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Sea Surface Temperatures and Atlantic Hurricanes
Reference
Michaels, P.J., Knappenberger, P.C. and Davis, R.E. 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters 33: 10.1029/2006GL025757.

Background
Emanuel (2005) and Webster et al. (2005), in the words of Michaels et al., "have suggested that rising sea surface temperatures (SSTs) in the North Atlantic hurricane formation region are linked to recent increases in hurricane intensity, and that the trend of rising SSTs during the past 3 to 4 decades bears a strong resemblance to that projected to occur from increasing greenhouse gas concentrations." Taken together, these two claims imply that mankind's CO2 emissions have played a significant role in promoting the recent spate of intense Atlantic hurricanes. But is this implication a true reflection of reality?

What was done
In an effort designed to answer this important question, the three researchers used weekly-averaged 1 latitude by 1 longitude SST data together with hurricane track data (developed by the National Hurricane Center) that provide hurricane-center locations (latitude and longitude in tenths of a degree) and maximum 1-minute surface wind speeds (both at six-hour intervals) for all tropical storms and hurricanes in the Atlantic basin that occurred between 1982 (when the SST data set begins) through 2005.

What was learned
Plotting maximum cyclone wind speed against the maximum SST that occurred prior to (or concurrent with) the maximum wind speed of each of the 270 Atlantic tropical cyclones of their study period, Michaels et al. found that for each 1C increase in SST between 21.5C and 28.25C, the maximum wind speed attained by Atlantic basin cyclones rises, in the mean, by 2.8 m/s, and that thereafter, as SSTs rise still further, the first category-3-or-greater storms begin to appear. However, they report that "there is no significant relationship between SST and maximum winds at SST exceeding 28.25C."

What it means
From these observations, Michaels et al. correctly conclude that "while crossing the 28.25C threshold is a virtual necessity for attaining category 3 or higher winds, SST greater than 28.25C does not act to further increase the intensity of tropical cyclones." Hence, it would appear that the comparison of SSTs actually encountered by individual storms performed by Michaels et al. - as opposed to the comparisons of Emanuel (2005) and Webster et al. (2005), which utilized basin-wide averaged monthly or seasonal SSTs - refutes the idea that anthropogenic activity has detectably influenced the severity of Atlantic basin hurricanes over the past quarter-century.

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 31 May 2006