jet stream is that wavy, distorted band of wind, but the average position changes over longer time scales,” says Matthew Osman, a climatologist at the University of Arizona and lead author of the study.
The jet stream is held in place by polar air to the north and tropical wind to the south. And it pushes surface-level storms in its path, in what’s called the storm track, reshaping precipitation in huge areas. “When the jet stream or the path of storms is located further south, the already semi-arid regions of southern Europe receive a lot of humidity and mild temperatures,” says Osman. “But as the jet stream moves further north, it takes the storm track and precipitation and brings them into northern Europe.”
“It’s not a big deal when we think of daily and weekly timescales, but if we think of projections of where deserts might go in the future, the displacement of moisture from arid regions. towards humid regions is a problem. “
By sampling cores from the Greenland ice sheet, which show records of precipitation and temperature dating back hundreds of years, the researchers were able to piece together a record of storm tracks across the North Atlantic.
The researchers found a surprising amount of variability. “Observations suggested that over the past few decades the jet stream has started to migrate north,” Osman said. But the jet stream has moved so north and south over the past 1,200 years that “it seems that the jet has not emerged from what one would expect from the variation alone. natural ”.
Still, climate models suggest the stream will head north and, according to this research, exit the historic zone by 2060 in the highest emission scenarios.
This means that as dramatic as this summer’s smoke, rains and fires are, there are many more unknowns on the horizon as more climate systems move into uncharted territory.
The research has not been able to provide the same kind of historical context on the ripple of the stream, which would help unravel the role of climate change in precipitating extreme weather conditions. But he could still establish a link between certain historical catastrophes and the jet stream. “1728, 1740, those were years when the winds blew at almost half of their normal intensity,” says Osman. “We know from historical records that these were really cold years, where there was a very bad lack of precipitation.” In 1740, this miserable weather sparked a massive famine in Ireland, killing millions of people – more per capita than during the Irish Potato Famine of the 1840s and 1950s.
“It’s a good test to see if the models can realistically reproduce the conditions we see in this distant period in terms of human civilization,” says Jennifer Francis, acting deputy director of the Woodwell Climate Research Center, who has was not involved in the study. “Anything that brings the jets closer to the pole is going to leave Central Europe high and dry, which will promote drought conditions and is associated with famines.”
But she offered two notes of caution. First, she says climate models can be flawed in several key ways that end up overestimating the jet stream’s tendency to move north. “There is a lot to ask in terms of what the models project for the future latitude of the jet stream.”
Second, she noted that historical data ends in 2000, when the most noticeable effects of warming on the jet stream would have appeared in the past two decades. “That would only leave a few years, say, for the Arctic to start to warm up, and that’s just not enough to see an impact on the jet stream.”
Osman says he still thinks the climate “probably hasn’t yet reached the emergence of a new kind of climate regime.” And, he points out, that means humans can still avoid changes in the jet stream – after all, the system has only escaped the historic norm in a high emissions scenario. “It doesn’t have to be the truth. It’s just a model.