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Hurricanes and Global Warming – Is There a Connection?

Filed under: — group @ 2 September 2005 - (Français) (Español)

by Stefan Rahmstorf, Michael Mann, Rasmus Benestad, Gavin Schmidt, and William Connolley

On Monday August 29, Hurricane Katrina ravaged New Orleans, Louisiana and Missisippi, leaving a trail of destruction in her wake. It will be some time until the full toll of this hurricane can be assessed, but the devastating human and environmental impacts are already obvious.

Katrina was the most feared of all meteorological events, a major hurricane making landfall in a highly-populated low-lying region. In the wake of this devastation, many have questioned whether global warming may have contributed to this disaster. Could New Orleans be the first major U.S. city ravaged by human-caused climate change?

The correct answer–the one we have indeed provided in previous posts (Storms & Global Warming II, Some recent updates and Storms and Climate Change) –is that there is no way to prove that Katrina either was, or was not, affected by global warming. For a single event, regardless of how extreme, such attribution is fundamentally impossible. We only have one Earth, and it will follow only one of an infinite number of possible weather sequences. It is impossible to know whether or not this event would have taken place if we had not increased the concentration of greenhouse gases in the atmosphere as much as we have. Weather events will always result from a combination of deterministic factors (including greenhouse gas forcing or slow natural climate cycles) and stochastic factors (pure chance).

Due to this semi-random nature of weather, it is wrong to blame any one event such as Katrina specifically on global warming – and of course it is just as indefensible to blame Katrina on a long-term natural cycle in the climate.

Yet this is not the right way to frame the question. As we have also pointed out in previous posts, we can indeed draw some important conclusions about the links between hurricane activity and global warming in a statistical sense. The situation is analogous to rolling loaded dice: one could, if one was so inclined, construct a set of dice where sixes occur twice as often as normal. But if you were to roll a six using these dice, you could not blame it specifically on the fact that the dice had been loaded. Half of the sixes would have occurred anyway, even with normal dice. Loading the dice simply doubled the odds. In the same manner, while we cannot draw firm conclusions about one single hurricane, we can draw some conclusions about hurricanes more generally. In particular, the available scientific evidence indicates that it is likely that global warming will make – and possibly already is making – those hurricanes that form more destructive than they otherwise would have been.

The key connection is that between sea surface temperatures (we abbreviate this as SST) and the power of hurricanes. Without going into technical details about the dynamics and thermodynamics involved in tropical storms and hurricanes (an excellent discussion of this can be found here), the basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes. This is why they only arise in the tropics and during the season when SSTs are highest (June to November in the tropical North Atlantic).

SST is not the only influence on hurricane formation. Strong shear in atmospheric winds (that is, changes in wind strength and direction with height in the atmosphere above the surface), for example, inhibits development of the highly organized structure that is required for a hurricane to form. In the case of Atlantic hurricanes, the El Nino/Southern Oscillation tends to influence the vertical wind shear, and thus, in turn, the number of hurricanes that tend to form in a given year. Many other features of the process of hurricane development and strengthening, however, are closely linked to SST.

Hurricane forecast models (the same ones that were used to predict Katrina’s path) indicate a tendency for more intense (but not overall more frequent) hurricanes when they are run for climate change scenarios (Fig. 1).

Figure 1. Model Simulation of Trend in Hurricanes (from Knutson et al, 2004)

In the particular simulation shown above, the frequency of the strongest (category 5) hurricanes roughly triples in the anthropogenic climate change scenario relative to the control. This suggests that hurricanes may indeed become more destructive (1) as tropical SSTs warm due to anthropogenic impacts.

But what about the past? What do the observations of the last century actually show? Some past studies (e.g. Goldenberg et al, 2001) assert that there is no evidence of any long-term increase in statistical measures of tropical Atlantic hurricane activity, despite the ongoing global warming. These studies, however, have focused on the frequency of all tropical storms and hurricanes (lumping the weak ones in with the strong ones) rather than a measure of changes in the intensity of the storms. As we have discussed elsewhere on this site, statistical measures that focus on trends in the strongest category storms, maximum hurricane winds, and changes in minimum central pressures, suggest a systematic increase in the intensities of those storms that form. This finding is consistent with the model simulations.

A recent study in Nature by Emanuel (2005) examined, for the first time, a statistical measure of the power dissipation associated with past hurricane activity (i.e., the “Power Dissipation Index” or “PDI”–Fig. 2). Emanuel found a close correlation between increases in this measure of hurricane activity (which is likely a better measure of the destructive potential of the storms than previously used measures) and rising tropical North Atlantic SST, consistent with basic theoretical expectations. As tropical SSTs have increased in past decades, so has the intrinsic destructive potential of hurricanes.

Figure 2. Measure of total power dissipated annually by tropical cyclones in the North Atlantic (the power dissipation index “PDI”) compared to September tropical North Atlantic SST (from Emanuel, 2005)

The key question then becomes this: Why has SST increased in the tropics? Is this increase due to global warming (which is almost certainly in large part due to human impacts on climate)? Or is this increase part of a natural cycle?

It has been asserted (for example, by the NOAA National Hurricane Center) that the recent upturn in hurricane activity is due to a natural cycle, e.g. the so-called Atlantic Multidecadal Oscillation (“AMO”). The new results by Emanuel (Fig. 2) argue against this hypothesis being the sole explanation: the recent increase in SST (at least for September as shown in the Figure) is well outside the range of any past oscillations. Emanuel therefore concludes in his paper that “the large upswing in the last decade is unprecedented, and probably reflects the effect of global warming.” However, caution is always warranted with very new scientific results until they have been thoroughly discussed by the community and either supported or challenged by further analyses. Previous analysis of the AMO and natural oscillation modes in the Atlantic (Delworth and Mann, 2000; Kerr, 2000) suggest that the amplitude of natural SST variations averaged over the tropics is about 0.1-0.2 ºC, so a swing from the coldest to warmest phase could explain up to ~0.4 ºC warming.

What about the alternative hypothesis: the contribution of anthropogenic greenhouse gases to tropical SST warming? How strong do we expect this to be? One way to estimate this is to use climate models. Driven by anthropogenic forcings, these show a warming of tropical SST in the Atlantic of about 0.2 – 0.5 ºC. Globally, SST has increased by ~0.6 ºC in the past hundred years. This mostly reflects the response to global radiative forcings, which are dominated by anthropogenic forcing over the 20th Century. Regional modes of variability, such as the AMO, largely cancel out and make a very small contribution in the global mean SST changes.

Thus, we can conclude that both a natural cycle (the AMO) and anthropogenic forcing could have made roughly equally large contributions to the warming of the tropical Atlantic over the past decades, with an exact attribution impossible so far. The observed warming is likely the result of a combined effect: data strongly suggest that the AMO has been in a warming phase for the past two or three decades, and we also know that at the same time anthropogenic global warming is ongoing.

Finally, then, we come back to Katrina. This storm was a weak (category 1) hurricane when crossing Florida, and only gained force later over the warm waters of the Gulf of Mexico. So the question to ask here is: why is the Gulf of Mexico so hot at present – how much of this could be attributed to global warming, and how much to natural variability? More detailed analysis of the SST changes in the relevant regions, and comparisons with model predictions, will probably shed more light on this question in the future. At present, however, the available scientific evidence suggests that it would be premature to assert that the recent anomalous behavior can be attributed entirely to a natural cycle.

But ultimately the answer to what caused Katrina is of little practical value. Katrina is in the past. Far more important is learning something for the future, as this could help reduce the risk of further tragedies. Better protection against hurricanes will be an obvious discussion point over the coming months, to which as climatologists we are not particularly qualified to contribute. But climate science can help us understand how human actions influence climate. The current evidence strongly suggests that:
(a) hurricanes tend to become more destructive as ocean temperatures rise, and
(b) an unchecked rise in greenhouse gas concentrations will very likely increase ocean temperatures further, ultimately overwhelming any natural oscillations.
Scenarios for future global warming show tropical SST rising by a few degrees, not just tenths of a degree (see e.g. results from the Hadley Centre model and the implications for hurricanes shown in Fig. 1 above). That is the important message from science. What we need to discuss is not what caused Katrina, but the likelyhood that global warming will make hurricanes even worse in future.

