The growing debate about the Arctic’s influence on hurricanes

In the devastating wake of back-to-back hurricanes Harvey and Irma, climate scientists are discussing the link between climate change and extreme storms with renewed vigor. And on some points, they widely agree – for instance, many researchers believe a warmer atmosphere will produce hurricanes with stronger rainfall.

But other theories about climate change and extreme weather are far more controversial. And now, recent events have once again raised one of the biggest debates in climate science – that is, whether the rapidly warming Arctic may be influencing the tracks of hurricanes and other weather patterns around the world.

Over the last few years, some climate scientists have begun to suggest that warming in the Arctic could affect the flow of large-scale atmospheric currents – notably the jet stream, a huge, wavy air current that flows from west to east around the world and has a major influence on global weather patterns.

Some scientists have suggested this theory could help explain the remarkable trajectory of Hurricane Sandy, which crashed into the New Jersey coast in 2012. And now, they suggest that the unusual behavior of Hurricane Harvey – which stalled over Texas, dumping record amounts of rain – could be related to Arctic-driven changes in the jet stream as well.

In a much-cited Facebook post, published in the aftermath of Harvey’s landfall, climate scientist Michael Mann of Pennsylvania State University noted that Harvey’s devastating stalling behavior was made possible by weak prevailing winds, which were associated with “a greatly expanded subtropical high pressure system over much of the U.S. right now, with the jet stream pushed well to the north.”

And he added that, while it’s still a “tenuous” idea, the phenomenon of weather patterns becoming locked in place for unusually long periods of time “appears to be favored by human-caused climate change.”

In a recent statement, climatologist Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, made a similar observation, noting that the effects of Arctic warming can slow atmospheric circulation in the mid-latitudes during the summer, allowing weather systems to “move less and stay longer in a given location – which can significantly enhance the impacts of rainfall extremes, just like we’re sadly witnessing in Houston.”

This is not to say that Harvey’s behavior was caused by Arctic warming, alone – it’s notoriously difficult to pin individual weather events on any single factor, and Mann cautioned in a follow-up email to The Post that we can’t say Arctic warming was responsible for Harvey. But some scientists do suggest that this type of event, in general, may be more likely to occur as a result of changes in the Arctic.

It’s a theory that has been drawing greater attention among climate scientists for the last five years, but some experts believe there’s still not enough evidence for it to be credible.

“Is this high-pressure system, this blocking pattern, is that becoming more likely as the climate warms? I’d say the jury is out on that,” said Tapio Schneider, a climate scientist at California Institute of Technology, referring specifically to the conditions in Harvey. “The theories that have been proposed are not convincing in themselves. Climate modeling results are ambiguous. I would say that it’s very unclear at present.”

The whole idea of the jet stream becoming wavier or more meandering thanks to Arctic warming, he added, is a theory with “no dynamical basis.”

The jet stream theory

The idea, largely the brainchild of Rutgers University climate scientist Jennifer Francis, revolves around the atmospheric differences between higher latitudes and lower latitudes. Closer to the equator, air tends to be warmer – and because warm air takes up more space than cold air, the atmosphere is thicker there than it is closer to the Arctic. The result is a kind of gradient, or atmospheric slope, over which air flows.

This phenomenon plays a major role in the typical behavior of the jet stream. Although the current generally flows from west to east, the air is constantly rolling over this atmospheric gradient on its way, creating a kind of meandering, up-and-down pattern as it flows along.

Francis, along with other scientists, has suggested that because the Arctic is warming at a faster rate than the rest of the Earth – a fact that’s well-documented by climate scientists – this gradient may be growing gradually less steep, potentially weakening the jet stream’s flow.

“The idea in general is that when that north-south temperature difference gets smaller, we expect to see weaker west-east winds because that temperature difference is what creates that sort of ‘hill’ in the atmosphere,” Francis told The Washington Post.

Because the jet stream drives so many other atmospheric processes, including wind flow throughout the North Atlantic, some recent research has begun to suggest that this weakening may be increasing the frequency of certain atmospheric phenomena, such as “blocking events” – high-pressure systems that sit in one place for long periods of time, rather than moving on – or otherwise changing wind patterns, which can significantly affect the behavior of storms.

