If the world is getting warmer, why is it so cold outside? That’s a question asked by climate change deniers who take the sub-zero temperatures, piling snow, and record low temperatures recorded this winter as a sign that this whole “global warming” thing is just a bunch of hot air.

But Rutgers climate scientist Jennifer Francis has another explanation for extended spells of cold weather: climate change. That’s right: global warming is making the American Northeast cooler. It’s also responsible for the extended dry spell in the West, according to a hypothesis that she proposed several years ago, and provided new evidence for in a paper published last month with her collaborator, Stephen Vavrus of the University of Wisconsin-Madison, in the journal IOPscience.

Francis’s hypothesis is that the well-documented increase in temperatures around the North Pole, and the subsequent melting of sea ice, is causing changes in the atmosphere that make the jet stream unstable.

“We know the Arctic is warming faster than everywhere else on the planet,” Francis says. “The temperature difference between the Arctic, which is cold, and areas farther south is one of the main forces that creates the jet stream. The jet stream is this very fast-moving river of air high up where jets fly that is responsible for the weather experience on the surface. It is a critical part of our existence in terms of the weather. Because the arctic is warming so fast, the temperature difference is getting smaller, so the jet stream is getting weaker.”

And how does Francis know the jet stream is getting weaker?

“We’ve got data going back to the late 1940s of the entire atmosphere at all levels,” she says. “When we measure the speed of winds of the jet stream, we see a decrease, especially in the fall, which is when this rapidly warming arctic is most pronounced. Meteorologists have observed for a long time that when the jet stream is weaker, it tends to meander more north and south.”

Normally, the jet stream keeps the arctic winds swirling, confined in the polar regions. But when the stream dips south, it can cause that polar air to enter such inconvenient places as New Jersey where it is not welcome.

“There is now a big southward dip over the northeast, and it has been locked in place for a long time. You can argue that it’s basically been in this pattern since December, 2013, more or less. It keeps coming back to this same general pattern. If you look at a map of departures from the norm, in 2014, the West was warm and the East was colder than normal.”

The jet stream slipped north in the West, causing an extended drought (and less snow than usual — see separate story, page 29).

As Francis noted, the connection between the “waviness” of the jet stream and cold weather was well known before she came along. But what Francis has added to the research is to establish the connection between the behavior of the jet stream and climate change, and also to link weak jet stream winds to its “waviness.” Francis believes that since the 1990s, the jet stream has been less stable, and that because of this, there have been more extreme weather events that have been more persistent. She believes the weather observed in 2014 and this year are consistent with the predictions she made years ago.

“This particular winter, it’s been particularly noticeable, because it’s caused dramatic weather impacts such as a rainless winter out in California, which is a huge problem, as is this cold, snowy situation we’ve had over East. This has been an incredibly persistent pattern.”

Meteorologists have dubbed the part of the jet stream that hems in dry air over the West the “Ridiculously Ridiculous Ridge,” and Francis has named its Eastern counterpart the “Terribly Tenacious Trough.” (A term that does not appear in her research paper.)

Francis’s paper blames this effect on a phenomenon known as Arctic Amplification. Calling the 1990s onward the “AA era,” Francis links the warmer polar region to the weakening of the jet stream and hence, the incursion of polar air into the Northeast. You may have seen the arctic air called the “polar vortex” on newscasts. Francis’s theory is based on the observation that the effects of climate change are most extreme at the north and south poles of the planet.

Francis doesn’t claim her theory is ironclad, or that her hypothesis is fully proven. Her paper was based on observation of climate conditions, and has yet to show up in computer climate models. Furthermore, with so many things changing around the globe — sea ice coverage, water temperatures, ground temperatures all over the world — it can be difficult to figure out exactly what is causing any particular weather effect.

“The climate system is a very complicated beast,” Francis says. “It’s difficult to pin this pattern on any particular cause. But this wavy jet stream is exactly what we would expect. It’s consistent with my predictions.”

Francis writes in her paper: “These results reinforce the hypothesis that a rapidly warming Arctic promotes amplified jet-stream trajectories, which are known to favor persistent weather patterns and a higher likelihood of extreme weather events. Based on these results, we conclude that further strengthening and expansion of AA in all seasons, as a result of unabated increases in greenhouse gas emissions, will contribute to an increasingly wavy character in the upper-level winds, and consequently, an increase in extreme weather events that arise from prolonged atmospheric conditions.”

