Background
The authors note that several studies have raised concerns that tropical cyclones, or TCs -- and especially the severe ones -- "have become more frequent in many places in response to global warming," citing Emanuel (2005) and Webster et al. (2005). In addition, Callaghan and Power write that the Intergovernmental Panel on Climate Change, or IPCC, highlights several studies that conclude that "tropical cyclones are likely to become more intense in the future in response to global warming," citing Alley et al. (2007); but rather than accepting this set of claims on the basis of the IPCC's "authority," they go on to provide a much more compelling analysis of their own.

What was done
Callaghan and Power, as they describe it, developed and used "a new data base of severe land-falling TCs for eastern Australia derived from numerous historical sources, that has taken over a decade to develop." This data base, as they continue, includes: "peer-reviewed publications; Bureau of Meteorology publications, including comprehensive case histories for a large number of TCs -- including all TCs since the mid-1950s, Monthly Climatological Bulletins and Monthly Weather Reviews, unpublished TC season reports, bounded operational analysis charts back to the 1890s stored in the National Archives, unpublished internal Bureau documents; publications by state and local governments; archives of several Queensland newspapers; newspaper clippings held by the Bureau of Meteorology; books describing land-falling TCs; information held by the Cairns and Townsville Historical Societies; a report to the QLD parliament (1918); and extensive unpublished information from the public including numerous damage photographs," as well as "reports on storm surge, wave action and shipwreck data from an extensive Australian shipwreck data base."

What was learned
The two researchers with Australia's Bureau of Meteorology first note that their new data base allows them "to document changes over much longer periods than has been done previously for the Southern Hemisphere," and among the host of results they describe, two of them stand out with respect to their significance to the global warming debate. First, they report that "the sign and magnitude of trends calculated over 30 years periods vary substantially," highlighting the fact that "caution needs to be taken in making inferences based on e.g. satellite era data only." And second, they report that "the linear trend in the number of severe TCs making land-fall over eastern Australia declined from about 0.45 TC/year in the early 1870s to about 0.17 TC/year in recent times -- a 62% decline." And they add that "this decline can be partially explained by a weakening of the Walker Circulation, and a natural shift towards a more El Niņo-dominated era." Thus, they conclude the abstract of their paper with the remark that "the extent to which global warming might also be partially responsible for the decline in land-falls -- if it is at all -- is unknown [bold and italics added to highlight the irony of the result]."

What it means
Callaghan and Power's analysis of their lengthy and comprehensive new data base reveals results that appear to be totally at odds with the contentions of the IPCC, which are based more on the output of numerical models of the atmosphere than on real-world observations. And their results also highlight the fact that even real-world observations may be misleading, especially if they do not cover a long enough time period to reveal the oscillatory nature of various aspects of earth's climate.

References
Alley, R., et al. 2007. Climate Change 2007: The Physical Science Basis. Summary for Policymakers. IPCC.

Emanuel, K.A. 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.