Background
A vigorous debate is currently raging over whether the frequency of occurrence and/or intensity of tropical cyclones (TCs) has increased in recent decades in response to CO2-induced global warming.

What was done
To shed further light on this important issue, Chan investigated "possible causes of the multi-decadal variability in intense TC [category 4 and 5] occurrence in the western North Pacific (WNP)," choosing this basin because it generally has the largest number of TCs every year.

What was learned
Based on data for the period 1960-2005, the Hong Kong researcher determined that decadal variations in intense typhoon activity largely result from a combination of the behavior of the El Niņo-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).

What it means
In discussing his findings, Chan says that "the view that global warming would lead to more intense TCs owing to the enhancement of thermodynamic factors ignores the fact that for TCs to intensify significantly, the dynamic factors must 'cooperate'," and that "the latter has not been demonstrated to be enhanced basin wide." Therefore, as he continues, "the more likely conclusion is that the major low-frequency variations in the frequency of intense TC occurrence is probably a multi-decadal one in response to similar variations in the factors that govern the formation, intensification and movement of TCs," and that "such variations largely result from modifications of the atmospheric and oceanographic conditions in response to ENSO and PDO." Consequently, "at least for the WNP," in the words of Chan, "it is not possible to conclude that the variations in intense typhoon activity are attributable to the effect of global warming."