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 topical cyclone numbers in several regions of the globe." Hence, they opined 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.

So 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." In addition, in another review of what real-world data have to say about the subject, Walsh (2004) was once again forced to report "there is as yet no convincing evidence in the observed record of changes in tropical cyclone behavior that can be ascribed to global warming." Nevertheless, Walsh continued to believe that (1) "there is likely to be some increase in maximum tropical cyclone intensities in a warmer world," (2) "it is probable that this would be accompanied by increases in mean tropical cyclone intensities," and (3) "these increases in intensities are likely to be accompanied by increases in peak precipitation rates of about 25%," putting the date of possible detection of these increases "some time after 2050," little knowing that two such claims would actually be made the very next year.

The historic contentions came from Emanuel (2005), who claimed to have found that a hurricane power dissipation index had increased by approximately 50% for both the Atlantic basin and the Northwest Pacific basin since the mid 1970s, and from Webster et al. (2005), who contended that the numbers of Category 4 and 5 hurricanes for all tropical cyclone basins had nearly doubled between an earlier (1975-1989) and a more recent (1990-2004) 15-year period. However, in a challenge to both of these claims, Klotzbach (2006) wrote that "many questions have been raised regarding the data quality in the earlier part of their analysis periods," and he thus proceeded to perform a new analysis based on a "near-homogeneous" global data set for the period 1986-2005.

So what did Klotzbach do? And what did he find?

Klotzbach first tabulated global tropical cyclone (TC) activity using best track data -- which he describes as "the best estimates of the locations and intensities of TCs at six-hour intervals produced by the international warning centers" -- for all TC basins (North Atlantic, Northeast Pacific, Northwest Pacific, North Indian, South Indian and South Pacific), after which he determined trends of worldwide TC frequency and intensity over the period 1986-2005, during which time global SSTs are purported to have risen by about 0.2-0.4°C. This work did indeed indicate, in his words, "a large increasing trend in tropical cyclone intensity and longevity for the North Atlantic basin," but it also indicated "a considerable decreasing trend for the Northeast Pacific," and combining these observations with the fact that "all other basins showed small trends," he determined there had been "no significant change in global net tropical cyclone activity" over the past two decades. With respect to Category 4 and 5 hurricanes, however, he found there had been a "small increase" in their numbers from the first half of the study period (1986-1995) to the last half (1996-2005); but he noted that "most of this increase is likely due to improved observational technology." Not mincing any words, therefore, Klotzbach declared that his findings were "contradictory to the conclusions drawn by Emanuel (2005) and Webster et al. (2005)," in that the global TC data did "not support the argument that global TC frequency, intensity and longevity have undergone increases in recent years."

Following close on the heels of Klotzbach's study came the paper of Kossin et al. (2007), who wrote that "the variability of the available data combined with long time-scale changes in the availability and quality of observing systems, reporting policies, and the methods utilized to analyze the data make the best track records inhomogeneous," and stated that this "known lack of homogeneity in both the data and techniques applied in the post-analyses has resulted in skepticism regarding the consistency of the best track intensity estimates." Consequently, as an important first step in resolving this problem, Kossin et al. "constructed a more homogeneous data record of hurricane intensity by first creating a new consistently analyzed global satellite data archive from 1983 to 2005 and then applying a new objective algorithm to the satellite data to form hurricane intensity estimates," after which they analyzed the resultant homogenized data for temporal trends over the period 1984-2004 for all major ocean basins and the global ocean as a whole.

In describing what they learned from this exercise, the five scientists who conducted the work said that "using a homogeneous record, we were not able to corroborate the presence of upward trends in hurricane intensity over the past two decades in any basin other than the Atlantic." Therefore, noting that "the Atlantic basin accounts for less than 15% of global hurricane activity," they concluded that "this result poses a challenge to hypotheses that directly relate globally increasing tropical sea surface temperatures to increases in long-term mean global hurricane intensity," delivering another major blow to the contentions of Emanuel (2005) and Webster et al. (2005) while stating that "the question of whether hurricane intensity is globally trending upwards in a warming climate will likely remain a point of debate in the foreseeable future."

As a result of the many investigations of the subject that have been conducted over the past several years, there currently appears to be no factual basis for claiming that planet-wide hurricane frequency and/or intensity will rise in response to potential future global warming. Nevertheless, parties pushing for restrictions on anthropogenic CO2 emissions continue to do so, citing the claims of Emanual (2005) and Webster et al. (2005) as major reasons for their unrelenting efforts, which necessitates our highlighting the findings of two additional papers.

Based on the demographic and economic findings of Pielke et al. (2000), Pielke et al. (2005) report that by the year 2050, "for every additional dollar in damage that the Intergovernmental Panel on Climate Change expects to result from the effects of global warming on tropical cyclones, we should expect between $22 and $60 of increase in damage due to population growth and wealth." This being the case, they merely state the obvious when they say that "the primary factors that govern the magnitude and patterns of future damages and causalities are how society develops and prepares for storms rather than any presently conceivable future changes in the frequency and intensity of the storms." Nevertheless, Pielke et al. note that many climate alarmists continue to claim a significant hurricane-global warming connection for the purpose of advocating massive anthropogenic CO2 emissions reductions, which in their estimation "simply will not be effective with respect to addressing future hurricane impacts," additionally noting that "there are much, much better ways to deal with the threat of hurricanes than with energy policies (e.g., Pielke and Pielke, 1997)."

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