In her paper, Francis calls for future research analyzing climate model predictions of stronger Arctic Amplification. Other climate scientists have reacted to Francis’s work, with the response falling broadly into two camps: some agreeing with it, and others remaining skeptical.

“There has been some pushback to the idea. I think we’ll find out over the next five years whether it is true or not. I always come back to the fact that we’re seeing that in the Arctic, 60 percent of the sea ice has been lost — only 40 percent remains. As we continue burning fossil fuels, we are going to see truly monumental changes happening in the Arctic at an incredible pace. There’s just no way that there won’t be some response in the large-scale circulation of the atmosphere, and of weather patterns. The question is how, and how much.”

Francis welcomes the debate. “It’s very complicated, and there’s still a lot we don’t understand,” she says. “As long as it’s done in a constructive way, I’m happy to engage in conversations.”

Francis is one of thousands of scientists studying the effects of global warming, but she is a standout for her work linking the jet stream fluctuations to climate change. It’s possible that her unique background gave her the tools to make that logical connection where other researchers might have overlooked it.

Francis grew up in southern Massachusetts where, she says, everybody is interested in the weather, especially her, her dentist father, and her teacher mother. “Whenever something was going on, we would jump in the car and go see the waves breaking on the beach and go down to the harbor and watch boats break loose from their moorings.”

Francis went to the University of New Hampshire in 1975, graduating in 1978 with a degree in zoology in preparation for a career following in her father’s footsteps as a dentist. But her longstanding interest in the weather drew her back to college in 1985 to study meteorology at the University of San Jose.

In between stints at college, her life depended on her ability to understand the weather. “My husband and I sailed around the world in the 1980s,” she recalls. “Some of the weather we experienced during that time was pretty amazing. We went around Cape Horn, and the swings in weather down there were like nothing that happens here. It was incredibly wild.”

Frances went to San Jose with the intention of becoming a forecaster after graduation. But along the way, she did an internship with a researcher at the NASA Ames Research Center where she was able to study Arctic weather, and she was gripped by a fascination with research. “Because of the sailing that we did, we realized that the weather information about the Arctic we could get our hands on was close to useless. I thought, ‘Well, here’s something that could use another hand. We could do a better job forecasting at high latitudes.’” Working with researcher Thomas Ackerman gave Francis a love of research that she never let go of.

Early on in her Arctic research, she was alarmed by the effects of man-made climate change and sought to understand it, and connected it with her understanding of meteorology. “I realized, OK, we are warming the Arctic really fast. The ice is changing really fast. This has got to have an effect on the jet stream.”

She went on to earn a doctorate in atmospheric sciences at the University of Washington in Seattle. She is now a research professor at the Institute of Marine and Coastal Sciences at Rutgers.

Francis is not one to sit in an ivory tower surrounded only by other academics. Instead, she gives numerous interviews to news outlets, and has published an open-source paper about her hypothesis about the jet stream (see page 30) for anyone to see. She has even engaged with outlandish Internet commenters who spin conspiracy fantasies on her articles blaming climate change on sinister jet contrails called “chemtrails”

Francis is more than willing to jump into the trenches to debunk chemtrail theories:

“There is absolutely no evidence or peer-reviewed studies that suggest that chem-trails are real, at least in the sense that they are somehow the result of some plot to poison people by injecting toxins into the atmosphere. Contrails, however, are real, and are nothing more than exhaust from aircraft. When conditions are right in upper levels of the atmosphere, those exhaust plumes (mostly water vapor and carbon dioxide) create clouds that can be long-lived. Having spent a lot of time in the middle of oceans, I have seen long-lived contrails thousands of miles from land. If someone wanted to spend the money to poison people this way, why would they do it in the middle of the ocean?” she wrote in one Internet post.

Francis believes engaging with the public — even conspiracy theorists — is important.

“I think it’s hugely important to get this message to the public because it’s saying that climate change is affecting us now, and it’s not this gradual, slow warming of the Earth on average. That’s the important thing. The fact that we are seeing the impacts of what we do to the climate system, we being humans, because of greenhouse gases, owing to fossil fuel burning, we are seeing huge impacts on the economy and to people’s property. All kinds of things are already happening. It’s not something that our grandchildren are going to worry about. We are already worrying about it.

“We are seeing more extreme weather events and we think that’s partially linked to the loss of Arctic sea ice. One of the ‘nice’ things about Arctic sea ice loss is that it’s such a huge signal that a first-grader can see that it’s happening. It’s a real visual tool that you can use to connect with the public and explain why and how the climate system is changing right before our very eyes.”

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