Without some historical context, it’s easy to over-interpret an unusual weather event, especially when it’s fresh in your mind. At this time of year in the US, that means cold snaps or unseasonably warm weather—and the storms that accompany them. Are they tied in with our changing climate?
There’s a legitimately controversial proposal that they are. The idea that warming in the Arctic (and shrinking sea ice coverage) has been making northern mid-latitude winters “weirder” has drawn a lot of attention in recent years. But does it explain the weather you complained about last week?
The idea suggests that the weirdness is driven by the fact that the Arctic is warming faster than any other region, which slightly decreases the temperature difference from equator to pole. A number of researchers think this can cause the jet stream (which separates frigid polar air from warmer midlatitude air) to get more wiggly—allowing cold air to spill southward more frequently. On the opposite side of those wiggles, warm air will get pulled north to normally frigid regions.
Many researchers remain unconvinced that this is a climate effect, as this behavior is naturally quite variable and could plausibly be influenced by other factors. Jennifer Francis—a leading proponent of the Arctic hypothesis—joined with Judah Cohen and Karl Pfeiffer to publish a new study focused on US winters. The work carefully acknowledges disagreements and unknowns, but it finds some interesting weather patterns over the last few decades.
The researchers work with some complex (but particularly useful) measures of weather conditions. For US weather, they use an index that tallies up cold snaps and snowstorms; it acts a bit like the “heating degree days” you may see on your energy bill. For the Arctic, they calculated average atmospheric pressure and temperature conditions north of 65° N latitude.
For 12 cities from Massachusetts to Washington—going back to 1950—the researchers compared the winter weather index to Arctic conditions. For the western US (which has often been the weather yin to the Northeast’s yang), there wasn’t much of a correlation. But a strong link was apparent in the eastern US. Higher Arctic upper air pressures—and warmer temperatures—have been followed by harsh winter weather in the eastern US a few days later.
(It’s important to note that snow storms aren’t indicative of especially cold temperatures, but rather the presence of water vapor and freezing temperatures.)
Applying a quick-and-dirty version of the winter weather index to the rest of the Northern Hemisphere shows that the linkage is similar from northern Europe across to northern Asia.
This connection may not seem like a shock, but one reason it’s interesting is that both the Arctic air pressure and temperature measures show an increase over the last few decades—especially in late winter. At the same time, places like the US Northeast have actually seen more severe winter weather. And that’s what you would expect if Arctic warming makes mid-latitude winters weirder.
The simplest expectation—and the one generally shown by model simulations—is that winter weather should become milder as the globe warms. But that’s not what’s happening in some places. These counterintuitive winter patterns could simply be the result of natural variability due to things like Pacific Ocean circulation, making it difficult to draw firm conclusions about the Arctic’s role.
For example, the researchers attempted to repeat their analysis for two time periods: 1950-1989 and 1990-2016. After 1990, the faster rate of warming in the Arctic became clear. So if the connection between Arctic conditions and mid-latitude winter weather was stronger after 1990, that would lend support to the idea that Arctic warming is in charge.
However, the correlation between the two is actually strongest before 1990. That leaves the researchers with some cautious conclusions. While there is a real link worth digging into, this type of study can’t say much about whether the Arctic is actually the cause of the increase in harsh eastern US winters—the authors describe their results as “only suggestive.” The proposed atmospheric connection makes sense, but that’s not enough.
If you’ve felt like the eastern US has seen some unusually harsh winter weather lately, you’re not wrong. But whether that’s due to human-caused warming in the Arctic or a separate factor isn’t quite clear just yet.
Nature Communications, 2018. DOI: 10.1038/s41467-018-02992-9 (About DOIs).
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