In 2000, Dan Sarewitz, Bobbie Klein and I published a paper titled “Turning the Big Knob: Energy Policy as a Means to Reduce Weather Impacts” (PDF) in which we calculate the relative sensitivity of future tropical cyclone damages to the independent effects of changes in storm behavior under climate change to changes in societal vulnerability. For the changes in both storm behavior and societal vulnerability we used the assumptions of the IPCC. Brian Schmidt, who works for an environmental organization in San Francisco, and who occasionally has visited our website with always-thoughtful comments, has taken the time to write up a critique of our paper and post it on his blog. We appreciate the engagement. Brian graciously asked us for a response, so here it is.


First a correction, Schmidt states that “Climate will increase hurricane costs by about 40-50%, according to the studies cited in the paper.” This is what those early-1990s studies said, however, they used assumptions about changing hurricane intensity that were later revised downward by the IPCC. Using in the damage projection methods the actual numbers cited by the IPCC SAR for changing “maximum potential intensity” of tropical cyclones the projected increase in damages due only to the effect of changes in storms related to climate change is in fact about 8-10%. For close followers of this subject the numbers used by the SAR were about twice those projected in a subsequent paper by Knutson and Tuleya. This error is repeated throughout Schmidt’s post. It is worth noting that the general conclusions are pretty much insensitive to an error of this magnitude.

Schmidt offers three critiques of our paper. First, he writes, “it ignores the combined effects of climate change and increased economic value.” We have heard this before. The response is that we are performing a sensitivity analysis and not generating a prediction of future damages. Our point in this paper was to identify the independent effects of climate change versus societal change. It Schmidt would like to go ahead and add another factor that recognizes the fact that future damages will indeed be the result of the combined effects, he should go right ahead. This does nothing to change the fact that the independent effect of societal changes is larger than the independent effect of climate change by a factor of between 22 to 1 and 60 to 1. This is the case if combined effects are included or not.

Schmidt offers up this as a second critique, “Another major problem is the implicit assumption that by controlling land-use, one can in effect relocate away from global warming.” We said no such thing. What we did say is that if societal factors are far more responsible than climate change for the expected growing impacts of tropical cyclones, then from a policy perspective it is only logical that decisions related to societal vulnerability are likely to have greater potential to address those impacts. We discuss land use, but also forecasts and warnings (to save lives), reducing environmental degradation, enforcement of building codes and other policies. Relocation is not something that we discuss.

Schmidt’s third critique is even less appropriate, “A third major problem parallels the problem with Bjorn Lomborg’s critique of a lack of economic analysis over global warming, particularly a lack of cost-benefit analysis.” We did not conduct a cost-benefit analysis, nor did we claim to, so it is hard to respond to this. We conducted a sensitivity analysis to ascertain where policy makers might have the greatest ability to influence future hurricane damages. What we found is that using the “big knob” of tuning global atmospheric carbon dioxide concentrations is unlikely to be able to affect anything more than a very small portion of future tropical cyclone losses, and that this finding is very robust under all combinations of scenarios of climate change and societal change present in the IPCC SAR.

Schmidt offers three other minor critiques:

“ Damage estimates are artificially low because they come primarily from a time when hurricanes were at a low point in their long-term cycle. Recalibrating the damages to include more intense hurricane cycles we are experiencing now would give much larger figures.

Let’s accept this as true. It is irrelevant. Our analysis focused on how damages would increase as a multiple of a particular base year. Changing the base has no effect on the multiple, and has no effect on our sensitivity analysis.

“It’s dated. It’s from 2000, and relies extensively on 1996 IPCC assessments, which rely on still earlier studies. This isn’t a flaw of the study itself, but rather than relying on it, the work should be done again with updated information.”

It is a 2000 paper. Nonetheless the analysis holds up exceedingly well as there has nothing that has occurred related to scientific understandings of tropical cyclones or projections of societal vulnerability that would change our basic conclusions. We do intend to update this analysis using the assumptions of the IPCC AR4 (we do have funded research on such sensitivity analyses under our SPARC project.). Better yet would be if the IPCC itself did such a sensitivity analysis, not just for tropical cyclones, but for every climate impact that is the joint function of climate and societal factors.

“It ignores costs resulting from redirecting land use.

We don’t claim to be doing a cost-benefit analysis, nor do we discuss relocation.

The bottom line is that the analysis is robust under a wide range of realistic and unrealistic scenarios for climate change and societal change. Even if we were to simply assume that the IPCC SAR underestimated changes in future tropical cyclone intensity by 100% or 200% the qualitative implications of our paper would remain unchanged. I appreciate Schmidt’s comments that changing land use behavior is difficult. But the reality is that there is no basis for expecting that a global energy policy focused on stabilizing greenhouse gas offers a meaningful tool with which to modulate future tropical cyclone damages.

The lure of a “big knob” that can be tuned to an ideal state is indeed appealing, but in the case of tropical cyclones the sooner we recognize that effective policy will take place on the ground in thousands of vulnerable locations around the world that experience damage, then the more effective policy responses will be.