_____________________
1. By ‘destructive’ we refer only to the intrinsic ability of the storm to do damage to its environment due to its strength. The potential increases that we discuss apply only to this intrinsic meteorological measure. We are not taking into account the potential for increased destruction (and cost) due to increasing population or human infrastructure.

References:

Delworth, T.L., Mann, M.E., Observed and Simulated Multidecadal Variability in the Northern Hemisphere, Climate Dynamics, 16, 661-676, 2000.

Emanuel, K. (2005), Increasing destructiveness of tropical cyclones over the past 30 years, Nature, online publication; published online 31 July 2005 | doi: 10.1038/nature03906

Goldenberg, S.B., C.W. Landsea, A.M. Mestas-Nuñez, and W.M. Gray. The recent increase in Atlantic hurricane activity. Causes and implications. Science, 293:474-479 (2001).

Kerr, R.A., 2000, A North Atlantic climate pacemaker for the centuries: Science, v. 288, p. 1984-1986.

Knutson, T. K., and R. E. Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17(18), 3477-3495.


317 Responses to “Hurricanes and Global Warming – Is There a Connection?”

  1. 101
    Michael Jankowski says:

    Re#98, Katrina is a tough example to use. Essentially the same $$$ amay have occurred had a much weaker storm hit. If the levees had held (and information I’ve seen said the levees were damaged by barges and that they were supposed to hold-up even to a Category 5 without breaching), the damage/cost would be immensely lower. Maybe I’m short-changing the damage Katrina did elsewhere (I’m under the impression the New Orleans devastation is going to be the bulk of the hurricane damage and that, in turn, the bulk of the New Orleans devastation was due to the post-storm levee failure and not the storm itself).

    Re#99, The future is supposed to be a warmer and subsequently wetter world due to AGW, so I’d say “wetter” is what you would call primary. Yet models seem to suggest many areas of the earth will become much drier, thanks to changing weather patterns (althought they often disagree on which areas will warm the most, which ones will get the wettest, etc). So obviously being “secondary” can have some substantial effects according to our climate models.

    Maybe you don’t like that climate-related analogy, so let’s talk about hurricanes specifically. As stated here http://www.sciencedaily.com/releases/2001/09/010924062015.htm : “During the 112-year period for which the NC State researchers have data, an average of 3.23 tropical cyclones pounded the East Coast each year. During El Nino years, that number dropped to an average of 2.47 storms. North Carolina saw an overall average of 0.81 tropical cyclones annually, and 0.56 during El Nino years.” We also know that we tend to have fewer hurricanes in general in the Gulf and Atlantic during El Ninos in general, not just East Coast landfalls. There are those who theorize global warming will lead to more frequent El Ninos (and the evidence suggests El Ninos became more frequent in recent decades) – and, hence, fewer Gulf and Atlantic hurricanes to worry about assuming everything else stays the same. Some even suggest El Nino will start lasting for periods like 18 yrs instead of 18 months! And as reported elsewhere and on this thread, there doesn’t seem to be any evidence of increased hurricane frequency overall due to 20th century warming. If AGW doesn’t intrinsically increase hurricane frequency (just strength) but does increase El Nino events, then we should see a decrease in Gulf and Atlantic hurricanes and landfalls. Going by the East Coast data above and assuming El Nino conditions account for roughly 1 yr out of every 4 in that 112 yr study period, you end up with about 40% less hurricanes under El Nino conditions. So if El Nino conditions become more frequent, I’d say the evidence suggests significant reductions in hurricane frequency and landfall. Will such a reduction negate the impact of stronger hurricanes, or possibly even overwhelm it? I don’t know, but I don’t see any evidence to suggest pooh-poohing it as “secondary” and assuming it’s inconsequential, either, as you seem to suggest doing.

    Re#100, I’d like to see the cost comparison between the results of Katrina^1.5, Katrina, and, say, Katrina^0.5. While the destruction outside of New Orleans may have been much different under each, I’m not sure the destruction of New Orleans itself wouldn’t have been basically the same (almost certainly wouldn’t have been much higher in New Orleans under Katrina^1.5, disputable in Katrina ^0.5 because of the levee-break situations), and I am under the impression (whether correctly or incorrectly) that the devastation of New Orleans is going to be the bulk of the “cost” of Katrina. So I’m not sure those costs are significantly different from each other (assuming the levees fail in each scenario).

    I found this statement of yours particularly interesting: “Katrina going 30 miles in one direction can change the cost of storm damage in 2005 by an order of magnitude without changing the intensity of the storm at all.” This was EXACTLY the major point I was making earlier about trying to assing a x% cost of Katrina’s damage to AGW and what the 55vs65vs70mph driving analogy was trying to demonstrate! Small changes in hurricane paths, as you indicated, drastically affect of the amount of devastation, and intensity is not the only major issue. So unless you feel AGW couldn’t/wouldn’t influence the path of a hurricane from formation to landfall by more than a few net miles along the coastline (which I have a very hard time believing…the width of paths of hurricane forecasts 48 hrs in advance often range in the scale of hundreds of miles alone), then you’ve been in agreement with my car-driving analogy all along! So what exactly have you been attacking it for?

  2. 102
    Mark Bahner says:

    Roger Pielke Jr. writes, “Don’t get me wrong, we should be taking more effective actions to decarbonize the global energy system.”

    According to Jesse Ausubel at Rockefeller University, we’re ALREADY decarbonizing the global energy system at a rate that will essentially *eliminate* carbon dioxide emissions by the end of this century:

    http://phe.rockefeller.edu/BrainNotChange/

    What’s more, methane atmospheric concentrations (judged the second-most-influential greenhouse gas) have ALREADY plateaued:

    http://www.greenhouse.gov.au/science/hottopics/pubs/topic10.pdf

    On your Prometheus weblog, Dr. Pielke, you asked for opinions on what the effect would be of cutting CO2 emissions to *zero, today* (ignoring that this is impossible). I responded that the earth would warm by about 0.5 degrees Celsius in the 21st century if worldwide emissions of CO2 were cut to zero today.

    I also noted that the warming that could be expected from “business as usual” would be about 1.2 degrees Celsius. (Such a warming is completely compatible with CO2 emissions estimates that would result from Jesse Ausubel’s projected CO2 decarbonization trend.)

    So I have some questions for you:

    1) Do you accept those estimates (0.5 degrees Celsius warming for cutting CO2 immediately to zero, and 1.2 degrees Celsius for “business as usual”)?

    2) If you do not accept those estimates, which do you think is too high or too low?

    3) If you DO accept those estimates, why do you think “we should be taking more effective actions to decarbonize the global energy system”?

  3. 103
    Pat Neuman says:

    Hurricanes, floods, other severe weather events and droughts often have regional or local economic impacts. Katrina’s impact is national and world. More hurricanes with national and world impacts are likely as global warming continues. Widespread heat waves, worse than Aug 2003 in Europe, will become numerous and more severe in the years and decades ahead, having critical national and world economic impacts. More importantly, what will the impacts be to Earth’s vegetation, needed for survival of all animals on the planet?

  4. 104
    vaughan thomas says:

    Re: 97

    Perhaps this is too basic, but I can’t see why discussing loaded dice isn’t “science” in terms of climate. Climate itself is an abstract, statistical concept, is it not? Climate as I understand it is the average of the weather conditions–it is inherently statistical. Otherwise, aren’t we just talking about the weather?

    [Response: Climate is more correctly the statistics of weather, rather than just the average. It is certainly a statistical concept, but I wouldn't see it as an abstract one - William]

  5. 105
    Lynn Vincentnathan says:

    RE #100, I just read an article about “How GW Will Burn Your 401K” in PLENTY (Oct/Nov 2005). So in addition to that recession, they write: “When a company has to pay for its direct ecological impact, compliance with environmental regulations, and higher energy costs–or even, in some cases, its reputation as a polluter and lawsuits that arise from its un-green practices–its stock price can plummet.”

