Background
Insurance costs related to life and property damages caused by extreme weather events have been steadily rising in the United States and elsewhere; and it is not uncommon for people in the insurance industry and government to place the blame for this development on what they claim are significant increases in the frequencies and intensities of severe weather events, since climate models suggest that these phenomena should be increasing as a consequence of CO2-induced global warming.

What was done
To determine if these claims have any merit, the author investigated trends in both severe weather events and changes in societal and economic factors over the last half of the 20th century in the United States.

What was learned
Trends in various weather extremes over the last 50 years of the 20th century were mixed.  Changnon reports, for example, that "one trend is upwards (heavy rains-floods), others are downward (hail, hurricanes, tornadoes, and severe thunderstorms), and others are unchanging flat trends (winter storms and wind storms)."  We might also add, however, that had the analysis of heavy rains-floods been extended back to the beginning of the 20th century, the longer-term behavior of this phenomenon would have been found to be indicative of no net change over the past hundred years, as recently demonstrated by Kunkel (2003).

So why did insurance losses rise so rapidly over the past several decades?  Changnon reports that "the primary reason for the large losses [was] a series of societal shifts (demographic movements, increasing wealth, poor construction practices, population growth, etc.) that collectively had increased society's vulnerability."

What it means
When properly adjusted for societal and economic trends over the past half-century, monetary loss values associated with damages inflicted by extreme weather events, in the words of the author, "do not exhibit an upward trend."  Thus, as he emphasizes, "the adjusted loss values for these extremes [do] not indicate a shift due to global warming."  And to make his point perfectly clear, he reiterates that these real-world observations "do not fit the predictions, based on GCM simulations under a warmer world resulting from increased CO2 levels, that call for weather extremes and storms to increase in frequency and intensity."

Reference
Kunkel, K.E.  2003.  North American trends in extreme precipitation.  Natural Hazards 29: 291-305.
Reviewed 6 August 2003