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.