    Re Katrina, I have been slow to blame people, because I don’t think anyone really understood just how intricate & interconnected everything is. Take us computer people, we might go bezerk with simply a computer failure. All systems failing and being wiped out in an area, even if people are left alive, is something we can’t calculate very well before hand. Now, in hindsight, there will be calcuations & an awareness of reverberating impacts.

  6. 106

    #101 comment about #99

    I am humbly inferior to model calculations per second, but current models lack resolution in a spatial sense, therefore our brains can outmatch them with concepts. The greatest inspiration for heat and moisture AGW effects can be seen daily on satellite animations with particular attention to the equator and the polar regions, the equator ocean air surface plays a daily explosion of convective clouds, almost like a pulse, while in the polar regions most clouds are flat and grey covering a wider geographical area. Convective activity needs stable air above, while in the Arctic the air is more chaotic with all kinds of winds and moisture aloft, truly lacking also heat and therefore convective clouds. For a perspective of a world warming , differential heat from zone to zones , the heat engine of the atmosphere, will change an adjust itself in a new balance regulated by the flow between warm and cold zones. Conceptually wise, it may be a mistake to say that when GW takes effect the air will be drier, but rather water vapour density increases with temperature and relative humidity will drop accordingly, it is perhaps this is what the models are calculating. It is another fallacy to say that there will be more clouds driven by AGW because of more evaporation, for the same reason R. Humidity should drop creating less clouds, reducing cloud albedo making the earth even more warmer. Hurricanes merely reflect the state of energy transferring from ocean to air, by way of water vapour, and are very good indicators of the state of energy of atmospheres in some near tropical locations. The greater the water vapour density of air, the more potential energy it has.

    [Response: The situation is too complex to allow simple reasoning to tell you which way the clouds will go. As it happens, just about all models predict that RH will stay about the same - William]

  7. 107
    Chuck Kutscher says:

    I am curious as to what sort of increase in Dr. Emanuelâ??s power dissipation one might see during an earlier peaking period in the Atlantic Multi-decadal Oscillation (e.g., from 1920 to 1940). Are there sufficient data to calculate this? This could tell us if the recent large rate of increase in power dissipation is unprecedented and thus likely linked to global warming.

    [Response: Kerry Emanuel's starting date (1950) is in the middle of the previous maximum of the AMO (see Sutton and Hodson, Science, 1 July 2005). Also, his SST data (see our Fig. 2 above) go back to 1930, thus covering the whole previous warm phase of the AMO. Assuming the high correlation of PDI and SST holds up, this suggests that current values are indeed unprecedented. -stefan]

    Regarding the question of why Dr. Emanuelâ??s increase in power dissipation over the last 30 years is greater than climate models would suggest, could this be due to the increases in subsurface sea temperatures reported by researchers at Scripps and Goddard? As Dr. Emanuel suggests in his paper, large hurricanes significantly churn up the ocean and bring subsurface, cooler water to the surface (and vice-versa). This cooler water should serve as a negative feedback mechanism and limit the hurricaneâ??s power. But if the subsurface water is now also warmer as a result of global warming-induced surface heat being transported downward by diffusion and mixing, this will allow a hurricane to continue to build in intensity. (It seems conceivable that the cooler subsurface water down to the hurricane mixing level has taken many years to capture the surface heating, but now that it has, we could experience rapidly increasing hurricane intensities.) Do any of the models account for the latest subsurface temperature profiles by Barnett et al. of Scripps and hurricane-induced mixing?

  8. 108
    Matthias Brun says:

    Just three weeks ago there was a century flooding along the alpine countries of Europe – sadly, it was the second so-called ‘century flooding’ in only 6 years. This means that it might be only a 6-year flooding instead of a 100 year one. Only one week later New Orleans is flooded by hurricane “Katrina” and apparently it just lost some power before impact. Knowing that the frequencies and intesities of flooding and the hurricanes are increasing do we really want to know what a real big incidence looks like under the New circumstances of climatic change? If so, then good night! I believe that it is time to
    wake up for everyone and think about what is really important to us. How about parting with growth and competitive thinking. How about saying good-bye to ego-materialism and beginning to shift in the direction of values likecollaboration and group spirit. How about we distribute work and wealth, getting more time for personal development and society, use less fuel, leave your car at home or get a smaller one and turn off television. Mother nature will reward us. The Knowledge has been there for quite some time, it is now time for mankind to take its responsibility.
    Matthias Brun, world citizen from Thun, Switzerland

  9. 109
    Matze Lasik Hunter says:

    Sometimes, the near term damage reduction potential from climate action is heavily oversold, when it should be clear that this is a long term issue with virtually all the pay-off of any action today and over the next two decades to be found later this century.

  10. 110
    Srini says:

    That a scientific analysis is being pursued to learn from Katrina is itself reassuring. Man has studied nature scientifically and achieved incredible moon trip, mars mission etc using science. Katrina may or may not have been caused by global worming, but two facts are self evident; one – that taking risks with continued abuse of nature till the abuse is proven guilty may not be a wise policy guaranteed to leave a safe globe for future generations, and, two – intensive research on cause and effects of global worming and on the remedial steps to sustain life harmonious with nature are, today, priority ONE for mankind.

  11. 111

    Fascinating discussion. The Why Files covered this topic in its most recent edition:
    http://whyfiles.org/227warm_hurricane/

  12. 112
    Tom Fiddaman says:

    Re #102

    > According to Jesse Ausubel at Rockefeller University, we’re ALREADY decarbonizing the global energy system at a rate that will essentially *eliminate* carbon dioxide emissions by the end of this century:

    Ausubel makes no such claim. In his own writings he says: “… the long-term, global rate of decarbonization is about 0.3 percent per year–gradual, to be sure, but enough to cut the ratio by 40 percent since 1860.” [Liberation of the Environment] That’s a decrease in intensity, not absolute emissions. I think that may be a typo, since he’s quoting Nakicenovic just above. Nakicenovic writes, “The average annual rate of decline is about 1.3 percent, meaning that every year about 1.3 percent less carbon is emitted to generate one dollar of value added. … At an average decarbonization rate of 1.3 percent per year, global CO2 emissions will increase about 1.7 percent annually, assuming the economic growth rate remains about 3 percent per year. … Thus, to stabilize global emissions at some (higher) level in the future, the decarbonization rate would have to at least double to offset the current rate of economic growth.” [Grubler, Nakicenovic & Nordhaus, Technological Change and the Environment, RFF & IIASA, 2002] So, depending on who’s number you prefer, extrapolating decarbonization results in emissions doubling in 26 or 40 years – a far cry from elimination by the end of the century.

    In any case, extrapolation is a risky business. In particular, magical increases in the decarbonization rate are highly suspect given that major reliance on coal is the de facto plan of the US and China in response to limited resources and that major exploitation of gas (e.g. hydrates) is highly uncertain, as is nuclear power.

    If you want to see technology unleashed on decarbonization, try putting a value on carbon.

  13. 113

    re 106, Hi William,

    Studying satellite data would say otherwise, is there not an increase in cloudiness in the winter for any Hemisphere compared with summer? RHumidity increases in winter because water vapour density decreases hence more clouds….. GW Models keeping same RH contradict the conclusion that models predict drier air.. Same RH near sea surface perhaps, but not above.

    [Response: I'm not sure about seasonal changes, I was talking about long-term trends deseasonalised. Winter/summer differences are probably due to a slightly different mechanism (guessing) - William]

  14. 114
    TCO says:

    What are the rules on commenting? Is Steve allowed to come over here and argue points? Why are comments cut off after a certain date of the post (and what is that duration)?

