Superstorm Sandy's left hook into the Jersey Shore. Flooding in Europe. Heat waves in Russia. In the past few years, extreme weather events like these have captured public attention as potential examples of a brave new world of weather, brought to you courtesy of climate change.
Last year, Rutgers climate scientist Jennifer Francis gained scientific -- and media -- attention for proposing an explanation for how climate change is leading to more events like these.
The basic idea goes like this: Rapid warming in the Arctic is changing the way the jet stream behaves. Since the jet stream is a driving force behind weather over the temperate part of the Earth, also known as the mid-latitudes, many people would be affected by this switch.
"You can't warm the Arctic two to three times faster than the rest of the Northern Hemisphere and not have an impact on the large-scale circulation," Francis said.
A warmer Arctic reduces the difference in temperature between the poles and the mid-latitudes. This decrease in temperature difference, Francis posits, slows the jet stream, which results in bigger, or wavier, atmospheric waves.
The impact of a slower, wavier jet stream can lead to weather getting stuck in place, resulting in extended heat waves or cold snaps, or a weather pattern like the block of high pressure in the Atlantic Ocean that pushed Sandy into New Jersey.
Francis' hypothesis is appealing, especially for those wanting to show the public what climate change means for them.
"We are seeing all of this very unusual weather, a lot of extreme events," she said. "I think people around the Northern Hemisphere are going, 'What the heck is going on?'"
Francis has given those people an answer. And in the past year, this explanation has become popular in media stories about climate change and extreme events.
Conflicting views on a 'wavy' jet stream
Yet the science behind the link between a warmer Arctic and weirder weather is far from settled. A recent paper in the journal Geophysical Research Letters, by Colorado State University atmospheric scientist Elizabeth Barnes, analyzed trends in wave amplitude over North America and the north Atlantic Ocean.
Barnes did not find that waves were getting wavier, as Francis has suggested. She also failed to find strong evidence for a slowdown in the speed of such waves.
Another paper by Barnes and researchers at Columbia University, published Monday in the journal Proceedings of the National Academy of Sciences, uses models to show that the atmospheric conditions that led to Superstorm Sandy's turn into the Jersey Shore are actually less likely as the climate changes, not more.
Other recent work, by climate scientist James Screen at the University of Exeter, also questions the idea that the jet stream is getting wavier because of a warming Arctic.
A 2013 analysis he published in the journal Geophysical Research Letters found some statistically significant trends in wave height in some seasons in some places: far from a ringing endorsement for the role of a warmer Arctic in the severe weather we've seen of late.
Screen pointed out that one of the reasons for results contradicting Francis' hypothesis is that there are many ways to test it.
"In my paper and also in the Barnes paper, I think one of the messages that comes out of both of those is that it depends exactly how you are defining this waviness and exactly what metrics you use to measure that," Screen said.
Screen has also used climate models to simulate the strong warming in the Arctic we have seen thus far, in an effort to tease out effects of that warming on changes in mid-latitude weather. He hasn't found much.
The researcher also said that since there are so many things influencing weather in the mid-latitudes, it's hard to pin down the Arctic amid them all.
"Based on these model runs, my take is the loss of sea ice so far hasn't been large enough to have a really big effect in the mid-latitudes over all these other things that influence weather [in that part of the world]," Screen said.
Polar changes may outpace analysis
But as Francis and others point out, the Arctic has only been this warm for a short time. Perhaps the failure to find statistically significant changes is simply due to that.
"It is only in the last 15 years or so that we have been able to see this really starting to kick in," Francis said. "And that's part of the reason that when you do trend analysis, it's hard to detect a trend."
National Oceanic and Atmospheric Administration scientist James Overland, who has long studied the Arctic, said he viewed the work by Barnes and Screen as too "conservative."
"Scientists are trained to be careful in their analysis and to prove results beyond a doubt. Their work is a contribution to the science. But in the real world of climate change we do not have this full luxury," Overland wrote in an email.
As Overland, Francis and many others in the community point out, the changes the Arctic is experiencing are fast, real and profoundly unsettling, especially for those who have studied the region for decades.
Arctic sea ice extent continues to plunge below average each summer. Measurements have shown that the winds that blow west to east and drive the jet stream are weakening because of the region's warming. By 2035, some scientists say, the Arctic could be ice-free in the summer.
All this, say Francis and others, has to have an effect on the jet stream and the weather. Even in the work of Barnes and Screen, whose research contradicts that of Francis, the Rutgers scientist sees support for her hypothesis. She says although they do not find statistically significant changes in waviness, the trends they document go in the right direction.
"As time goes on, I think those statistics will start to become more robust," Francis said.
As a pioneer of a compelling new explanation for how a warming Arctic could be changing how the atmosphere works, Francis will undoubtedly see more challenges to, and work in support of, her hypothesis.
She also frankly acknowledges that the story is not yet clear and the science is still emerging.
"I'm also the first to recognize there is a lot more going on than just the Arctic warming," Francis said.
Mounting interest in the linkage
This September, the National Academy of Sciences is sponsoring a workshop with around 50 scientists to discuss the topic of a warming Arctic and its affects on weather. Since the field is so young, one of the prerogatives is setting a research agenda, said Cecilia Bitz, a polar climate scientist at the University of Washington who is on the workshop's organizing committee.
Bitz said the idea of a linkage between a rapidly warming Arctic and changes in mid-latitude weather is intriguing and plausible, but her mind is still open about whether it is true.
"The back-and-forth you see in the literature doesn't surprise me, because the data sets are short and there are different ways of defining weather," she said.
Bitz believes more modeling studies could help sort out the issue. Both she and Exeter's Screen suggested that perhaps one of the reasons researchers are getting contradictory results is that the effect of a warmer Arctic on mid-latitude weather, even if it exists, just might not be big enough to measure.
"If there is a signal, it's quite small; it's quite weak; it's quite hard to identify with the noise in the climate system," Screen said.