Once Again Attributing Katrina’s Damages to Greenhouse Gases

March 29th, 2006

Posted by: Roger Pielke, Jr.

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.

26 Responses to “Once Again Attributing Katrina’s Damages to Greenhouse Gases”

  1. steve c Says:

    Minor note: do you think Trenberth actually said 12 inches instead of 305mm and 1 inch instead of 25.4mm? The conversions are almost exact, and using inches like that would be both more colloquial and imply much less precision (though still much more than you’re comfortable with, I understand).

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  3. Roger Pielke Jr. Says:

    Steve C.- Thanks .. could be. Though I’d hope that a newspaper would report that like this: “1 inch [25.4 mm]” rather than changing the quote.

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  5. Jim Says:

    I think NZ has been metric for some time so it would make no sense for the paper to report, or Kevin to speak, in terms of inches. As a native, Kevin would be aware of this. So I’m guessing he spoke in terms of “mm”. That’s interesting because I would have just rounded off to 25 mm. BTW, 8 percent of 305 mm is 24.4 mm. Oh well.

    What’s really interesting is that he was using a back-of-the-envelope calculation to make his argument. Pretending for a moment that we knew that GHGs increased total storm rainfall by 8 percent ON AVERAGE, applying that number to a particular storm or to a particular time and place within a storm is pretty hard to justify scientifically.

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  7. Mitch Says:

    Thanks for posting on this. This is a major pet peeve of mine and it disappoints me to see Dr. Trenberth going this route (unless there is a scientific basis for his claims, but I don’t think there is). I completely agree with your points regarding the potential for such public statements to damage the IPCC’s credibility and undermine the legitimacy of arguments for action.
    Best, Mitch

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  9. Steve Bloom Says:

    Roger, what’s really going on here is that you’re part of the NHC/Gray camp that believes the present increase in cyclone strength is the result of a natural cycle. Trenberth, the Curry/Webster team, Emanuel, Knutson and a number of others obviously think otherwise. We’ll see who’s left standing after the remaining papers and comments filter out over the rest of the year, but for now it’s perfectly justified for Trenberth to reduce the AGW component to a percentage of influence as he did. If that is illegitmate, then it’s equally wrong to say that a given cyclone is the result of natural causes since we cannot know the exact influence of such causes for a given cyclone. So, either we speak in terms of cyclones with no causation at all or we do what Trenberth did. There is no chance that this will reflect poorly on the IPCC or the AR4, although we can be confident you’ll keep saying so.

    BTW, I assume you saw Webster’s comment on RC with regard to the Bell+Chelliah paper. Do you think he would make a public statement of that sort without complete confidence in the outcome? I have to say that the strained nature of B+C’s argument was apparent even from the abstract.

    Finally, what happened to your follow-up on the NOAA/NHC gagging? I think you said you took Lautenbacher’s denial of censorship at face value, but as you know Jerry Mahlman disagrees, and said this in response to Lautenbacher: “Contrary to Dr. Lautenbacher’s assertions, I state emphatically that climate scientists within NOAA have indeed recently been systematically prevented from speaking freely. A number of NOAA scientists have directly and openly disagreed with Lautenbacher’s statements that deny his direct connection with censorship of climate science.” It doesn’t take a rocket scientist to figure out that the White House would be deeply unhappy if NHC and other NOAA scientists started speaking openly of a hurriciance-global warming connection, and in particular of a Katrina-global warming connection, during this election year.

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  11. Rabett Says:

    He had to be talking in terms of inches and feet. No one says ABOUT 305 mm, you do say about a foot. No one says ABOUT an extra 25.4 mm. NZ is metric, but inches and feet are rooted into the language (closely related to English).

    It looks to me more like what a copy editor would do than a reporter.

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  13. Roger Pielke Jr. Says:

    Steve- Thanks for your comments. I just spent the past three days at this workshop:


    I am sorry to report that your views of this community bear little correlation with reality, starting with my own views.

    I have yet to identify anyone who claims to have been censored by NOAA leadership, though I’d be interested in hearing from them. If you (or Jerry Mahlman) would like to name names, I’m all ears.


