The weather during the last decade has become increasingly weird and extreme. The occurrence of persistent weather events such as droughts, floods, heat waves and cold spells is increasing. The evidence that the most extreme events are linked to human-caused global warming is compelling. NASA climate scientists James Hansen, Makiko Sato and Reto Ruedy write  “we can state with a high degree of confidence that extreme summers, such as those in Texas and Oklahoma in 2011 and Moscow in 2010, are a consequence of global warming, because global warming has dramatically increased their likelihood of occurrence.” This is also the conclusion of several other peer-reviewed scientific journal articles . Such sentiment is also being reported in the main stream media for example by Alexandra Steel on CNN . Even television meteorologists such as Stu Ostro a Senior Meteorologist on The Weather Channel who were recently deniers have come around to this view .
Figure 1. Frequency of occurrence of local temperature anomalies in units of the local standard deviation. Anomalies are relative to the 1951-1980 base period. Source Hansen et al.
Figure 1 from the Hansen paper, shows the northern hemisphere summer temperature anomaly in standard deviations  from the mean 1951 to 1980 temperature. The summer temperatures are normally distributed. One standard deviation is exceeded 15.8 percent of the time in one of the two directions. Two standard deviations are exceeded only 2.3 percent of the time. Three standard deviations are exceeded only 0.13 percent of the time. The probability of exceeding four standard deviations is less than 0.003 percent and the probability of exceeding five standard deviations is less than 0.00003 percent of the time. Note that the distribution functions are getting broader with time, which means there will be more extremes and relative to the decade between 1951 and 1961, the mean temperature is getting warmer. Together, this means that warm temperatures will be set much more frequently than cold temperatures. In fact there is now a 10 percent probability that the summer temperature will exceed a value which in the 1950’s had less than a 0.13 percent chance of happening (greater than three standard deviations). Hansen et al write “Recent examples of summer temperature anomalies exceeding +3? [standard deviations] include the heat wave and drought in Oklahoma, Texas and Mexico in 2011 and a larger region encompassing much of the Middle East, Western Asia and Eastern Europe, including Moscow, in 2010.”
Identifying the strong correlation between extreme weather and global warming is not nearly as satisfying to the skeptical mind as understanding the physics that explains the correlation. The most obvious physical explanation is that in a warmer world there is more energy in the atmosphere, in the oceans and on the surface of the terrestrial earth and more energy will naturally fuel more energetic weather. Hurricanes have always been associated with high sea surface temperatures.
Another interesting phenomenon is polar amplification or more accurately arctic amplification. The poles tend to warm (or cool) faster than the tropics and temperate zones. When Svante Arrhenius first described the potential for human-caused global warming in 1896, he identified this feature of climate change . While the earth has warmed one degree in the last 40 years, the arctic has warmed more than two degrees. There are several explanations for this. The most important and most obvious reason is the change in surface albedo in the arctic and sub-arctic . The snow and ice reflect most of the incoming short wave solar energy back out into space while the dark ocean and land which are replacing it absorb most of this energy and as a consequence heat up. In the last three decades the arctic sea ice has declined by 40 percent. For a good illustration of the difference in albedo around Lake Superior, see figure 2 below from Jeff Master’s weather underground web site . Note the difference between a cold winter with lots of snow and ice and a relatively warm dry winter with no snow and ice.
In a recent paper appearing in a March 2012 issue of Geophysical Research Letters, Jennifer Francis of Rutgers University and Stephen Vavrus of the University of Wisconsin explain  that arctic amplification causes the west-to-east winds of the jet stream, the river of wind separating the warm temperate climate to the south from the frigid arctic climate to the north, to slow. In fact the jet stream’s west winds around the northern hemisphere have slowed about 14 percent since 1980. They write that, “the differential warming of the Arctic relative to midlatitudes is the key linking AA [Arctic Amplification] with patterns favoring persistent weather conditions in mid-latitudes.” And, “Slower progression of upper-level waves causes more persistent weather conditions that can increase the likelihood of certain types of extreme weather, such as drought, prolonged precipitation, cold spells, and heat waves.”
The jet stream flow of air follows a serpentine path as it drifts north and south between about 30 degrees and 60 degrees latitude. The southward dips are called troughs and the northern peaks are called ridges. This phenomenon is shown in Figure 3 below. The slower eastward migration of waves in the jet stream causes weather to stall, favoring persistent conditions that can lead to extreme weather if they last a long time. The south winds behind the ridges pull tropical humid warm air northward, while north winds behind the troughs pull arctic air south. As shown in Figure 3, the ridges are occurring further north, which slows down the wave progression even more.
