As the Arctic warms under the influence of climate change, data suggest the jet stream (red) tends to become more wavy. This causes weather patterns to persist longer than they otherwise would, causing prolonged hot and cold spells, as well as wet and dry spells.
Scientists call it Santa's revenge. It's the theory that persistent weather patterns at the mid-latitudes – like this winter's tediously long-lasting polar vortex or California's severe drought – are a direct consequence of climate change heating up the Arctic.
New evidence suggests the link is real, even as experts continue to argue over how much it is influencing the weather on a day to day basis. The effect has implications for severe weather predictions, food security and water use across the northern hemisphere. If the data are right, they suggest that climate change in the Arctic is coming home to roost in a big and expensive way.
"It's starting to get harder to say that something isn't happening," said Jennifer Francis, a research professor at Rutgers University who presented her findings Saturday at the annual meeting of the American Association for the Advancement of Science in Chicago.
Prof. Francis based her remark on a reanalysis of data used in weather forecasting over the past 35 years. The study supports the idea that Arctic warming is changing the jet stream, a high-altitude wind that rings the globe, and is a key factor influencing the weather experienced by much of North America and Eurasia.
The cause of the jet stream is well understood. Because warm air is less dense than cold air, warm air in the south has a tendency to rise and spill northward, onto the colder air that sits above the Arctic. The Earth's rotation bends that northward flow into an west-to-east direction. The greater the temperature difference, the faster the air flows and the straighter the jet stream's ribbon-like path across the northern hemisphere.
But as increased levels of carbon dioxide warm the planet, the loss of Arctic sea ice together with other factors mean that the Far North is warming much faster than areas that are farther south. This lessens the temperature difference and weakens the jet stream, causing it to meander like a slow moving river and follow a more convoluted path. Weather patterns near the surface get caught up among those convolutions, and so tend to linger in one place for longer periods of time. In theory, that means areas where the weather has become especially hot, cold, wet or dry tend to stay that way much longer than they otherwise would without the effect of climate change. This increases the chances of both flooding and drought or other extremes, like prolonged sequences of severe storms.
"What we're seeing is that the size of those waves does appear to be getting larger," Prof. Francis said. While it remains difficult to connect any single weather event to climate change, events like the extended cold spell in the east this winter and severe weather in the U.K. "are very consistent with the kind of pattern we expect to see related to the Arctic warming so fast."
Other studies have argued that it is not yet clear whether there is such a direct connection. In a letter published in the journal Science on Friday, a group of senior climate scientists wrote that the winter's bitter cold, which broke records from Chicago to New York, could be chalked up to coincidence, rather than sea ice loss. Prof. Francis said the letter misinterprets what she and her colleagues are saying.
"We're just talking about the persistence of the patterns, not more record-breaking cold," she said.
At the Chicago meeting, Jerry Hatfield, a laboratory director at the U.S. National Laboratory for Agriculture and the Environment in Ames, Iowa, said a changing jet stream has the potential to drastically affect international food supplies and prices.
"If you look around the world, we produce the bulk of our crops at that mid-latitude area," he said, adding that livestock productivity is also sensitive to prolonged heat.
David Titley, a retired rear admiral with the U.S. Navy and director of the Center for Solutions to Weather and Climate Risk, said that policy makers need to reframe their discussions about climate change so that people understand its effects are more immediate and local than is widely perceived.
"It's about people, not polar bears," he said. "The real question is whether we can adapt and change faster than the climate."
Ivan Semeniuk