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  15. Steve Hemphill Says:

    I posted this before, but I think it bears repeating here. Katrina’s damage was due to engineering failure, not “an extra inch” of rainfall:

    “relatively weak shear strengths in the clay layer under the inboard toe of the levee allowed the lateral translation of the levee from the floodwall back along a failure plane in the clay layer. The peat layer above the clay did not initiate the failure. This failure mechanism was not anticipated by the design criteria used.”

    and (buried in the above)

    “Flooding in New Orleans from Hurricane Katrina was much worse than models anticipated in part because of nearly 3 ft of subsidence of levees and storm protections in some locations, according an interim report from an expert task force of engineers and scientists released March 10.”

    and, last but not least (obviously not “official”)

    “The U.S. Army Corps of Engineers did a full-scale test 20 years ago on a floodwall like one that collapsed here during Hurricane Katrina, with results that predicted its failure, scientists working on an independent study of the failure said Monday.”

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  17. Rabett Says:


    “Thomas Knutson, a research meteorologist with the agency’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J., said he believes his views have been censored by the NOAA public-affairs office because of his view that global warming could be making hurricanes worse. Last October the public-affairs office said no to a scheduled interview with CNBC television, he said.” -By ANTONIO REGALADO and JIM CARLTON, Wall Street Journal February 16, 2006; Page A4

    Would you now like to debate the issue of what constitutes NOAA leadership?

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  19. Rabett Says:

    More amusement http://www.climatesciencewatch.org/index.php/csw/details/providence-journal/

    “Calls to NOAA’s public-affairs office led to Kent Laborde, who was described as the public-affairs person who focuses on climate-change issues.

    Laborde made it clear that the NOAA has discounted the research tying global warming to worsening hurricanes.

    “What we’ve found is, if you look at a couple segments of science, observational or modeling, there is no illustrated link between climate change and hurricane intensity,” Laborde said. “We actually have periods of intensity followed by periods of lower intensity. We have evidence of periods going back to the 1930s. It follows a clear pattern.”

    Laborde was asked if he would approve an interview with Knutson.

    What is the topic? he asked.

    Emanuel’s theories linking climate change to worsening hurricanes.

    “Chris Landsea would be better. He’s an observational scientist,” Laborde said.

    Landsea is a top meteorologist at the NOAA, often called upon for expert testimony to Congress or to speak at news conferences. He also very publicly quit an international climate-change panel last year, because one of its leaders had publicly linked global warming to hurricane severity.

    At Laborde’s request, Landsea cheerfully discounted Emanuel’s theories in an interview with The Journal….

    Landsea insisted that, although he represents the NOAA, there is no official NOAA stance on the impact of global warming on hurricanes.

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  21. Rabett Says:

    And, of course, it ain’t just hurricanes http://www.ucsusa.org/scientific_integrity/interference/survey-political-interference-at-noaa-fisheries.html

    Sometimes Roger, I think you just try it on.

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  23. Steve Bloom Says:

    Thanks, Eli. Roger, so attendees Bell, Emanuel, Knutson and Landsea all indicated agreement on the science, did they? I’ll look forward to their joint paper. With regard to your views, I said that you a) are part of the NHC/Gray natural cycle camp and b) can be relied upon to keep attacking Trenberth. What exactly did I mischaracterize?

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  25. llewelly Says:

    Am I the only one who finds irony in the fact that Knutson’s models of the effect of AGW on tropical cyclones find a much smaller increase than Emanuel’s analysis of the historical record? It seems to me Laborde’s efforts, whatever their intent, have had the effect of focusing attention on Emanuel’s findings.

    Beyond that, Laborde’s actions seem to have had the unfortunate effect of making Landsea look like a stooge. I don’t mind the focusing of attention on Emanuel’s findings, but while I think Landsea is probably mistaken about the influence of AGW on TCs, I’m don’t care for the uncomplimentary light this casts on Landsea.

    Does science inform policy well when one scientist appears to be co-operating in a bit of sleight-of-hand which prevents another scientist from being interviewed?

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  27. Roger Pielke Jr. Says:

    Eli, Steve B.- A few responses:

    1. Eli, you’re sharing dated materials. Both NASA and NOAA obviously have had some problems. I am sure they will continue to do have issues on the PR front, but Jim Hansen has gone back to work satisfied with NASA’s response, and since Lautenbacher’s statement, no one in NOAA I have spoken to has complained. Those earlier news articles will be online forever, but reality will move on.

