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Hurricane Activity of the Past Two Decades
Reference
Klotzbach, P.J. 2006. Trends in global tropical cyclone activity over the past twenty years (1986-2005). Geophysical Research Letters 33: 10.1029/2006GL025881.

Background
The author notes that "recent papers by Emanuel (2005) and Webster et al. (2005) have caused a flurry of debate about the relationship between increasing tropical sea surface temperatures (SSTs) and intense tropical cyclones (TCs)." Specifically, he says Emanuel found that a hurricane power dissipation index "had increased by approximately 50% for both the Atlantic basin and the Northwest Pacific basin since the mid 1970s," and that Webster et al. determined that the numbers of Category 4 and 5 hurricanes for all TC basins "had nearly doubled between an earlier (1975-1989) and a more recent (1990-2004) 15-year period." However, he reports that "many questions have been raised regarding the data quality in the earlier part of their analysis periods," and, hence, he proceeds to rectify this situation with a new analysis of a "near-homogeneous" global data set for the period 1986-2005.

What was done
Klotzbach tabulated global TC activity using best track data for all TC basins (North Atlantic, Northeast Pacific, Northwest Pacific, North Indian, South Indian and South Pacific) - which he describes as "the best estimates of the locations and intensities of TCs at six-hour intervals produced by the international warning centers" - after which he determined trends of worldwide TC frequency and intensity over the period 1986-2005, during which time global SSTs are purported to have risen by approximately 0.2-0.4C.

What was learned
Klotzbach says "the data indicate a large increasing trend in tropical cyclone intensity and longevity for the North Atlantic basin and a considerable decreasing trend for the Northeast Pacific." Combining these observations with the fact that "all other basins showed small trends," he finds "there has been no significant change in global net tropical cyclone activity" over the past two decades. With respect to Category 4 and 5 hurricanes, however, he finds there has been a "small increase" in their numbers from the first half of the study period (1986-1995) to the last half (1996-2005); but he says that "most of this increase is likely due to improved observational technology."

What it means
Not mincing any words, Klotzbach declares that "these findings are contradictory to the conclusions drawn by Emanuel (2005) and Webster et al. (2005)," in that the global TC data "do not support the argument that global TC frequency, intensity and longevity have undergone increases in recent years."

References
Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.

Webster, P.J., Holland, G.J., Curry, J.A. and Chang, H.-R. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846.

Reviewed 7 June 2006