In the introduction to their recently published paper that addresses this subject, Bender et al. (2010) write that "increasing amounts of greenhouse gases are a likely factor in recent warming of tropical Atlantic sea surface temperatures (SSTs, 1-3)," although they say that "internal variability (4) and reduced aerosol or dust forcing (5,6) may have also contributed." They additionally report that "some statistical analyses suggest a link between warmer Atlantic SSTs and increased hurricane activity (6-8)," but they add that "other studies contend that the spatial structure of the SST change may be a more important control on tropical cyclone frequency and intensity (9-11)." In addition, they report that "a few studies (6,8,12) suggest that greenhouse warming has already produced a substantial rise in Atlantic tropical cyclone activity," but they hasten to say that "others question that conclusion (9,11,13)." Clearly, therefore, there is no consensus on the subject; and the science is not settled, in contradiction of what so many would have us believe.

So what did Bender et al. do in their study? ... and what did they learn?

As the seven scientists describe it, they "explored the influence of future global warming on Atlantic hurricanes with a downscaling strategy by using an operational hurricane-prediction model that produces a realistic distribution of intense hurricane activity for present-day conditions," working with 18 models from the World Climate Research Program's Coupled Model Intercomparison Project 3, while employing the Intergovernmental Panel on Climate Change's A1B emissions scenario.

The result of this exercise was, in their words, "an increase in the number of the most intense storms for the warmer climate compared with the control climate." More specifically, Bender et al.'s modeling work predicted that for "category 4 and 5 hurricanes with maximum winds greater than 60 m/s, the total number increased sharply from 24 to 46," and that "hurricanes with winds greater than 65 m/s increased from 6 to 21." However, they report there were reductions in the total number of hurricanes of all categories, which seems a bit contradictory.

In further discussing their findings, the researchers comment on the wide range of variability in what the various models predicted. They note, for example, that an increase in hurricane-caused "damage potential" of +30% was projected for the 18-model ensemble, while a range of -50% to +70% was found for four models for which they did more detailed work. And this extreme range of variability also reduces confidence in their mean result.

On another point, however, Bender et al.'s findings fly in the face of those who have tried to link the occurrence of strong hurricanes of the recent past with what they have claimed was unnatural and unprecedented CO2-induced global warming (Al Gore and James Hansen, take note!). Quite to the contrary, and although the new model results suggest that "a significant anthropogenic increase in the frequency of very intense Atlantic hurricanes may emerge from the background climate variability," the researchers say that this development would likely not occur until "the latter half of the 21st century."

As is nearly always the case in climate modeling work, Kerr (2010) reports -- in a commentary on Bender et al.'s study -- that the researchers "are looking for yet more computer power and higher resolution to boost the realism of simulations." And if such occurs, and "if the models continue to converge as realism increases," Kerr writes that "the monster storms that seemed to be already upon us would be removed to decades hence."

But who really knows, when one is working with much-less-than-perfect models of a complex planetary climate/weather system? As Kerr reports, even the researchers themselves "caution" that their findings are still "far from the last word" on the subject. And so they are. So please stay tuned to the ever-flowing stream of ever-new climate research findings that we will continue to bring you ... and don't make any hasty decisions based on the "latest and greatest" climate model results. They will always be evolving.

Sherwood, Keith and Craig Idso

References
Bender, M.A., Knutson, T.R., Tuleya, R.E., Sirutis, J.J., Vecchi, G.A., Garner, S.T. and Held, I.M. 2010. Modeled impact of anthropogenic warming on the frequency of intense Atlantic hurricanes. Science 327: 454-458.

Kerr, R.A. 2010. Models forsee more-intense hurricanes in the greenhouse. Science 327: 399.

Citations in Bender et al.'s Text
1. S. Solomon et al., Eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, Cambridge, 2007).

2. B.D. Santer et al., Proc. Natl. Acad. Sci. U.S.A. 103, 13905 (2006).

3. N.P. Gillett, P.A. Stott, B.D. Santer, Geophysical. Res. Lett. 35, L09707 (2008).

4. R. Zhang, T.L. Delworth, Geophys. Res. Lett. 33, L17712 (2006).

5. A.T. Evan, D.J. Vimont, A.K. Heidinger, J.P. Kossin, R. Bennartz, Science 324, 778 (2009).

6. M. Mann, K. Emanuel, EOS 87, 233 (2006).

7. J.B. Elsner, J.P. Kossin, T.H. Jagger, Nature 455, 92 (2008).

8. K.A. Emanuel, Nature 436, 686 (2005).

9. T.R. Knutson, J.J. Sirutis, S.T. Garner, G.A. Vecchi, I.M. Held, Nat. Geosci. 1, 359 (2008).

10. M. Zhao, I.M. Held, S.J. Lin, G.A. Vecchi, J. Clim., 22, 6653 (2009).

11. G.A. Vecchi, K.L. Swanson, J.J. Soden, Science 322, 687 (2008).

12. G.J. Holland, P.J. Webster, Philos. Trans. R. Soc. 365, 2695 (2007).

13. G.A. Vecchi, T.R. Knutson, J. Clim. 21, 3580 (2008).