The Island of Doubt

Rasmus provides an excellent overview of how much we know, and how much we don't know, about the hurricane-climate relationship, which is the subject of a new paper in Nature Geoscience. And the accompanying comments are, as usual, almost as useful as the post itself.

At the heart of the discussion is Rasmus' observation that

Of most fundamental significance to assessing the reliability of these current projections, in our view, is the "junk in/junk out" factor. The detailed projections made using either the RCM [regional climate model ] approach of Knutson et al or the 'random seeding' approach of Emanuel et al, can only be as good as the large-scale scenarios used to drive them. And since key aspects of those large-scale scenarios as far as Atlantic TC [tropical cyclone = hurricane] activity is concerned (i.e. what really happens to the ENSO [El Nino-Southern Oscillation, a natural, irregular climate cycle that sucks heat from the ocean depths and dumps it into the atmosphere] mean state and amplitude of variability) are currently not confidently known, neither can we be confident using the model projections to say what will happen to Atlantic TC activity in the future.

Given that level of uncertainty, Rasmus concludes that

"coastal homeowners, insurers, the re-insurance industry, and every other potential stakeholder in this debate would be wise not to take false comfort from the notion (which the headlines resulting from this paper will inevitably feed) that climate change poses no future Atlantic hurricane threat.

After you've read the entire thing (and as many of the comments as you have time for), only then should you venture into the mainstream media coverage. The AP's Seth Borenstein does an admirable job, for example, but first you have to get past the headlines that various AP subscribers are giving his piece. ABC, sadly, get sthe story completely wrong by topping Borenstein's balance with "Study Says Global Warming Not Worsening Hurricanes."

This despite the fact that the new study, by Knutson, et al, actually forecasts an increase in intensity, along with a decrease in frequency in tropical cyclones because of global warming. And then there's little proviso, which is found not too far past the abstract of the paper: "A limitation of the model is that it does not simulate hurricanes as intense as observed." I'd call that Doubt Alert No. 1.

Another element that should be kept in mind is the fact that Knutson's predictions are not actually new, although the methods by which they were produced are. Which is the subject of Rasmus' analysis. Understanding the limitations of those methods, and those of previous papers, is key to determining how citizens and policy-makers should react.

And of particular interest in the reaction category is the first comment, from one Maiken Winter, who, like me, is a member of Al Gore's Climate Project: slide show army:

Thank you for that! I had cut out all slides on hurricans from my presentation on climate change after reading the Nature Geoscience paper, because my sense of not trusting the conclusion of the paper was not good enough to stand ground in front of a skeptic. I will now immediately put those slides back in. I love your blog - it is absolutely essential for non-climatologists like me to stay informed and well equipped to help educate the public.

This issue has also plagued my own deliberations for each showing of the presentation. Follow the subsequent comments for further discussion on her decision.

And, just in case you were wondering, here's the concluding line of the Knutson abstract:

Here we assess, in our model system, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.

Knutson, T.R., Sirutis, J.J., Garner, S.T., Vecchi, G.A., Held, I.M. (2008). Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience DOI: 10.1038/ngeo202