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Extreme Tropical Cyclone Trends
Reference
Landsea, C.W., Harper, B.A., Hoarau, K. and Knaff, J.A. 2006. Can we detect trends in extreme tropical cyclones? Science 313: 252-254.

Background
The authors note that recent studies have claimed to have found a large increase in tropical cyclone intensities linked to increasing sea surface temperatures that may be associated with global warming (Emanuel, 2005; Webster et al., 2005; Hoyos et al., 2006); but they say that modeling and theoretical studies suggest only small temperature-induced changes to tropical cyclone intensity on the order of ~5% near the end of the 21st century (Knutson and Tuleya, 2004; Emanuel, 2004). Hence, they ask the question: "Are the global tropical cyclone databases sufficiently reliable to ascertain long-term trends in tropical cyclone intensity, particularly in the frequency of extreme tropical cyclones (categories 4 and 5 on the Saffir-Simpson Hurricane Scale)?"

What was done
In an attempt to answer this question, Landsea et al. analyzed the history of a number of operational changes at various tropical cyclone warning centers that they feel have led to "more frequent identification of extreme tropical cyclones," as well as an unreal "shift to stronger maximum sustained surface wind," investigating in particular in this regard the Dorvak Technique for estimating tropical cyclone intensity.

What was learned
In the words of the four researchers, "trend analyses for extreme tropical cyclones are unreliable because of operational changes that have artificially resulted in more intense tropical cyclones being recorded [with the passing of time], casting severe doubts on any such trend linkages to global warming." In addition, they note that "data from the only two basins that have had regular aircraft reconnaissance - the Atlantic and Northwest Pacific - show that no significant trends exist in tropical cyclone activity when records back to at least 1960 are examined (Landsea, 2005; Chan, 2006)," and that "Klotzbach (2006) has shown that extreme tropical cyclones and overall tropical cyclone activity have globally been flat from 1986 until 2005, despite a sea surface temperature warming of 0.25C."

What it means
The results of this study, in our opinion, raise sufficient serious questions about the validity of the analyses of Emanuel (2005), Webster et al. (2005) and Hoyos et al. (2006) as to render their primary conclusions unproven and, in fact, doubtful.

References
Chan, J.C.L. 2006. Comment on "Changes in tropical cyclone number, duration, and intensity in a warming environment." Science 322: 1713-1713b.

Emanuel, K. 2004. In: Murnane, R.J. and Liu, K.-B. (Eds.) Hurricanes and Typhoons: Past, Present and Future. Columbia University Press, New York, NY, USA, pp. 395-407.

Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.

Hoyos, C.D., Agudelo, P.A., Webster, P.J. and Curry, J.A. 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science 312: 94-97.

Klotzbach, P.J. 2006. Trends in global tropical cyclone activity over the past twenty years (1986-2005). Geophysical Research Letters 33: 10.1029/2006GL025881.

Knutson, T.R. and Tuleya, R.E. 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate 17: 3477-3495.

Landsea, C.W. 2005. Hurricanes and global warming. Nature 438 (22 December 2005) doi:10.1038/nature04477.

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.

Reviewed 25 October 2006