    Response: Anyone can comment as long they follow the comment policy. Discussion closes after a month generally or earlier if we feel it’s necessary. -gavin]

  15. 115
    Bernie Masters says:

    Excellent interaction between all contributors to this site which I’ve just visited for the first time.
    A recent report quoted in Science (26 August, 2005 – page 1302) refers to a study of the March 2004 cyclone in the South Atlantic that turned into a hurricane and struck the southern coast of Brasil at about latitude 27 degrees south. This was the first hurrican ever reported in the South Atlantic.
    The authors of the report in Geophysical Research Letters state that the hurricane developed because of an unusual combination of high sea surface temperatures, low vertical wind shear and strong mid-to-high latitude blocking which intereferes with normal east-west atmospheric flow.
    As a person living on the west coast of Australia at longtitude 33 degree south, can anyone offer advice on whether I am more or less likely than in the past to have strong hurricanes (we call them cyclones) come this far south?

  16. 116
    Tom Rees says:

    Re #97. Roger, presumably the losses due to societal changes, increased hurricane wind speeds and rising sea levels will interact. In order to make a rational judgement, you need an estimate of 1) losses with societal changes and no AGW 2) losses from AGW and no societal change, and 3) losses with both.

    Then you need to look at the costs of minimizing AGW, costs of minimizing these societal changes (which are presumably also economically driven), versus the benefits (which, in the case of minimising AGW, are generally considered to be much more than simply reducing storm-related costs).

    Until such an analysis is available, I think that it is pointless conflating these two different issues (societal changes vs AGW). They are not mutually exclusive, after all. It is possible to address both simultaneously. In fact, it may be optimal to do so.

  17. 117
    Pat Neuman says:

    Re: 108 Mathias, do you think that increases in floods with global warming should be accounted for in developing flood predictions? Potential increases as global warming continues are currently being ignored by government agencies. See Probabilistic Forecast Updates: A Case Study at: http://www.crh.noaa.gov/ncrfc/

  18. 118
    TCO says:

    Why do you cut the comments off? I think this is the only thread I can comment on. There are interesting ones in the archives and all. I wasn’t here then, when they were open.

    Has Steve been here? Did you let him comment? Is there a policy that (even if germane to the topic of discussion) one can not link to “anti” webpages.

    Response: Comments are cut off to prevent spam and since people tend to only read recent posts. No one is barred from commenting, but comments are moderated for relevance and tone. If you have comments related to the topic of this post, please continue to comment. Future postings may be more to your taste… – gavin]

  19. 119
    Roger Pielke, Jr. says:

    Re: #116

    Tom- Good points. Here you go:

    [Note, yes the paper below is in that politically incorrect outlet, Energy and Environment, which is unfortunate, because the paper is solid. Those interested in substance will read it for its merits. Those interested in avoiding the issues can dismiss the paper because of its venue]

    A Climate Policy for the Short and Medium Term: Stabilization or Adaptation?

    Author: Goklany, Indur M. Source: Energy & Environment, Volume 16, Numbers 3-4, July 2005, pp. 667-680(14)

    Abstract:
    An evaluation of analyses sponsored by the predecessor to the U.K. Department for Environment, Food and Rural Affairs (DEFRA) of the global impacts of climate change under various mitigation scenarios (including CO2 stabilization at 550 and 750 ppm) coupled with an examination of the relative costs associated with different schemes to either mitigate climate change or reduce vulnerability to various climate-sensitive hazards (namely, malaria, hunger, water shortage, coastal flooding, and losses of global forests and coastal wetlands) indicates that, at least for the next few decades, risks and/or threats associated with these hazards would be lowered much more effectively and economically by reducing current and future vulnerability to those hazards rather than through stabilization. Accordingly, over the next few decades the focus of climate policy should be to: (a) broadly advance sustainable development (particularly in developing countries since that would generally enhance their adaptive capacity to cope with numerous problems that currently beset them, including climate-sensitive problems), (b) reduce vulnerabilities to climate-sensitive problems that are urgent today and might be exacerbated by future climate change, and (c) implement “no-regret” emission reduction measures while at the same time striving to expand the universe of such measures through research and development of cleaner and more affordable technologies. Such a policy would help solve current urgent problems facing humanity while preparing it to face future problems that might be caused by climate change.

  20. 120
    Tom Rees says:

    Thanks Roger – unfortunately the abstract is a bit low on detail and I can’t access the paper itself. However, one obvious fly in the ointment is this line: “…indicates that, at least for the next few decades, risks and/or threats associated with these hazards would be lowered much more effectively and economically by reducing current and future vulnerability to those hazards…”

    I don’t doubt that, if your horizon is limited to the next few decades, there’s little point in attempting to reduce current GHG emissions. Reducing GHG is a long-term project that at best hopes to achieve stabilization within 50 years. What would be useful is an estimate of the relative cost-effectiveness in, say, 2030 and 2080. That would show how the relative importance shifts according to the time-horizon.

    Furthermore, I don’t see any indication in the abstract of a test for the interaction of factors.

    For example, in your paper, you ascribe the increased costs of storm damage primarily to increased population density and property. By projecting these changes into the future (with a 50 yr time horizon, which again is probably a bit short), you predict that the costs due to AGW will be small compared with the costs due to population changes. You use this result to downplay the importance of emissions reductions. However, if more and more people are going to live on the coasts, then that will in fact make the need to tackle AGW greater, not lesser.

    Furthermore, most authors present a false dichomtomy. Both actions can be undertaken. The question is: “Will each action be cost-effective?” Not “Is one action more cost-effective than the other.”

    One other lesson from Katrina is that adapdation may not be as effective as many pundits propose. After all, the inundation of New Orleans was the one natural disaster that everyone saw coming. And yet even in one of the wealthiest nations on earth the defences were inadequate and the response plan poor. This isn’t a criticism of the federal/state govts – more a criticism of human nature. How much more difficult will it be for developing countries to pre-emtively adapt to a novel environmental situation.

  21. 121
    Mark Bahner says:

    I wrote, “According to Jesse Ausubel at Rockefeller University, we’re ALREADY decarbonizing the global energy system at a rate that will essentially *eliminate* carbon dioxide emissions by the end of this century:…”

    Tom Fiddaman replied, “Ausubel makes no such claim.”

    Well, you’ll have to take that up with Dr. Ausubel or William K. Stevens of the New York Times. Here is what Stevens wrote in the October 30, 1999 issue of the Times (note: the Solstice environmental website places the date of the article as October 31, 1999):

    “However that may turn out,’”the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis’ of the system, said Dr.Jesse H. Ausubel, an expert on energy and climate at Rockefeller University in New York City. Dr. Ausubel predicts that this evolution will produce a carbon-free energy system by the end of the 21st century.”

    http://www2.sunysuffolk.edu/westn/carbon1.html

    “Dr. Ausubel predicts that this evolution will produce a carbon-free energy system by the end of the 21st century.”

    So says the New York Times.

    Tim Riddaman concludes, “If you want to see technology unleashed on decarbonization, try putting a value on carbon.”

    “Technology” is already being “unleashed” on decarbonization, Tom. That was my very point. The world-wide trend towards decarbonization has been going on for almost 200 years. It started long before any government was ever worried about global warming. It’s not going to stop if governments start paying attention to more important problems (e.g., making sure that no strong hurricanes hit any country).

    By the end of this century, even a “business as usual” scenario puts CO2 emissions near zero…or at least to the point where atmospheric CO2 concentrations are no longer rising.

    In that NY Times article, Jesse Ausubel predicts that the (peak) atmospheric CO2 concentration will be 500 ppm in 2100. That is very close to my own prediction of approximately 560 ppm:

    http://markbahner.typepad.com/random_thoughts/2005/01/prujections_ipc.html

    The current concentration is about 380 ppm. So we have Jesse Ausubel’s (peak) projection of 500 ppm by 2100, and my (peak) projection of approximately 560 ppm by 2100.

    My question for Dr. Pielke (and you, and anyone else) remains…if y’all accept Dr. Ausubel’s and my projections, why is it that, “…we should be taking more effective actions to decarbonize the global energy system.”

    What benefit would be achieved by limiting the peak CO2 concentration not to 500-560 ppm (“business as usual” values), but instead to 450 or 480 ppm?

    This contrasts very strongly with (for example) limiting ***black carbon*** emissions, e.g., from diesel vehicles and residential coal and wood stoves. Black carbon (aka, “soot”) kills.