    2. Steve, there is nothing sinister in scientific disagreements about complicated subjects. One difference between how these various hurricane researchers interact with one another from your apparent views is that they give each other the respect of knowing that each is sincere and focused on understanding the science. As much fun as it might be to have good guys/bad guys, here we just have a bunch of smart people with different views. And there is no crime in that, it is in fact a strength of science.

    3. For my views on hurricanes/climate, please have a look at our BAMS paper from last year.

    4. The irony here of course is that both of your efforts to discredit Landsea and others in NOAA who you for some odd reason have some antagonism towards represents a threat to open scientific exchange. If you’d like to debate their scientific ideas, that is one thing, but if you’d like to discredit through sinister implication that is another. That I see nothing here from you except the latter suggests a bit of kettle-pot when it comes to support for science.

    It is not just the Bush administration which holds the irrational view that politically uncomfortable science should be supressed!!


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  29. Roger Pielke Jr. Says:


    Thanks for your comments. The large discrepancy between theoretical understandings (which inform the creation of climate models to look at hurricanes) and recent observational studies (e.g., Emanuel 2005) was a big focus of attention at the Lamont meeting. Everyone there agreed that the discrepancy exists and needs to be addressed.

    Tom Knutson nicely presented the options while not advocating one over another:

    1. The data is bad
    2. The theory is wrong
    3. Some part of 1 + 2

    The folks at the meeting vigorously discussed all of 1, 2, and 3. Emanuel presented some new thinking on theory, while also saying that there are indeed some data problems. Landsea explained some concerns about data, while also saying that Emanuel’s new work presents a new perspective on the theory.

    Landsea also made a strong statement about scientific openness and criticized NOAA. It would be unfair and uniformed to characterize his actions otherwise.


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  31. Chris Mooney Says:

    Great to meet you at the Lamont event.

    Irrespective of Trenberth’s claim (which I’m not fit to evaluate), I wonder whether we will ever be able to attribute any fraction of the impact of a single storm to GW. It seems to me that this might be possible with storm surge in a world with higher sea levels due to GW, but maybe I’m looking at the matter too simplistically. Interested in your reaction:

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  33. llewelly Says:

    Roger, thank you for description of the discussion that went on at the Lamont meeting. To me, that’s very good news all around.

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  35. Jim Says:

    Regarding the attribution of GW to Katrina, or more specifically, the warmer Gulf waters to Katrina, has anyone looked at this with a model instead of an envelope. It seems like you could take some flavor of the MM5 model, run a simulation with the actual SSTs from Katrina. Then do the same simulation with long-term average SSTs and compare the differences. Any differences could then be attributed to the elevated SSTs of last year. I realize it’s a little trickier than that to set up but it seems like an obvious first step in resolving this issue.

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  37. Steve Bloom Says:

    Roger, regarding your point 4 above, recall that it was Chris Landsea who took this whole thing negative when he used his IPCC resignation letter to attack Trenberth. Given how recent Mahlstrom’s comments are (you say in effect “what does he know, he was only the head of GFDL for 15 years” — huh?) and Judy Curry’s slightly less recent ES&T remarks about the negative role played by you (among others), I think your papering-over exercise is less than a success. As for the failure of concerned NOAA folks to contact you, why in the world would any of them trust you with their careers? I sure wouldn’t.

    All of that said, I’m not at all interested in seeing public fights among hurricane scientists, but your campaign to pretend that the differences over the science are less than they are is getting a little tired.

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  39. Roger Pielke Jr. Says:

    Steve Bloom- You are welcome to post here. However, continued misrepresentations of my views will be ignored. You’ve made your views known. Thanks.

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  41. Rabett Says:

    Come on Roger, the Providence Journal article was dated March 26, five days ago, that was really dated man. The Wall Street Journal article was dated February 16, a whole month and a half ago, ancient man, ancient. The article about how NOAA is manipulating science about fisheries was June 2005, 10 months, but what do you want to bet there will be similar articles this summer when water is low in the NW. Here is another from Nov 2004 http://www.fishsniffer.com/dbachere/041104noaa.html.

    And I take it you have heard directly from Jim Hansen that he is fully satisfied. Funny, my grapevine says different.

    Since this is a family blog I won’t use the barnyard description of what you are trying to do.

