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Tropical Storms of the North Atlantic Ocean
Villarini, G., Vecchi, G.A., Knutson, T.R., Zhao, M. and Smith, J.A. 2011. North Atlantic tropical storm frequency response to anthropogenic forcing: Projections and sources of uncertainty. Journal of Climate 24: 3224-3238.

The authors write that "the impact of future anthropogenic forcing on the frequency of tropical storms in the North Atlantic basin has been the subject of intensive investigation," but they say that "whether the number of North Atlantic tropical storms will increase or decrease in a warmer climate is still heavily debated and a consensus has yet to be reached."

What was done
In an effort to help resolve the issue, Villarini et al. (2011) used the statistical model developed by Villarini et al. (2010), in which "the frequency of North Atlantic tropical storms is modeled by a conditional Poisson distribution with a rate of occurrence parameter that is a function of tropical Atlantic and mean tropical sea surface temperatures (SSTs)," in order to (1) examine "the impact of different climate models and climate change scenarios on North Atlantic and U.S. landfalling tropical storm activity," and (2) reconcile "differing model projections of changes in the frequency of North Atlantic tropical storms in a warmer climate."

What was learned
The five researchers report, first of all, that their results "do not support the notion of large increases in tropical storm frequency in the North Atlantic basin over the twenty-first century in response to increasing greenhouse gases." Second, they report that "the disagreement among published results concerning increasing or decreasing North Atlantic tropical storm trends in a warmer climate can be largely explained (close to half of the variance) in terms of the different SST projections (Atlantic minus tropical mean) of the different climate model projections." Third, they say that "for the SRES A1B scenario and 24 climate models, over the twenty-first century there is a large spread among projected trends in tropical storm activity in the North Atlantic basin, with a mean of -0.83 tropical storm per century and a standard deviation of 2.48 tropical storms per century." And finally, with respect to U.S. land-falling tropical storms, they say that "results based on 7 climate models point to a statistically significant increasing trend, while 6 point to a decreasing trend," which suggests, once again, that ...

What it means
... we really don't know what will happen over the course of the current century. And thus it is that Villarini et al. (2011) conclude, among several other things, that "there is a considerable level of uncertainty in climate change projections that will remain effectively 'irreducible,' as no current prospects exist for skillful century-scale predictions of unforced climate variability."

Villarini, G., Vecchi, G.A. and Smith, J.A. 2010. Modeling of the dependence of tropical storm counts in the North Atlantic basin on climate indices. Monthly Weather Review 138: 2681-2705.

Reviewed 14 September 2011