    Mark Bahner (environmental engineer)

  22. 122

    First off, I want to thank the RC crew for their wonderful site. It is definitely one of the most informative sites on the web and the discussions are also of a high calibre. It gives me some hope for the future.

    I thought the readers might be interested in a page I put up documenting how I managed to reproduce Emanuel’s results for the North Atlantic using the data from National Hurricane Center (also referenced by RealClimate and on the page itself.) For the impatient, the plot is here, and the entire summary can be found here. The pages also includes a few plots I did of hurricane activity for various seasons, including the 2004 season. (Incidentally, we are looking for beta testers for the product used to generate the plots, so if anyone is interested – especially if they have a big pile of data they are trying to look at – please contact me at hawkfish at tableausoftware dot com.)

    Anyway before I go, let me just add that I really appreciate that other scientists (like Roger Pielke, Jr.) join in the conversation here – it really adds to the quality of the presentations.

  23. 123
    Paul Emberger says:

    Although there is not universal agreement in the scientific community on the global warming and its anthropogenic causes, the political community seems to have accepted it and proposed solutions. My concern is the solutions themselves. For example if we replace significant numbers of internal combustion engines with fuel cells then we trade carbon dioxide emmisions for water vapor. What is the effect of increasing the propoprtion of water vapor in the atmosphere? Is is better or worse than CO2 in terms of the effect on climate. Likewise, what is the impact of diverting significant energy from surface winds with massive wind farms? While we are clearly getting a better handle on climate study, it is not clear that we know enough to start making changes we do not understand or have at least evaluated.

    [Response: You want http://www.realclimate.org/index.php?p=142, at least for a start - William]

  24. 124
    Isaac Held says:

    Continuing my cautionary remarks about overemphasizing the role of SSTs vs. upper level temperatures in this issue, a useful reference is

    Shen, Weixing, Tuleya, Robert E., Ginis, Isaac. A Sensitivity Study of the Thermodynamic Environment on GFDL Model Hurricane Intensity: Implications for Global Warming. Journal of Climate 2000 13: 109-121

    Part of the abstract reads
    “The results indicate that stabilization in the environmental atmosphere and sea surface temperature (SST) increase cause opposing effects on hurricane intensity. The offsetting relationship between the effects of atmospheric stability increase (decrease) and SST increase (decrease) is monotonic and systematic in the parameter space. This implies that hurricane intensity increase due to a possible global warming associated with increased CO2 is considerably smaller than that
    expected from warming of the oceanic waters alone.”

    I am harping on this point becuase i see it a central to an informed discussion of the relative ability of internal Atlantic SST variations and global warming to affect hurricane intensity: an internally generated increase in Atlantic SST is likely to be associated with a more unstable atmosphere than the same increase due to global warming.

  25. 125
    Steve Bloom says:

    Re #120: Five years is a long time in climate science. I think http://www.gfdl.noaa.gov/reference/bibliography/2004/tk0401.pdf (Knutson & Tuleya 2004), cited in the post and the source of the first graph, is more reflective of current thinking. Note that it uses an ensemble of models as contrasted to the single one used in the study you cited.

    You can use Google Scholar to try to check if a paper is still entirely current. Unfortunately, sometimes it’s a bit hard to figure out if this is the case or not since often the terms of rejection of the prior work are subtle.

  26. 126
    Lynn Vincentnathan says:

    RE #118, I see no problem with your suggestions. I hope you are aware of Amory Lovins’s works, such as NATURAL CAPITALISM, and works of other such experts & businesses that have actual examples of greatly reducing GHGs in cost-effective ways. As I’ve mentioned I’m just waiting for a plug-in hybrid, then I can drive almost entirely on wind power, and probably at a cheaper price.

    I think that building stronger buildings & higher levees actually costs more, but will be worth it in the long run if destruction from hazards is mitigated. On other hand, we can also build using “green” products (like reused wood, and natural pozzalons in cement), or planning room/building sizes to available materials (without much wastage), and using passive solar & other energy/resource efficient/conservative techniques & products. If we have to rebuild, let’s do it right this time, with awareness of most likely to worse-case GW scenarios. It seems the science keeps racheting up predictions of GW harms as time goes on – compare FAR, SAR, TAR & the one coming up. We’ve got to be aware of what way the wind it blowing.

    I think congress wanting to open up more oil drilling in the Gulf of Mexico is very unwise, considering we are entering a period of greater natural hurricane activity, plus the overlay of GW enhancement, perhaps even category 6 sometime in the future. Maybe they really do not know about GW.

  27. 127
    Roger Pielke, Jr. says:

    Re: Response to #97

    Stefan-

    Thanks for your continued comments, and my apologies if you interpret condescension and insinuations, none are intended. I will admit to pushing you to engage this discussion in the context of a robust literature on climate impacts. That literature does suggest some answers and raise some further questions. Most importantly to your latest point, how does one explain the fact that the same hurricane damage record shows a clear and statistically significant ENSO signal (in relation to ASO Nino 3.4) shows absolutely no correlation with the Atlantic annual PDI? This question must be addressed satisfactorily before asserting a future relationship of PDI and damage. And until the PDI is shown to be a metric of “destuctiveness,” forward-looking statements about the relationship of changes in the PDI to changes in damage remain purely speculative.

    FYI, as interesting as this thread has been, it is becoming a bit difficult to follow. Perhaps you might revisit this subject when Webster et al. comes out later this week, which will have a little something for everyone.

  28. 128
    Matthias Brun says:

    RE # 117 Pat: Flooding is definitely an issue in Europe and also Landslides an avalanches once you get to steeper terrain. In Switzerland every community has to develop a map of natural hazards-
    which is relevant for buildung actions. So if you want to build a new
    house on the wrong slope you are either not allowed or you have to get further inquieries. I work in an office for natural hazards and I can tell you we got a lot of work right now because of the recent flooding event here. In certain areas boulder of 15 feet in diameters where transported in small riverbeds – and you can imagine the damage to the houses.
    The events we got in some place were a lot bigger than the 300-year-incidents we calculated our hazard maps with. So as you can figure out easely the statistics have to be rewritten or don’t work anymore under the changing climate!! Extrapolation and modelling of rainfall events is all you can do – and the picture doesn’t look good. All you can do is abandon some of the dangerous areas in the moutains or along the rivers – not very easy in the highly populated alpine regions in Europe.
    Same problem for New Orleans. Climatic change is taken as a fact in Europe. And I still think the discussion should be on values and human duties for gonverments and everyone and how to attain the inescapable post fossil society instead of technical and security measures
    because that is closing the eyes to origin of the problem.

  29. 129
    Tom Rees says:

    Re #127, Roger, you ask: “how does one explain the fact that the same hurricane damage record shows a clear and statistically significant ENSO signal (in relation to ASO Nino 3.4)”

    Have you ruled out the possibility that ENSO changes the storm track, such that it is correlated with the proportion of landfalling storms (as well as hurricane intensity). If this were so, then it would explain why it is a better correlate of economic loss than storm intensity alone. I have skimmed your paper, but can’t see where this issue is addressed.

  30. 130
    Joel Shore says:

    Re #123:

    (1) While it may be true that there is “no universal agreement” in the sense of complete unanimity in the scientific community on global warming, there technically isn’t that on evolutionary theory or just about anything else. There is a general concensus and, while uncertainties remain, I think that at least here in the U.S., the politicians are actually well behind the scientists, not ahead of them, in their acceptance of the science and willingness to act on it.

    (2) As you can find discussed in a previous post here on RealClimate, direct anthropogenic emissions of water vapor are basically a “red herring” in the context of global climate change. First of all, fossil fuels are hydrocarbons and hydrocarbons contain hydrogen that gets converted to water vapor upon combustion, so our current fossil fuel burning is already producing plenty of water vapor. Second of all, humans can’t easily have nearly as significant effect on water vapor in the atmosphere as carbon dioxide by direct emissions of these gases. This is because the time scales for the gases to remain in the atmosphere are much different. (A CO2 perturbation lasts on the order of a hundred years or more while water vapor gets rained out in about a week.) It is also because there is much more water vapor in the atmosphere than CO2 and, since the amount of climate forcing goes approximately logarithmically with concentration that means the concentration of water vapor has to change by a significantly larger ABSOLUTE amount to produce the same effect. (Admittedly, a given FRACTIONAL change in water vapor concentration is more potent than the same FRACTIONAL change in CO2 concentration but this difference is not large enough to offset the larger difference due to their relative concentrations in the atmosphere.) Where water vapor is important is as a feedback effect…whereby the warming of the atmosphere due to increased CO2 causes the “equilibrium” concentration of water vapor to increase and this then enhances the warming because of water vapor’s absorption of infrared radiation.

