The Current Wisdom is a series of monthly articles in
which Senior Fellow Patrick J. Michaels reviews interesting items
on global warming in the scientific literature that may not have
received the media attention that they deserved, or have been
misinterpreted in the popular press.
The Current Wisdom only comments on science appearing in the refereed, peer-reviewed literature, or that has been peer-screened prior to presentation at a scientific congress.
Prior to April, 2011, issues of this Wisdom, which began in 2010, are available at our blog Cato@Liberty (www.cato-at-liberty.org/).
Greenhouse-effect theory has predicted—first written by Svante Arrhenius in 1895—that lower atmospheric warming from increasing carbon dioxide would preferentially warm high latitude (polar) regions of the Northern Hemisphere. A hundred years later, computer models still predict the same, and it is a fact that this region has warmed much more than the rest of the globe.
Back-of-the-envelope reasoning (usually not a good idea when addressing the incendiary minefield of global warming) suggests that this should produce weaker midlatitude storms. These are the garden-variety low-pressure systems (cyclones) that form as a result of bursts of kinetic energy (motion) supplied by the jet stream. The jet is a fast-moving current in the mid-and-upper troposphere that is nature’s way of dissipating the potential energy difference between the (hot) tropics and the (cold) poles. Warm the poles preferentially and this temperature contrast declines, and so does the energy for cyclones. Less energetic cyclones generally means less severe weather, like floods, tornadoes and other pestilences.
Well, maybe not.
There is a new paper about to be published in Geophysical Research Letters that claims a positive relationship between the “Arctic amplification” of global warming and extreme weather outbreaks in the Northern Hemisphere mid-latitudes. This is a significant finding if it proves to be true. However, the authors, Jennifer Francis and Steven Vavrus—despite being very enthusiastic—fail to convincingly make their case in light of other previous publications in the climate literature.
In their paper “Evidence Linking Arctic Amplification to Extreme Weather in Mid-Latitudes” Francis and Vavrus argue that the differential rates of warming between the Artic and the mid-latitudes of the Northern Hemisphere cause a slowdown in the eastward progression of weather systems driven by the jet stream, “which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.”
(This statement is reminiscent of the infamous phrase in the United Nations’ Intergovernmental Panel on Climate Change Second Assessment Report in 1995 that moved global warming from science to what the philosopher Karl Popper called “pseudo-science.” The difference is that science (example: classical physics) can be “falsified” by critical observations, and pseudo-science purports to explain everything. As the IPCC said, “Warmer temperatures will lead to a more vigorous hydrological cycle; this translates into prospects for more severe droughts and/or floods in some places and less severe droughts and/or floods in other places.” So much for falsifiability.)
The jet stream circumnavigates the North Pole, and “waves” move along it, such that its shape in not perfectly circular, but rather more meandering and ever changing. Like waves on a stretched string, short-wavelength ones (of about 400 miles from peak-to-peak) move through the jet stream faster than long waves (which can cover thousands of miles). In fact, long waves can get ”stuck,” for various and sundry reasons, with little net movement. This is a situation known as “blocking,” and results in the same type of weather occurring over a particular region for a prolonged period of time. If that’s good weather, don’t look for scientists to blame greenhouse gases (where’s the incentive for that?); but if it’s bad weather, well, global warming!
Francis and Vavrus prefer the latter approach. From their conclusions:
Can the persistent weather conditions associated with recent severe events such as the snowy winters of 2009/2010 and 2010/2011 in the eastern U.S. and Europe, the historic drought and heat-wave in Texas during summer 2011, or record-breaking rains in the northeast U.S. of summer 2011 be attributed to enhanced high-latitude warming? Particular causes are difficult to implicate, but these sorts of occurrences are consistent with the analysis and mechanism presented in this study. As the Arctic sea-ice cover continues to disappear and the snow cover melts ever earlier over vast regions of Eurasia and North America (Brown et al, 2010), it is expected that large-scale circulation patterns throughout the northern hemisphere will become increasingly influenced by Arctic Amplification. Gradual warming of the globe may not be noticed by most, but everyone — either directly or indirectly — will be affected to some degree by changes in the frequency and intensity of extreme weather events as greenhouse gases continue to accumulate in the atmosphere. Further research will elucidate the types, locations, timing, and character of the weather changes, which will provide valuable guidance to decision-makers in vulnerable regions.
Hello, Karl Popper!
Francis and Vavrus hypothesize that two processes are involved in slowing the progression of the weather; 1) a weakening of the mid-level atmospheric winds; and 2) increased amplitude of the waves moving through the flow in jet stream.
To demonstrate a weakening of the winds in the mid-levels of the atmosphere, Francis and Vavrus take the difference between a measure of integrated temperature of the lower atmosphere (called the “1000-500mb thickness”) in the latitude zones 60-80°N and 30-50°N.
