Some of the songs are earwigs – catchy whether you like them or not. (I won’t spoil the rest of your day with an example.) Some explanations in science seem to be the equivalent of earwigs: inherently intuitive, making them easy to get stuck in the mind. That is clearly the case for the hypothesis that a warming Arctic leads to a wobbly jet stream, causing extreme weather events in the mid-latitudes, such as the recent epic cold snap in the central US.
The cold arrived after the spinning “polar vortex” in the upper atmosphere over the Arctic was disrupted in January, releasing its contents south as the jet stream made a detour from its usual commute. Could this behavior actually be a result of global warming? The suggestion has appeared in news articles and Twitter threads across the country. But the idea is tackier than science says it should be.
While the details differ, 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 becomes slightly smaller. The jet stream forms on the boundary between the North Pole and the mid-latitude sky, so a smaller temperature difference would weaken the jet stream. And a weaker jet stream is more prone to large, twisting meanders that can bring you cold air from the north or warm air from the south.
Weather data from the past few decades includes 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 “correlation is not causation” mantra comes in handy. Climate scientists don’t just chase trends and then blame them all for human-caused climate change. They study the mechanisms that may drive these trends to evaluate which hypothesis (sometimes among many) can actually explain them.
Some models have found plausible links between certain patterns in our weather and things like Arctic warming, sea ice loss and even snow cover decline. But more often – as described by Carbon Brief after a new winter cold in the US in 2019 – models show no connection. It is certainly possible that the models are not doing well, but this should at least give us a break with regard to any connections.
For example, a 2017 study concluded that trends in the stratospheric polar vortex were likely the result of natural variability rather than human-induced sea ice loss. And a paper published last November in the journal Nature Climate Change noted that observations and studies from recent years have not strengthened the case. “The short-term trends from the late 1980s to early 2010 that fueled initial speculation about Arctic influence have not continued over the past decade,” the authors wrote. “Long-term trends in the Arctic Oscillation and [jet stream] undulation, updated to winter 2019/20, are small and indistinguishable from internal variability.”
IPCC reports have also evaluated the state of the art on this point. The 2019 Special Report on the Ocean and Cryosphere in a Changing Climate checked in quite recent. “There is only low-to-medium confidence in the current nature of the weather relationships between the Arctic and mid-latitudes because conclusions from recent analyzes have been inconsistent,” the report said. “In general, changes in the stratospheric polar vortex and [Arctic Oscillation] are inseparable from natural variability and thus cannot be attributed to greenhouse gas-forced sea ice loss.”
That said, some climate scientists to do find evidence that climate change caused the event convincingeven if others do not† And this isn’t a fake debate involving a few contrarian hornets, like those who reject man-made warming altogether. In any case, there is no consensus, as the research does not dictate a clear answer yet. That means confident statements linking jetstream craziness to a man-made trend to make loads of climate scientists surly†
So what about the chill that just stretched through the central US into Texas? Although rare and extreme, it was not unprecedented† And as the Arctic warms, the air that sometimes flows south won’t be as frigid as it used to be. But science doesn’t yet have a clear answer as to whether the mechanisms that control that spillage change with global warming. Instead, multiple competing answers are tested.
Aside from assigning blame, this is important in determining 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 winding jetstream hypothesis makes sense. Climate science canconfirm, however, that winter still exists. It is always wise to prepare for weather extremes that can lead to cascading failures of the systems intended to keep us safe.
List image by Gillian Henry | Getty Images