The effect on hurricanes?

Francis and other scientists have suggested that this amplified warming in the Arctic could affect all kinds of different weather patterns, from droughts to floods. But the event that first led Francis to begin raising the idea was another devastating storm – Hurricane Sandy, which ravaged the Northeast in 2012.

Sandy, too, was a storm that behaved in an unusual way. Instead of curving back out to sea as it rolled up the coast – the typical trajectory for most Atlantic hurricanes – it veered sharply to the west as it approached New Jersey, trapped by a high-pressure blocking event in the Labrador Sea. Shortly after veering inland, the storm collided with a developing extratropical cyclone, or “nor’easter,” which caused it to morph into a full-blown hybrid superstorm.

In a subsequent paper, Francis, along with colleagues Charles Greene and Bruce Monger of Cornell University, suggested that climate change-driven losses in Arctic sea ice may “stack the deck” in favor of a shifting jet stream, unusual movements of Arctic air through the mid-latitudes and more frequent blocking events, like the one that forced Sandy’s westward track. In other words, they suggested that the combination of events that led to Sandy’s devastating trajectory and strength could occur by chance, alone – albeit a very small chance – but may be made more likely by climate change.

And according to Francis, the pattern of weak winds that stalled Harvey “is very consistent with some new work that’s come out in the last couple years suggesting that in the summertime, over North America in particular, the jet stream is tending to be farther north and so kind of leaving the Gulf of Mexico region high and dry.”

In an email to The Washington Post, Penn State’s Mann cautioned that we can’t say that Hurricane Harvey’s stalling was caused by these processes, but added that Arctic warming constitutes “one of several potential mechanisms that may be behind the increase we’ve seen in persistent and extreme weather events.”

And while Francis agreed that we can’t pin Harvey’s individual behavior definitively on Arctic warming, she added, “What you can say is we do expect to see these kinds of bizarre behaviors happen more often in the future, whether it’s a very slow-moving storm like Harvey, or whether it’s a westward-going track like Sandy.”

The controversy

These ideas have met with a very skeptical reception from some scientists. Although some research indicates a relationship between Arctic warming and global weather patterns, multiple other studies have suggested the relationship is weak or nonexistent.

Elizabeth Barnes, an atmospheric scientist at Colorado State University, has published several papers challenging the theory in the last few years, including one 2014 paper concluding that “the link between recent Arctic warming and increased Northern Hemisphere blocking is currently not supported by observations.”

Another 2014 paper, published by a separate group of researchers also suggested that the theory just doesn’t have sufficient evidence behind it.

One of the authors of that paper, Kevin Trenberth of the National Center for Atmospheric Research, told The Washington Post by email that he is “highly skeptical about this idea.” In the Arctic, he said, “there is nothing that adds up to produce much in the way of persistent heating of the atmosphere to cause the kinds of changes observed.”

As for hurricanes, he added, “There is no model that simulates the changes that have been postulated by a few individuals. There is no physical process or mechanism that can do it either.”

But Francis suggests that different research methodologies – or natural climate variations, such as fluctuations in ocean temperatures or volcanic eruptions – may account for some of the inconsistencies between studies.

Judah Cohen, the director of seasonal forecasting at analytics group Atmospheric and Environmental Research, agreed that differences among the methods may account for some of the inconsistencies.

Observational studies suggest that there are, indeed, atmospheric changes occurring that may be influenced by the warming Arctic, he said, and he suggested that many of the models used to further investigate still “have some serious flaws.”

For the time being, the science clearly remains unsettled. The controversy stems mainly from the fact that evidence is still coming out on both sides of the issue, and “scientists sort of fall out with one another over what level of confidence they’re willing to go with” when it comes to emerging theories, said Charles Greene, an atmospheric scientist at Cornell University, who has collaborated with Francis in the past.

“What I’m trying to do, and what several people have been doing just in the last couple of days, is saying, ‘Hey we should look at this blocking pattern situation and see if that’s associated with Arctic amplification,” Greene said. “The intensification of this storm and the additional rainfall, I think a majority of climate scientists would agree that the warming that’s occurred has likely made those things more intense for Hurricane Harvey. This other suggestion is much more tenuous at this time, but it’s worth raising.”