    (3) Without having done the calculation (or seen it done), I would conjecture that your concerns about wind are another red herring. The amount of wind farms needed to extract a noticeable amount of the total energy available from the wind would probably be huge. Furthermore, the perturbation to the winds that we cause with wind farms will, at least for quite some time, likely be smaller than the perturbations just due to other structures that we have built (buildings, etc.)

  31. 131
    Roger Pielke Jr. says:

    Re: #129

    Tom- Thanks. Actually this paper is more directly relevant:

    Pielke, Jr., R.A., and C.W. Landsea, 1999: La Nina, El Nino, and Atlantic Hurricane Damages in the United States. Bulletin of the American Meteorological Society, 80, 10, 2027-2033.
    http://sciencepolicy.colorado.edu/admin/publication_files/resourse-85-1999.14.pdf

    We do find that the ASO Nino 3.4 index is related independently to both frequency and intensity. With respect to the latter we find that a 20% difference in intenstity (36.3 m/s vs. 30.6 m/s average winds) between the two phases manifests itself in twice the losses per storm during La Nina than in El Nino, and we can see this effect in the seasonal totals. So, yes, we can identify the changes in the damage record related to intensity that are considerably smaller than those identified in the PDI.

    Rick Katz quantified these effects more rigorously in this paper:

    Katz, R.W., 2002: “Stochastic modeling of hurricane damage.” Journal of Applied Meteorology, 41, 754-762.
    http://www.isse.ucar.edu/HP_rick/damage.pdf

    [Response: Just a clarification: a 20% increase in wind speed (as cited above) corresponds to a 73% increase in the cubed wind speed. The PDI is the cubed wind speed, integrated over surface area and life-time of a hurricane. If size and life-time of hurricanes increase as well with increasing mean wind speed, then the PDI will increase more than 73% for a 20% increase in wind speed. What I conclude from this:
    - A doubling in losses for a 20% wind speed increase seems consistent with losses being proportional to the PDI.
    - You don't know whether the ENSO related changes in hurricane power that you can identify are indeed "considerably smaller"; rather they are similar in magnitue to the changes that Kerry talks about.
    -stefan]

  32. 132
    Roger Pielke Jr. says:

    Re: #131

    Stefan-

    Let’s assume that you are correct that the ENSO-related and PDI-related effects on TC power are of similar magnitude (but do note that Emanuel uses max winds and I referred to mean winds). The question remains, if this is the case why do we see a very strong ENSO signal and no PDI signal in the damage record?

    [Response: This is just two bits of speculation on my part, but it is conceivable that a) the influence of ENSO is of a different character than the influence from SST (i.e. there is more happening than a similar increase in hurricane intensity/number), and ii) the different frequency distribution of ENSO events compared to variations in SST (or PDI) mean that the signal is stronger compared to the noise in that frequency band. i.e. the SST influence maybe harder to discern in the data because of the coastal development contamination which clearly dominates the longer timescale changes. -gavin]

  33. 133
    Steve Bloom says:

    A quick Google Scholar finds http://garnet.fsu.edu/~jelsner/PDF/Research/Elsner2003.pdf . If this paper is correct, the upshot is that ENSO does have a major impact on tracking of hurricanes, and thus on the damage they do. (Interestingly, there’s a related paper showing that ENSO does the same thing on the other side of the planet with East Pacific typhoons — that’s one dominant cycle!) I think Emanuel addressed why it might be a long time (50 years) before any connection between PDI and damage would become apparent.

  34. 134
    Steve Bloom says:

    From Emanuel’s FAQ:

    7.) Q: Does this [increase in PDI] mean that we are seeing more hurricane-caused damage in the U.S. and elsewhere?

    A: There is a huge upward trend in hurricane damage in the U.S., but all or almost all of this is due to increasing coastal population and building in hurricane-prone areas. When this increase in population and wealth is accounted for, there is no discernible trend left in the hurricane damage data. Nor would we expect to see any, in spite of the increase in global hurricane power. The reason is a simple matter of statistics: There are far too few hurricane landfalls to be able to discern any trend. Consider that, up until Katrina, Hurricane Andrew was the costliest hurricane in U.S. history. But it occurred in an inactive year; there were only 7 hurricanes and tropical storms. Data on U.S. landfalling storms is only about 2 tenths of one percent of data we have on global hurricanes over their whole lifetimes. Thus while we can already detect trends in data for global hurricane activity considering the whole life of each storm, we estimate that it would take at least another 50 years to detect any long-term trend in U.S. landfalling hurricane statistics, so powerful is the role of chance in these numbers.

  35. 135
    Roger Pielke, Jr. says:

    Re: 132 and 134

    Gavin-

    The data has been adjusted to remove any development signal. But I do lean toward a view that the relationship is weak, for whatever reason (and there are good ones why this might be so).

    Steve-

    My question seeks to address how Kerry’s explanation can be correct, yet we still can identify and ENSO signal in the database. There only only a few possibilities, and Kerry’s simply reinforces the argument that the PDI is only weakly, if at all, related to damages.

    [Response: Detecting a trend and detecting an oscillatory signal of the same magnitude in the same noisy data set are not the same thing - I suspect finding the oscillatory signal is easier. This would be easy to check for anyone with a couple of hours time at their disposal (unfortunately, that's not me right now). Cheers, Stefan]

  36. 136
    Pat Neuman says:

    Re: 128 – Hi Matthias, there’s a commentary on the overtopping of dikes in New Orleans and Grand Forks at:
    http://www.startribune.com/stories/562/5610904.html … and at: http://groups.yahoo.com/group/ClimateArchive/

    Flood prediction, preparedness and design of structures with a rapidly warming world interests me. If you want to talk more I suggest you join: http://groups.yahoo.com/group/ClimateArchiveDiscussion/

  37. 137
    Steve Bloom says:

    Roger, Stefan (among others) has already responded to the same issue in comments 49, 73 and 82 in considerable detail. The key point seems to be that ENSO affects tracking and PDI does not. According to the paper I linked, ENSO results in more “straight-moving” hurricanes that track through the Caribbean and Gulf before landfalling. This seems to me to be a very large effect since pretty much all of those hurricanes landfall, many of them in the US, plus (I assume) they tend to be a lot stronger when they do so since they spend more time over warmer waters. Stefan’s view is that PDI could already be having a substantial but still undetectable influence, so perhaps it would be worth looking at how strong of a signal could exist now and remain undetectable. I assume that this must be the calculation Emanuel did in coming up with that 50 year number. Would it help resolve things to have that?

  38. 138
    Tom Rees says:

    The Association of British Insurers use an Insurance Catastrophe Model to estimate that a 6% increase in hurricane wind speeds will increase the annual insured losses by $3.9 billion in the US and $1.6 billion in Japan (2004 $). For a 1-in-100-year storms, the cost will be an extra $41 billion in the USA. This assumes that there are no changes in demographics, precipitation, storm surges or adaptation. They point out that, on the one hand, higher insurance premium will encourage more risk-conscious development but that, on the other hand, increases in precipitation and storm-surge height are also likely and will inflate the figure.