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  43. Roger Pielke Jr. Says:

    Eli- Yes, actually a lot of people heard from Jim Hansen on this. Here you go:

    Date: Mon, 13 Mar 2006 18:11:54 -0500
    From: James Hansen
    To: jhansen@giss.nasa.gov
    Cc: jhansen@giss.nasa.gov
    Subject: “Political Inclinations” and “Back to Science”

    To be removed from Hansen’s e-mail distribution list, respond to sender with “Remove” as the subject.

    “Political Inclinations”
    Following statement was placed on my web site in response to journalist requests and (moderate number of) e-mails/letters from the public (not colleagues).


    “Back to Science”

    Dear Colleague,
    Last December, following a talk on global warming that I gave at the AGU meeting, NASA Public Affairs attempted to place rigid constraints on my ability to communicate with the media and the public. When I objected publicly to their proposed constraints, and ignored them, I received several offers of pro bono counsel. I accepted advice from the Government Accountability Project (GAP), headed by Louis Clark. I especially appreciate the practical, insightful guidance of GAP Legal Director Tom Devine with regard to my legal rights of free speech and how to protect myself in exercising them. I urge anybody with similar concerns to contact GAP.
    The situation in NASA regarding free speech (albeit not science funding) is promising. There is no doubt that Administrator Griffin recognized the problem, fully supports openness and free speech, and intends to have supportive rules and procedures. If implementation by Public Affairs differs from that spirit, you will hear about it.
    NOAA’s Admiral Lautenbacher also expressed support for openness. I am unaware of whether actions are being taken to insure free speech in NOAA. It will be interesting to query NOAA colleagues to see if there is still selective use of government ‘minders’ to monitor interactions with the media on topics such as global warming. The situation in EPA, where double-speak (“sound science”, “clear skies”, …) has achieved a level that would make George Orwell envious, is much bleaker, based on the impression that I receive from limited discussion with colleagues there. The battle to achieve open communication between government scientists and their employer, the public, is far from won.
    Nevertheless, I agree with the opinion of colleagues that the focus should be on discussing solutions to global warming. Unless some new event demands it, I am not going to participate further in “whistle-blower”
    activities. In particular, I decline the generous offer of GAP and Rick Piltz to share in a whistle-blower award and ceremony in April. I think that it would be most useful and effective for the spotlight to shine brightly on Rick Piltz and EPA, where the rubber meets the road.
    My personal aim is to get back to science research full time, especially on quantifying options for dealing with global warming. Sincere apologies for overdue book reviews, workshop reports, and science manuscripts. As of now, I am working on these.
    Best regards, Jim Hansen

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  45. Roger Pielke Jr. Says:


    Committee on Science
    Bart Gordon, Tennessee, Ranking Democrat

    Press Contacts:
    Joe Pouliot
    (202) 225-4275


    WASHINGTON, March 30, 2006 – House Science Committee Chairman Sherwood Boehlert (R-NY) today issued the following statement in response to the release of a new public affairs policy for NASA:

    “I am very pleased with NASA’s efforts to develop a clear public affairs policy that puts a premium on open communication. This policy should become a model for the entire federal government. I am particularly pleased to see that the policy allows scientists to communicate their views as long as they make clear they are not speaking for the agency. I applaud Administrator Mike Griffin and Deputy Administrator Shana Dale for acting swiftly and forcefully when presented with information indicating that NASA scientists were not always able to speak their minds. I also greatly appreciate their efforts to keep Congressman Bart Gordon and me up-to-date as the policy was being drafted. Obviously, the full test of the policy will occur once it is implemented. I will continue to carefully review NASA’s activities to ensure that the policy truly results in an agency that fully complies with the principles of openness that are stated in the policy.”

    # # #


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  47. Roger Pielke Jr. Says:

    New NASA information policy:


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  49. Roger Pielke Jr. Says:

    Lest this thread be completely hijacked, we can now return to regularly scheduled programming;-)

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  51. Rabett Says:

    Roger, you got suckered. Hansen said in that Acrobat link:

    “The constraints placed on scientists, preventing them from informing the public about their concerns, is much worse in places such as the National Institutes Health and the Environmental Protection Agency than it is in NASA. My quotation on 20 April 2006 Freedom Forum calendar, In my more than three decades in government, I have never seen anything approaching the degree to which information flow from scientists to the public has been controlled as it is now”, is accurate.”

    If anyone wants to continue elsewhere http://tinyurl.com/ekabt