Last fall we took issue with Kevin Trenberth, NCAR scientist and IPCC lead author responsible for the chapter on hurricanes, when he gave a presentation to policy makers that attributed part of Katrina’s rainfall to greenhouse gas emissions, and suggested that the added rainfall may have caused the New Orleans flooding and damage.

When I blogged this, some folks accused me of misquoting him or “putting words in his mouth.” Fortunately, Kevin has spoken to the media this week in New Zealand on this subject, and we can let his own words speak for themselves. Here is a how the Dominion Press reported Kevin’s thoughts on Katrina:

“Hurricane Katrina, which ripped through New Orleans last year, leaving a repair bill of up to US$200 billion, is a sign that humans ought to be paying more attention, he says. “This is not to say Katrina was due to global warming . . . There is an influence of global warming, something like an 8 per cent influence.

“So if about 305 millimetres of rain falls in New Orleans that means about an extra 25.4mm of rainfall more than might have occurred anyway. Often it’s the straw that broke the camel’s back. Was 25.4mm of extra rainfall enough to cause the levees to break? The big flooding event in Manawatu, that may be an example of the same kind of thing . . .”

Scientists routinely tell us that we cannot attribute the effects of anthropogenic climate change to any one particular event, and while Trenberth does say that we cannot attribute the existence of Katrina to greenhouse gas emissions (GHGs), he does suggest that once it does exist we can attribute precisely 25.4 mm of rainfall to GHGs, and that this difference could have led to the disastrous flooding of New Orleans (as well as another flood event in New Zealand). Here is what we said about this last fall:

Perhaps there are good scientific reasons for making such a claim, but they probably should go through peer review before being announced as fact (with no sense of uncertainty!) before policy makers. Especially when other scientists, like Kerry Emanuel, assert that such precise attribution is not possible. And further, given that the IADWG has published in JOC May 2005 that attribution of trends in precipitation to GHGs has not yet occurred, it stretches the credulity of this non-climate scientist to think that such precise attribution is possible for specific events.

I am a supporter of scientists speaking their minds, and I support Trenberth’s right to say what he believes. Kevin is a prolific scientist who is widely cited in the community. However, on this issue he is way out on a limb, especially in his comments about damage, a subject on which I have some expertise. In a 1999 paper (PDF) we responded to Kevin’s frequent invocation of the straw/camel metaphor to draw a very different conclusion:

With respect to the relative contribution of climate and societal factors to the flood damage record, Trenberth’s (1997) metaphor of increasing precipitation being the ‘‘straw that breaks the camel’s back’’ is particularly apt. The camel (representing society) is already burdened by the weight of past decisions that have placed people and property into harm’s way. Thus, when hydrologic floods do occur, they can lead to enormous damage. In many instances, such damage is avoidable— society need not wait for the ‘‘last straw’’ to act.

Kevin’s statements on hurricane science and impacts are especially troubling because of his role as the lead author of the IPCC chapter on hurricanes. For better or worse, his credibility, and thus that chapter’s legitimacy, is at risk on this particular issue. [Note: Kevin has an open invitation to respond to this post here, which we will gladly publish.] It is important for the climate science and policy communities to recognize the reality that out-on-a-limb statements by leaders in the IPCC community can seriously damage the credibility of the IPCC and the legitimacy of arguments for action on climate change even if, as some assert, such views have no bearing on the content of the report or are tangential to more important reasons for action.