Climate models have long suggested that the intensity and frequency of hurricanes or tropical cyclones (TCs) may be significantly increased in response to global warming, as noted by Free et al. (2004), who have written that "increases in hurricane intensity are expected to result from increases in sea surface temperature and decreases in tropopause-level temperature accompanying greenhouse warming," citing in support of this statement the studies of Emanuel (1987), Henderson-Sellers et al. (1998) and Knutson et al. (1998). Before accepting this climatemodel-based projection, however, it is important to see what the world of nature has to say about the issue.
In an early review of empirical evidence related to the subject, Walsh and Pittock (1998) concluded that "the effect of global warming on the number of tropical cyclones is presently unknown," and that "there is little relationship between SST (sea surface temperature) and tropical cyclone numbers in several regions of the globe." Hence, they said there was "little evidence that changes in SSTs, by themselves, could cause change in tropical cyclone numbers."
In a second early analysis of the topic, Henderson-Sellers et al. (1998) determined that (1) "there are no discernible global trends in tropical cyclone number, intensity, or location from historical data analyses," (2) "global and mesoscale-model-based predictions for tropical cyclones in greenhouse conditions have not yet demonstrated prediction skill," and (3) "the popular belief that the region of cyclogenesis will expand with the 26°C SST isotherm is a fallacy."
Six years later, in yet another futile attempt to find the long-sought global warming signal in hurricane data, Free et al. (2004) looked for increases in potential hurricane intensity, because, as they put it, "changes in potential intensity (PI) can be estimated from thermodynamic principles as shown in Emanuel (1986, 1995) given a record of SSTs and profiles of atmospheric temperature and humidity." This they thus did, using radiosonde and SST data from 14 island radiosonde stations in the tropical Atlantic and Pacific Oceans, after which they compared their results with those of Bister and Emanuel (2002) at grid points near the selected stations. And what did they find?
As Free et al. describe it, "our results show no significant trend in potential intensity from 1980 to 1995 and no consistent trend from 1975 to 1995." What is more, they report that between 1975 and 1980, "while SSTs rose, PI decreased, illustrating the hazards of predicting changes in hurricane intensity from projected SST changes alone."