    I know insurers have an incentive to talk up risks, but they don’t have an incentive to talk up risks due to climate change compared with other risks. In this regard, this Sept 8 report (coauthored by a paid-up scientist from LLNL) is interesting Availability and Affordability of Insurance Under Climate Change. They quote one of your papers, Roger! “One can easily hypothesize that increasing population and urbanization in the United States has led to a commensurate increase in population at risk. Yet, one can also hypothesize that the various societal responses may have more than compensated for population growth and in fact fewer people are today at risk.” ;)

  39. 139
    Roger Pielke Jr. says:

    Re: 137

    Steve- Thanks. We do find an independent intensity effect in the ENSO results, so we can simply ignore the frequency effects for present purposes, and the question remains. Emanuel’s analysis suggests that PDI is not at all a good proxy for damages, i.e., there is historically no relationship with damages, and in the future this relationship will continue to be weak. (Another way to think about it is that the PDI includes a whole lot of information not relevant to hurricane landfalls, so of course the relationship to damage is weak.) The ENSO analysis helps us to understand what magnitude of climate signals might be observed in the damage record via a simple climate index. What this suggests to me is that climate scientists should continue to develop indicies of hurricane behavior, but to use as one criterion for their development a close correlation with damages. Emanuel’s work is an important step in this direction, but I’d bet we can improve upon it. We have discussed using V^3 at landfall as such an index, but it is not well correlated with the annual/basin PDI. The ACE index is similarly poorly correlated with damages. But let’s keep looking, the search really has just begun. There is no need to force the PDI to carry more significance than the data allow, there are going to be better indicies from the standpoint of understanding the relationship of climate changes and damages.

  40. 140
    Tom Rees says:

    Re #137 Roger, are you really saying that, when a hurricane hits land, there’s no relationship between PDI and likely destructiveness?. I presume not. After all, we know that a mean 20% increase in wind speed in La nina years gives a doubling of damage per hurricane Pielke & Landsea, 1999.

  41. 141
    Dan Allan says:

    The debate here has focussed on two issues:

    1) correlation between SST and hurricane strength / PDI.

    2) correlation between PDI and damage.

    Obviously, both of these correlations need to exist to conclude that AGW will lead to increased hurricane damae.

    Regarding the 1st point:

    I admit that, like most of the rest of us, I am not an expert on hurricane formation. But it seems quite clear empirically that the SST correlates much more closely with hurricane strength than does temperature gradient / instability. For instance, it is well known that hurricanes require SSTs of ~80 degrees F to form. Period. This is the minimum. There is no exception in the case of a very high degree of instability, or very low shear, or whatever, where a hurricane will form with a SST of 76 degrees F. It simply doesn’t happen. Moreover, each category of storm has a similar minimal SST under which it will form. I wish I had a link for this, but I can tell you that I have read enough hurricane forecast discussions to feel certain of what I am saying. This suggests to me, personally, that a correlation between AGW and hurricane strength / PDI, is highly likely – unless someone can show an additional likely effect of AGW that is so strong that it cancels this factor out entirely.

    Regarding the second point:

    In some cases, it seems to me that the argument in favor of a correlation is so strong that, if we don’t see a correlation historically, we have to simply wonder if there is enough data yet to see the correlation, if we are asking the right questions, if there is so much background noise that it masks why is potentially a very significant correlation – or some combination thereof. I think this is just such a case.

    The issue of how we pose the question seems especially pertinent here. Unless I misunderstood, Roger is looking at total annual PDI versus annual damage, rather than each storm’s PDI versus that storm’s damage. To me this is likely to obscure correlations with lots of noise. Suppose we wanted to test the correlation between pitching velocity and strikeout potential. Would we look at the net average velocity across all pitchers in year A, and compare it to total strikeouts? Or would we look at the average velocity of each pitcher and compare it to that pitcher’s strikeouts? To me, the first method has a great deal of noise inherent in it.

    Finally, if Katrina really causes 200 billion in damage, and given that it was strong storm in a year of strong storms, it is likely to bring out a correlation between PDI and damage that had not previously been evident. And the fact that a single event could do this (assuming it does do this) is just an indication of how few data points we have.

  42. 142
    Tom Fiddaman says:

    Re #119 (A Climate Policy for the Short and Medium Term: Stabilization or Adaptation?)

    I don’t have the journal, but assume the article is similar to this one by Goklany: Reducing Climate-Sensitive Risks in the Medium Term: Stabilisation or Adaptation?. I agree with some of Goklany’s observations, e.g. if you want to stop malaria, tackle it directly, not via climate. However, in other ways it’s shaky, including those observed by Tom Rees (#120, the false dichotomy, short horizon, interactions).

    Pielke writes, “There are much, much better ways to deal with the threat of hurricanes than with energy policies … . There are also much, much better ways to justify climate mitigation policies than with hurricanes … .” (#46). By contrast, Goklany seems to conclude, “there are better ways to deal with malaria than climate policy, so let’s work on malaria and skip the climate policy for now.”

    Kyoto is used as the usual straw man. But Kyoto fails conventional cost-benefit tests because it’s inefficient, not because emissions control is inherently a bad idea. The low payoff to Kyoto in conventional analyses is also contingent on discounting the welfare of future generations, hardly a forward-looking ethical policy. If you don’t do that, Kyoto looks like a good deal, and more efficient measures really pay.

    Fundamentally, if avoided emissions have a value, we ought to impose it uniformly now. It’s hard to imagine how one would “broadly advance sustainable development” without doing so. To think there’s no value to avoided emissions requires some rather extreme views on the distribution of impacts and reversibility of emissions vs. investments.

    If you don’t put a value on carbon, market forces will overwhelm the no-regret and technology policies Goklany proposes by redirecting them to business-as-usual purposes. Witness that vehicle technology has advanced greatly since the 80s, but absent policy, fuel economy has slightly declined, driving has increased, and vehicle power/weight ratios are way up.

    In a similar letter to Science Goklany suggests:
    Thus, it would be far more beneficial, and cost-effective, at least for the next several decades, to reduce vulnerabilities to current problems, especially if they might be exacerbated by climate change (e.g., hunger, malaria, drought, and flooding) (3). Even with a lagtime of 50 years to account for the inertia of the climate and energy system, the aforementioned analyses suggest we may have at least a quarter century window (2080s minus 50 years) before deciding on the depth and extent of mitigation. Meanwhile, we should focus on improving mitigation and adaptation technologies and our knowledge of climate change science, economics, and responses. This way we can advance sustainable development and solve the problems of today while furthering our ability to solve the problems of the day after tomorrow.

    Goklany’s compression of the time constants of the economy and climate to 50 years is rather extreme given infrastructure and power plant lifetimes on the order of 40 years, carbon cycle time constants >100 years, and long adjustment times to radiative imbalance. Goklany’s suggested “wait and see” approach doesn’t produce good results in a system with long delays.

    Perhaps a better approach would be to look for synergies between near and long term measures, e.g. land use policies that put people out of harm’s way and lower transport energy intensity, or recycling carbon tax revenue into disease prevention.

  43. 143

    I’ve noticed Katrina and Orphilia’s ocean path speeds, fascinatingly slow, Is this normal? Or is there a record of ocean path speeds to compare them with? I would suggest that there is no t enough continental cold air generating dominant winds to cary systems at more “normal” speeds. This abnormality would explain Katrina’s intensity cauused by significantly warmer SST over the gulf of Mexico, and also would explain Orphilia’s longevity, over somewhat cooler sea surface temperatures .

    The answers we seek, seem to be right in front of us, and SST’s have a direct impact as explained by Orphilia’s weaker winds.

  44. 144
    Roger Pielke, Jr. says:

    Lots of good comments here, a few quick replies,

    Re: #138

    Tom- On the ABI report, see this post:

    http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/000568of_blinders_and_innu.html

    And the quote that Mills et al. cite comes from a 1999 paper and refers to floods. It is true that there are no studies (at least that I am aware of) that quantify trends in floodplain occupancy or trends in wealth in floodplains, so in that paper we used overall trends in population and wealth as proxies. But were I a betting man I’d be “all in” on the first hypothesis!

    On their discussion of attribution, see this post:

    http://sciencepolicy.colorado.edu/prometheus/archives/climate_change/000537the_other_hockey_sti.html

    Re: #140

    Tom- Emanuel (2005) focused on the PDI as measured on an annually accumulated basis. The PDI at landfall doesn’t quite make sense because the PDI integrates the max winds (cubed) with the lifetime of the storm. I suppose we could define landfall as a period of time when the storm begins to have effects on land to when it moves on and calculate the PDI over that time. I would fully expect that such an index would be strongly correlated with damage, conditioned upon the population/wealth at risk in the path of the storm. This is exactly the sort of research that ought to be done.

