A paper published today in the Bulletin of the American Meteorological Society presents a review of the current state of knowledge regarding extreme weather. The authors, most of whom are from NOAA, find no significant trends in severe thunderstorms, overall ice storms, or change in the percentage of the contiguous US impacted by extreme snowfall. According to the authors, trends in tropical cyclones are controversial and indeterminate, although other authors find global cyclone activity is at a historical low. The authors state, "attribution of trends to anthropogenic forcing [man-made global warming] remains controversial."
Although the authors do find an increase in extreme precipitation, they are uncertain of the cause, "although there is evidence that increasing atmospheric water vapor may be one factor." Satellite and weather balloon records, however, indicate atmospheric water vapor has been on a generally declining trend since 1948.
Bulletin of the American Meteorological Society 2012 ; e-View
Monitoring and Understanding Trends in Extreme Storms: State of Knowledge
Kenneth E. Kunkel1, Thomas R. Karl2, Harold Brooks3, James Kossin2, Jay H. Lawrimore2Derek Arndt2, Lance Bosart4, David Changnon5, Susan L. Cutter6, Nolan Doesken7, Kerry Emanuel8, Pavel Ya. Groisman2, Richard W. Katz9, Thomas Knutson10, James O'Brien11, Christopher J. Paciorek19, Thomas C. Peterson2, Kelly Redmond12, David Robinson13, Jeff Trapp14, Russell Vose2, Scott Weaver15, Michael Wehner16, Klaus Wolter17, Donald Wuebbles18
Abstract: The state of knowledge regarding trends and an understanding of their causes is presented for a specific subset of extreme weather and climate types. For severe convective storms (tornadoes, hail storms, and severe thunderstorms), differences in time and space of practices of collecting reports of events make the use of the reporting database to detect trends extremely difficult. Overall, changes in the frequency of environments favorable for severe thunderstorms have not been statistically significant. For extreme precipitation, there is strong evidence for a nationally-averaged upward trend in the frequency and intensity of events. The causes of the observed trends have not been determined with certainty, although there is evidence that increasing atmospheric water vapor may be one factor. For hurricanes and typhoons, robust detection of trends in Atlantic and western North Pacific tropical cyclone (TC) activity is significantly constrained by data heterogeneity and deficient quantification of internal variability. Attribution of past TC changes is further challenged by a lack of consensus on the physical linkages between climate forcing and TC activity. As a result, attribution of trends to anthropogenic forcing remains controversial. For severe snowstorms and ice storms, the number of severe regional snowstorms that occurred since 1960 was more than twice that of the preceding 60 years. There are no significant multi-decadal trends in the areal percentage of the contiguous U.S. impacted by extreme seasonal snowfall amounts since 1900. There is no distinguishable trend in the frequency of ice storms for the U.S. as a whole since 1950.