Francis and Vavrus conclude: “As the Arctic sea-ice cover continues to disappear and the snow cover melts ever earlier over vast regions of Eurasia and North America, 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.”
The slowing of waves in the jet stream may explain both persistent cold events, such as the 2009/2010 winter over the eastern United States, as well as persistent droughts and heat waves such as over Texas and Oklahoma during 2011. Francis has written a more accessible explanation on the Yale Environment 360 web site . I asked Dr. Francis in an e-mail if climatologists expect this trend to continue and she responded: “Our results suggest that the enhanced warming in the Arctic relative to the rest of the globe is causing our weather patterns to move more slowly. This means that the types of extreme weather that are caused by prolonged weather conditions — such as droughts, cold spells, heat waves, and floods — are becoming more likely. This tendency is expected to continue as greenhouse gases accumulate in the atmosphere and as additional sea ice is lost.”
I also asked if accumulation of atmospheric carbon dioxide would have a more pronounced impact on polar climate than global climate. My observation was that carbon dioxide and water vapor absorb long wave heat radiation at some of the same wavelengths thus blunting a bit the impact of additional carbon dioxide. But the arctic being cold has very little water vapor so the impact of an increase in carbon dioxide is much stronger than in the temperate zones. Dr. Francis confirmed this hypothesis and sent me a paper describing just this phenomenon .
Figure 3. Region of study: 140oW to 0o. Schematic of ridge elongation (dashed vs. solid) in upper-level heights caused by enhanced warming in Arctic relative to mid-latitudes. Higher amplitude waves progress eastward more slowly, as indicated by arrows. Source: Francis and Vavrus.
Figure 2. A tale of two winters: Lake Superior was choked with ice at the end of the winter of 2008 – 2009 (top), but was virtually ice-free at the end of the winter of 2011 – 2012 (bottom.) Image credit: NASA. Source: Jeff Masters.
 James Hansen, Makiko Sato and Reto Ruedy, “Perceptions of Climate Change: The New Climate Dice”, submitted to Proceedings of the National Academy of Sciences http://www.columbia.edu/~jeh1/mailings/2012/20120105_PerceptionsAndDice.pdf
 That the recent extreme heat events are caused by anthropogenic global warming is in fact the conclusion of a PNAS paper by Stefan Rahmstorf and Dim Coumou http://www.pnas.org/content/early/2011/10/18/1101766108.abstract That the Moscow drought was caused by global warming is the conclusion of yet another recently published paper in The Geophysical Research Letters http://www.agu.org/news/press/pr_archives/2012/2012-10.shtml. That the droughts in the Mediterranean region are a result of climate change has been reported by NOAA scientists in the journal Climate Change http://www.noaanews.noaa.gov/stories2011/20111027_drought.html .
 Stu Ostro, Senior Meteorologist, The Weather Channel see http://climatecrocks.com/2010/12/03/stu-ostro-the-weather-channels-former-skeptic/ by the way, in this video, Ostro, in my opinion misuses the word skeptic. Skepticism is indeed a healthy part of science but it requires one to question everything, not simply that with which one disagrees. Deniers do not do this because no denier argument can stand even the weakest scrutiny. A real skeptic frames her skepticism as a testable counter hypothesis and being testable requires that a real skeptic is willing to be wrong as pointed out by Gabrielle Walker in her book Snowball Earth. For another recent convert see http://thinkprogress.org/romm/2012/03/29/454476/a-message-from-a-republican-meteorologist-on-climate-change/
 Svante Arrhenius, “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground”, Philosophical Magazine and Journal of Science, Series 5, Volume 41, April 1896, pages 237-276. http://www.rsc.org/images/Arrhenius1896_tcm18-173546.pdf
 Jeff Masters, http://www.wunderground.com/blog/JeffMasters/show.html
 Francis, J. A. and S. J. Vavrus, “Evidence linking Arctic amplification to extreme weather in mid-latitudes, Geophysical Research Letters, Vol. 39, March, 2012.
 V. A. Alexeev, P. L. Langen, J. R. Bates, Polar ampli?cation of surface warming on an aquaplanet in ‘‘ghost forcing’’ experiments without sea ice feedbacks, Climate Dynamics (2005), DOI 10.1007/s00382-005-0018-3