Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”
Come the cold season, whenever there is some type of strong storm system near the U.S. Eastern Seaboard—be it a Nor’easter, a blizzard, or ex-hurricane Sandy—you don’t have to look very hard to find someone who will tell you that this weather is “consistent with” expectations of climate change resulting from human greenhouse gas emissions. The worse the storm, the more “consistent” it becomes.
The complete collection of climate science describes just how complex the physical processes are governing such storm systems. Teasing out any anthropogenic influence, including even the direction of any influence, is darn near impossible. Claims to the contrary are usually based on a highly selective assessment of the science or the data.
A case in point:
The latest en vogue explanation linking human greenhouse gas emissions to strong winter-season East Coast storms involves changes in the characteristics of the jet stream—a river of fast moving air in the atmosphere that influences both the strength and the forward speed of extratropical storm systems. A prominent (in the media, anyway) research study last year by Rutgers’s Jennifer Francis and University of Wisconsin’s Stephen Vavrus suggests that the declining temperature difference between the Arctic and the lower latitudes (adding greenhouse gases into the atmosphere warms colder, drier regions more so than warmer, wetter ones—with the notable exception of Antarctica) has led to changes in the jet stream which result in slower moving, and potentially stronger East Coast winter storm systems.
Just Google “Jennifer Francis global warming” to see how this mechanism is supposedly tied to all sorts of extreme weather events.
Even before the Francis and Vavrus study made it to print, we noted that their findings ran afoul of other existing literature which painted a far murkier picture of the influence (if any) that anthropogenic global warming was having in extratropical cold-season storm systems. After reviewing the literature, we cautioned:
So where does this leave us? When the new paper by Francis and Vavrus comes to the attention of the mainstream press, it’ll play as if a warming Arctic and declining sea ice—an asserted consequence of human greenhouse gas emissions—has been definitively tied-in to all sorts of weather extremes across the U.S. No mention will be made to the fact that other research, which is many cases is more robust and detailed, has concluded nearly the opposite.
Conflicting research findings continue to be published.
One was published several months ago in Geophysical Research Letters by James Screen and Ian Simmonds, who looked for changes in jet stream characteristics using a different methodology than that of Francis and Vavrus. A robust signal should be apparent no matter how you look at it (within reason). But Screen and Simmonds found few statistically robust changes and what changes they did find were contrasting depending on the methodology they used. They noted that the changes they found were much smaller (and non-significant) than the large (and significant) changes reported by Francis and Vavrus. Screen and Simmonds concluded that their findings held “different and complex possible implications for midlatitude weather, and we encourage further work to better understand these.”
In other words, the picture is far less clear than that described by Francis and Vavrus.
This point is further driven home by a another paper just accepted in Geophysical Research Letters by Colorado State University’s Elizabeth Barnes. Barnes, too, examined the relative warming in the Arctic and its possible link to extreme weather events in the Northern Hemisphere mid-latitudes. In a nutshell, she found little if any definitive relationship—again in contrast to the Francis and Vavrus results. Barnes discussed this discrepancy directly:
We conclude that the mechanism put forth by previous studies (e.g. Francis and Vavrus [FV12]; Liu et al. ), that amplified polar warming has led to the increased occurrence of slow-moving weather patterns and blocking episodes, is unsupported by the observations….A recent study by Screen and Simmonds  also provides evidence that the trends in planetary waves suggested by FV12 may be an artifact of their methodology… The Arctic is changing rapidly, and these changes will likely have profound effects on the Northern Hemisphere. This study, however, highlights that the relationship between Arctic Amplification and midlatitude weather is complex.
The more folks look the less robust the popular global-warming-is-leading-to-more-extreme-winter-storms finding by Jennifer Francis and Stephen Vavrus seems to be.
It’ll be interesting to see during this upcoming winter season how often the press—which seems intent on seeking to relate all bad weather events to anthropogenic global warming—turns to the Francis and Vavrus explanation of winter weather events, and whether or not the growing body of new and conflicting science is ever brought up.
If you don’t see it in the morning paper, you will most certainly find it here!
Barnes, E., 2013. Revisiting the evidence linking Arctic Amplification to extreme weather in the midlatitude. Geophysical Research Letters, in press, doi: 10.1002/grl.50880.
Francis, J. A. and S. J. Vavrus, 2012: Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters, 39 (6), L06 801, doi:10.1029/2012GL051000.
Screen, J. and I. Simmonds, 2013: Exploring links between Arctic amplification and midlatitude weather. Geophysical Research Letters, 40, 1–6, doi:10.1002/GRL.50174.