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Blaming a wiggly jet stream on climate change? Not so fast – Ars Technica


The weather map on February 15.
Enlarge / The weather map on February 15.

Some songs are earworms—catchy whether you like them or not. (I won’t infect the rest of your day with an example.) Some explanations in science seem to be the earworm equivalent: inherently intuitive, making them stick readily in the mind. That’s obviously the case for the hypothesis that a warming Arctic leads to a wigglier jet stream, producing weather extremes in the mid-latitudes like the recent epic cold snap in the central US.

The cold arrived after the spinning “polar vortex” in the upper atmosphere above the Arctic was disturbed in January, unleashing its contents southward as the jet stream detoured from its usual commute. Could this behavior actually be a consequence of global warming? The suggestion has appeared in news articles and Twitter threads across the land. But the idea is stickier than the science says it should be.

Jet setting

Although the specifics vary, the general idea is based on the fact that the Arctic is warming faster than the mid-latitudes. As a result, the temperature difference between them is getting a little smaller. The jet stream forms at the boundary between the Arctic and mid-latitude air, so a smaller temperature difference would weaken the jet stream. And a weaker jet stream is more prone to great, wiggling meanders that can bring you cold air from the north or warm air from the south.

Weather data from the last few decades contains some trends in the mid-latitudes, implying that the warming Arctic could be messing with weather patterns there. However, this is a case where the mantra “correlation is not causation” serves well. Climate scientists don’t just hunt for trends and then blame them all on human-caused climate change. They study the mechanisms that could drive those trends to evaluate which hypothesis (sometimes among many) can actually explain them.

Some modeling has found plausible linkages between certain patterns in our weather and things like Arctic warming, sea ice loss, and even snow-cover decline. But more commonly—as detailed by Carbon Brief after another US winter chill in 2019—models fail to demonstrate a linkage. It’s certainly possible that the models aren’t getting it right, but this should at least give us pause regarding any connections.

A 2017 study, for one example, concluded that trends in the stratospheric polar vortex were likely to be the result of natural variability rather than human-caused sea ice loss. And an article published in the journal Nature Climate Change last November noted that observations and studies in the last few years haven’t strengthened the case. “The short-term tendencies from the late 1980s through to early 2010s that fueled the initial speculation of Arctic influence have not continued over the past decade,” the authors wrote. “Long-term trends in the Arctic Oscillation and [jet stream] waviness, updated to winter 2019/20, are small and indistinguishable from internal variability.”

IPCC reports, too, have evaluated the state of the science on this question. The 2019 Special Report on the Ocean and Cryosphere in a Changing Climate checked in quite recently. “There is only low to medium confidence in the current nature of Arctic/mid-latitude weather linkages because conclusions of recent analyses are inconsistent,” the report stated. “Overall, changes in the stratospheric polar vortex and [Arctic Oscillation] are not separable from natural variability, and so cannot be attributed to greenhouse gas forced sea ice loss.”

Not conclusive

That said, some climate scientists do find the evidence that climate change helped cause the event convincing, even as others don’t. And this isn’t a fake debate involving a few contrarian gadflies, like those who reject human-caused warming altogether. There truly isn’t a consensus either way because the research doesn’t dictate a clear answer yet. That means that confident statements linking jet stream weirdness to a human-caused trend make many climate scientists grumpy.

So what can be said about the chill that just stretched down through the central US to Texas? While rare and extreme, it was not unprecedented. And as the Arctic gets warmer, the air that sometimes spills southward won’t be quite as frigid as it used to be. But science does not yet have a clear answer as to whether the mechanisms that control that spilling are changing with global warming. Instead, multiple competing answers are being tested.

Apart from assigning blame, this is significant for defining what weather extremes we can expect in the future. For now, climate science can’t tell you whether opening a winter coat business in Texas is a smart long-term investment—even if the wiggly jet stream hypothesis seems to make sense. Climate science can, however, confirm that winter still exists. It’s always wise to prepare for weather extremes that can trigger cascading failures of the systems that are meant to keep us safe.

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