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No Consensus on Hurricanes and Global Warming
Reference
Klotzbach, P.J. and Gray, W.M. 2006. Causes of the unusually destructive 2004 Atlantic basin hurricane season. Bulletin of the American Meteorological Society 87: 1325-1333.

What was done
To provide some context to the lively and ongoing debate over whether or not CO2-induced global warming leads to increases in the frequency and intensity of hurricanes, the authors report that "two papers in 2005 indicated a large increase in power dissipation index for the northwestern Pacific and north Atlantic basins since the 1970s (Emanuel, 2005) and a nearly 50% increase in global category 4-5 hurricanes since the mid-1970s, respectively (Webster et al., 2005)." However, they note that "other papers question the validity of these findings due to potential bias-correction errors in the earlier part of the data record for the Atlantic basin (Landsea, 2005)," and that "while major hurricane activity in the Atlantic has shown a large increase since 1995, global tropical-cyclone activity, as measured by the accumulated cyclone energy index, has decreased slightly during the past 16 years (Klotzbach, 2006)." Hence, they go on to more fully describe their non-CO2-influenced view of the subject.

What was learned
As a result of data and reasoning described in their paper, Klotzbach and Gray say they "attribute the heightened Atlantic major hurricane activity of the 2004 season as well as the increased Atlantic major hurricane activity of the previous nine years to be a consequence of multidecadal fluctuations in the strength of the Atlantic multidecadal mode and strength of the Atlantic Ocean thermohaline circulation." In addition, they say that "historical records indicate that positive and negative phases of the Atlantic multidecadal mode and thermohaline circulation last about 25-30 years (typical period ~50-60 years; Gray et al., 1997; Latif et al., 2004)," and that "since we have been in this new active thermohaline circulation period for about 11 years, we can likely expect that most of the next 15-20 hurricane seasons will also be active, particularly with regard to increased major hurricane activity."

What it means
Since both views of the subject suggest that upcoming hurricane seasons could well be extraordinarily active ones, there would appear to be little hope of the question of "Why?" being definitively answered any time soon, which pretty much refutes the climate-alarmist claim that "the science is settled," and that consensus exists among hurricane experts on this issue, especially, we might add, since the 2006 hurricane season turned out be pretty much a no-show, complicating matters even more.

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

Gray, W.M., Sheaffer, J.D. and Landsea, C.W. 1997. Climate trends associated with multi-decadal variability of Atlantic hurricane activity. In: Diaz, H.F. and Pulwarty, R.S., Eds. Hurricanes: Climate and Socioeconomic Impacts, Springer-Verlag, pp. 15-52.

Klotzbach, P.J. 2006. Trends in global tropical cyclone activity over the past 20 years (1986-2005). Geophysical Research Letters 33: 10.1029/2006GL025881.

Landsea, C.W. 2005. Hurricanes and global warming. Nature 438: E11-13, doi:10.1038/nature04477.

Latif, M., Roeckner, E., Botzet, M., Esch, M., Haak, H., Hagemann, S., Jungclaus, J., Legutke, S., Marsland, S., Mikolajewicz, U. and Mitchell, J. 2004. Reconstructing, monitoring, and predicting multidecadal-scale changes in the North Atlantic thermohaline circulation with sea surface temperature. Journal of Climate 17: 1605-1614.

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 21 March 2007