The “1000-500mb thickness” is related to the distance that a weather balloon must rise in order for its onboard barometer to read one-half of an atmosphere. Thanks to the ideal gas law (pv=nRt), the volume of the atmosphere varies proportionally with temperature (p, pressure, is constant at ½ an atmosphere, R is a constant, and, in this case so is n, the number of molecules). So all that is left is v (volume, or “thickness”) being scaled by t (temperature). Izzat clear?
The difference in the thickness in these two regions is therefore a direct measure of the atmospheric temperature difference which is directly related to the strength of the mid-atmospheric wind. However, the difference between the average temperature in two rather large areas is a very gross estimate of the average wind speed.
Further, it is really not the average wind speed that drives weather systems along, but rather the wind speed in the jet stream—a much narrower and closer defined region of fast flowing air that is usually located somewhere in the general region that Francis and Vavrus are averaging over. But just because the average wind speed in the region between 60-80°N and 30-50°N may be slowing down, doesn’t mean that the winds in the region of fastest flow have declined.
In fact, my former colleague Bob Davis at the University of Virginia as worked with several of his grad students through the years to investigate changes in the Northern Hemisphere jet stream and circumpolar flow. And the general finding from that research is that the speed of the jet stream flow has been increasing over the past several decades (after a period of decreasing flow ending in the early, to mid-1970s).
For instance, from research published in the Journal of Geophysical Research (Frauenfeld and Davis, 2003), they found that:
“During the early period [1949-1969] when the vortex was expanding, the westerly flow was therefore weaker, and during the later period [1970-2000] when the vortex was contracting, the westerly flow across the Northern Hemisphere has been much stronger.”
And from more recent research published in the Quarterly Journal of the Royal Meteorological Society (Strong and Davis, 2007), they reported:
[Jet stream] core speed… increased over the midlatitudes (40-60 °N), with changes as large as 15%….
The reason that the jet stream winds have increased despite a weakening mid-latitude to polar temperature gradient is that the width of the jet stream has contracted somewhat. Davis and co-researchers have found that during cooler periods the jet stream is located further south, but that it is also wider in spatial extent (with lower wind speeds), and that during warmer periods, the jet stream is located further north and is contracted (with high wind speeds).
While the north/south migration of the jet stream is in accordance with the findings of Francis and Vavrus concerning the arctic amplification of a general warming of the hemisphere, the more detailed research of Davis et al. shows that the behavior of the jet stream flow is the opposite to that which was reported by Francis and Vavrus who used a less detailed methodology—a methodology which Strong and Davis suggest “can generate false, or mask actual, variability patterns including trends.”
So when it comes to Francis and Vavrus’s first explanation for more extreme weather events—a weakening of the mid-level atmospheric wind—it turns out that a more detailed and comprehensive analysis finds that the speed of the winds of import (those in the jet stream) are in fact, speeding up.
When it comes to Francis and Vavrus’s second mechanism—increased amplitude of the waves moving through the flow—the Davis and colleagues’ work gives mixed results. Fraunfeld and Davis (2003) reported that over North America (the primary region examined by Francis and Vavrus), that the changes to the position of the jet stream “can also be argued to represent and enhancement of the PNA pattern.”
The PNA, or the Pacific-North America pattern, is a typically winter atmospheric flow pattern that is characterized by the jet stream flowing northeastward towards Alaska and then southeastward to the eastern U.S. Such a pattern can lead to frequent cold outbreaks and snowstorms in the eastern portions of the country and protracted pleasant and mild conditions in the western half of the country. If the PNA pattern is enhanced, it would tend to exacerbate these conditions.
However, in Strong and Davis (2007), not only was an enhancement of the PNA pattern much less pronounced but it also was somewhat dependent on which upper-air data set was being analyzed. (just like the situation with the surface temperature, there are several different research groups producing compilations of upper-air data).
The dataset with a slight PNA enhancement (produced by the U.S. National Center for Atmospheric Research, NCAR) was the one used by Francis and Vavrus. The other dataset (produced by the European Centre for Medium Range Weather Forecasts, ERA-40) showed a tendency for reduced PNA conditions and instead of an amplified wave pattern, and the general flow flattened from west-to-east, which moves storms quickly across North America and produces weaker cyclones, contrary to Francis and Vavrus’s assertion.
So where does this leave us? When the new paper by Francis and Vavrus comes to the attention of the mainstream press (the paper is still in the “in-press” stage at Geophysical Research Letters), 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.
As with most issues related to climate science, the truth is significantly more complex than what is in the media. To increase your chances of getting “the rest of the story,” check your monthly “Current Wisdom”.
Francis, J., and S. Vavrus, 2012. Evidence linking arctic amplification to extreme weather in mid-latitudes. Geophysical Research Letters, in press, doi:10.1029/2012GL051000.
Frauenefeld, O., and R. Davis, 2003. Northern Hemisphere circumpolar vortex trends and climate change implications. Journal of Geophysical Research, 108, doi:10.1029/2002JD002958.
Strong, C., and R. Davis, 2007. Winter jet stream trends over the Northern Hemisphere. Quarterly Journal of the Royal Meteorological Society, 133, 2109-2115, doi:10.1002/qj.171