At the end of the 2004 Atlantic hurricane season, many scientists, reporters and policymakers looked for simple answers to explain the extent of the devastation, which totaled more than $40 billion according to the National Hurricane Center. Some prominent scientists proposed that the intense 2004 hurricane season and its considerable impacts, particularly in Florida, could be linked to global warming resulting from the emissions of greenhouse gases into the atmosphere (e.g., Harvard Medical School 2004; NCAR 2004). But the current state of climate science does not support so close a linkage.
Tropical cyclones can be thought of to a first approximation as a natural heat engine or Carnot cycle (Emanuel 1987). From this perspective global warming can theoretically influence the maximum potential intensity of tropical cyclones through alterations of the surface energy flux and/or the upper-level cold exhaust (Emanuel 1987, Lighthill et al. 1994, Henderson-Sellers et al. 1998). But no theoretical basis yet exists for projecting changes in tropical cyclone frequency, though empirical studies do provide some guidance as to the necessary thermodynamical and dynamical ingredients for tropical cyclogenesis (Gray 1968, 1979).
Since 1995 there has been an increase in the frequency and in particular the intensity of hurricanes in the Atlantic.. But the changes of the past decade are not so large as to clearly indicate that anything is going on other than the multi-decadal variability that has been well documented since at least 1900 (Gray et al. 1997; Landsea et al. 1999; Goldenberg et al. 2001). Consequently, in the absence of large or unprecedented trends, any effect of greenhouse gases on the behavior of hurricanes is necessarily very difficult to detect in the context of this documented variability. Perspectives on hurricanes are no doubt shaped by recent history, with relatively few major hurricanes observed in the 1970s, 80s and early 90s, compared with considerable activity during the 1940s, 50s and early 60s. The period from 1944 to 1950 was particularly active for Florida. During that period eleven hurricanes hit the state, at least one per year, resulting in the equivalent of billions of dollars in damage in each of those years (Pielke and Landsea 1998).
Globally there has been no increase in tropical cyclone frequency over at least the past several decades (Lander and Guard 1998; Elsner and Kocher 2000). In addition to a lack of theory for future changes in storm frequencies, the few global modeling results are contradictory (Henderson-Sellers et al 1998; IPCC 2001). Because historical and observational data on hurricanes and tropical cyclones are relatively robust, it is clear that storm frequency has not tracked recent tropical climate trends. Research on possible future changes in hurricane frequency due to global warming is ambiguous, with most studies suggesting that future changes will be regionally-dependent, and showing a lack of consistency in projecting an increase or decrease in the total global number of storms (Henderson-Sellers et al. 1998, Royer et al. 1998; Sugi et al. 2002). These studies give such contradictory results as to suggest that the state of understanding of tropical cyclogenesis provides too poor a foundation to base any projections about the future. While there is always some degree of uncertainty about the future and model-based results are often fickle, the state of current understanding is such that we should expect hurricanes frequencies in the future to have a great deal of year-to-year and decade-todecade variation as has been observed over the past decades and longer.