In An Inconvenient Truth (pp. 94-95), Al Gore blamed global warming for Hurricane Katrina and the devastation of New Orleans. Not in so many words but through heavy-handed insinuation no movie goer could miss.
It seemed plausible because Gore invoked an “emerging consensus linking global warming to the increasing destructive power of hurricanes . . . based in part on research showing a significant increase in the number of category 4 and 5 hurricanes.”
The research to which Gore alluded was Webster et al. (2005), a study which found a significant increase in the number and percentage of category 4 and 5 hurricanes during 1970-2004. The study was hotly debated at the time. For example, on the same day Science magazine published the Webster study, climatologist Patrick Michaels published a critique. Michaels showed that, in the Atlantic basin—the hurricane formation area with the best data over the longest period—the “trends” observed by Webster et al. disappeared once data going back to 1940 were included. Roughly the same number and percentage of intense hurricanes occurred during 1940-1970 as occurred during 1970-2004.
This week’s edition of CO2Science.Org reviews “Extremely Intense Hurricanes: Revisiting Webster et al. (2005) after 10 Years,” a study by Phil Klotzbach of Colorado State University and Christopher Landsea of NOAA/NWS/National Hurricane Center.
Klotzbach and Landsea examine whether the “trends” found by Webster et al. continue after an additional 10 years of data. The two researchers find that “the global frequency of category 4 and 5 hurricanes has shown a small, insignificant downward trend while the percentage of category 4 and 5 hurricanes has shown a small, insignificant upward trend between 1990 and 2014.” They further report that “Accumulated cyclone energy globally has experienced a large and significant downward trend during the same period.” In other words, there has been a large decrease in the overall destructive power of hurricanes based on an assessment of the number, strength, and duration of all individual hurricanes worldwide.
Klotzbach and Landsea conclude that the intense-hurricane trends observed by Webster et al. were primarily due to “observational improvements at the various global tropical cyclone warning centers, primarily in the first two decades of that study.”