    Re: #141

    Dan- Be careful directly comparing estimates of Katrina’s damages with other historical storms, it is not yet clear what the apples to apples number should be, we are working on this and hope to have some recommendations this fall. Meantime, see these posts:

    http://sciencepolicy.colorado.edu/prometheus/archives/environment/000557making_sense_of_econ.html
    http://sciencepolicy.colorado.edu/prometheus/archives/disasters/000563part_ii_historical.html

  45. 145
    Lynn Vincentnathan says:

    RE #141, the social sciences use (I think it’s called) “structured sampling.” If they understand that, say income, has an impact, and they are not likely to randomly draw enough folks from a particular income bracket (proportionate to their percent in the population), then they will arrange to have the same percent of each income group in their sample as in the population.

    I also vaguely remember reading about Love Canal, and the official studies found no significant correlation between ill health & living in Love Canal (partly due to low numbers), but when a resident along with some university scientists found an old geological map of the natural swale network underlying the community, and added that variable into the study — finding that most of the sick people lived along the swales — they got a high and significant correlation. They also tested the soil in the swales and found greater toxicity.

    So, of course, we have to compare PDIs only of hurricanes that make landfall in roughly the same population/built areas, to see if PDI correlates positively with damage (I think this is a no-brainer). Then we have to see whether greater PDI has been caused by natural causes, random & unpredictable fluctuations, or perhaps in part by AWG.

    The problem now seems to be that we can only talk about hurricane PDI on the whole for the entire world, but if we ever get to a point where we can figure what (including AGW) went into each hurricane’s PDI (including what went into SST in its vicinity), then we’d be able with more confidence to attribute a portion of the damage from specific hurricanes to AGW.

    I’m not sure, but is it correct to say that it is just as likely as not that AGW increased Katrina’s PDI and the damage (without hazarding to guess by how much)? That’s the way I plan to put it, unless someone here says that’s a wrong assessment. I also say that AGW has increased hurricane intensity in general, and is expected to increase it more so in the future, perhaps giving us even Category 6 hurricanes.

    I think there are several AGW factors & other natural factors that go into higher Mexican Gulf SST, and we need to consider all of them – more shallow waters being heated more, & more rapidly than deep seas; the slowing of the thermo-haline ocean conveyor from fresh water pouring into the North Atlantic, leaving hot waters more stuck in place in the south.

    Personally, my gut feeling is the Gulf waters were heated by AGW more than the .5 degree average for world SST (which I think is averaged for the whole year, so even particular times of year may have greater warming from the greenhouse effect). I think they said the Gulf SST when Katria struct was 6 or 7 degrees F(or C ?) higher than usual. I’d wildly guess maybe two-thirds of that was from natural fluctuations, and maybe a third from AGW factors (direct & slowing of the thermo-haline).

  46. 146
    J. Sperry says:

    Re #143:

    I followed the links in #122 above, and found an abundance of data on ocean path speeds of storms measured at 6 hour intervals. The mean ocean speed is 12.9 mph, with a st. dev. of 8.7 mph, and the data can be summarized as follows:
    0-5 mph, 19% (0.5% were 0, i.e., stationary when measured)
    5-10 mph, 27%
    10-15 mph, 24%
    15-20 mph, 16%
    20-90 mph, 14%

    I’m not having luck finding data on the ocean path speeds for the recent storms (maybe 15 mph for Katrina and 7 mph for Ophelia), but I doubt that they could be considered “fascinatingly slow” or “abonormal” since one fifth of the measurements are below 5 mph (a slow jog in human terms).

  47. 147
    Hank Roberts says:

    Katrina passed over and picked up heat energy from a 200-foot-deep “loop current” that’s tracked in the Gulf every year, containing warmer water — so the hurricane pulls up warm water instead of cold water as it travels across that area.

    http://www.nola.com/news/t-p/frontpage/index.ssf?/base/news-4/1125214307249320.xml

    QUOTE (from befor the storm came onshore)


    “Katrina could turn out to be the perfect hurricane, much to the dismay of south Louisiana residents.

    Not only is there little to keep it from strengthening on a dangerous scale, but it is expected to create a dome of storm surge that could flood much of eastern New Orleans, the 9th Ward and Mid-City in New Orleans, swamp much of the West Bank and Plaquemines and St. Bernard parishes, and flood north shore areas.

    “I don’t remember seeing conditions as favorable as this for a strengthening hurricane for a long time,” National Hurricane Center Director Max Mayfield said Saturday.

    He was referring to a unique combination of physical and meteorological conditions that are expected to cause Katrina to spin up to Category 4 or 5 strength, like a top accelerates when you pull its string.

    The northern Gulf of Mexico is unusually warm, rising past 85 degrees this year. And while that could, in part, be due to less cool freshwater entering the Gulf from the Mississippi River, National Hurricane Center meteorologist Eric Blake said it’s more likely the result of relentless high temperatures in recent weeks along Louisiana’s Gulf Coast.

    ****** That warmth extends more than 200 feet deep in one area in advance of Katrina, thanks to a donut-shaped pool of hot water called the loop current, which breaks off from the Gulf Stream and floats around the Gulf each year. ******

    The warm water acts like a flame, heating the atmosphere above it. In this case, that atmosphere already has formed into the circular system of thunderstorms that make up a hurricane.

    Above the storm circle, an upper-level high-pressure system helps squeeze out the rising hot air, a process called outflow, which increases the wind speeds and keeps the storm healthy.

    Pressure builds

    Not present near the hurricane on Saturday were two things that could reduce its intensity: wind shear caused by upper-level disturbance blowing across the cloud tops and reducing their warmth and height; and dry air that could swirl into the circular storm and sap it of the moisture needed to keep it strong.

    And by Saturday, Katrina had grown so large and strong that it had begun entering what’s known as an “eye-wall replacement cycle.”

    As a hurricane spins counterclockwise, the clouds at its center move faster and faster in a circle, which narrows until it falls apart, to be replaced by a new circle of eye-wall clouds.

    The eye wall contains the hurricane’s highest winds, and as the inner circle collapses, those winds can decrease a bit. But with the collapse happening Saturday, it provides an indicator that the wind speeds will increase today.

    The result, Mayfield said, is that Katrina is expected to be a strong Category 4 hurricane, with winds of 145 mph, as it approaches Louisiana’s coastline Monday morning.
    ….

    Researcher Ivor van Heerden warned his model is probably understating the water’s effects, though. The surge will be topped by wind-driven waves that could be half again as high as the surge, which could mean water pouring over levees all along the south shore of Lake Pontchartrain.

    ‘A worst-case scenario’
    ….

    All of eastern New Orleans outside the levee system, including the Irish Bayou community, will be underwater.

    . . . . . . .

    Mark Schleifstein can be reached at mschleifstein@timespicayune.com or (504) 826-3327.

    © 2005 The Times-Picayune. All rights reserved.
    .
    END QUOTE

  48. 148
    Pat Neuman says:

    Re: 147 That’s interesting data on ocean path speeds. Do you have any ideas for making a table on hurricane size? e.g. a table showing average annual max diameter for Atlantic-Gulf hurricanes.

  49. 149
    Gerald Machnee says:

    Re #145 – in the last paragraph. Guessing at fractional contributions of AGW is not good science. I did find the quotes in #147 very interesting as I had heard most of them as Katrina was approaching. It does shed light on why Katrina was that strong – almost all factors coming together.

  50. 150

    re 147, Although helpful, my fascination was not based on those stats you just gave.

    What is Interesting is two hurricanes having roughly close points of origins , lets say on average slowish (if these stats are right), but at times nearly stalled or looping , abnormal perhaps? Both hurricanes having plenty of time to strenghten over warmer water, but one, Orphilia languishing longer over a little bit cooler water.

    This comparison shows with ease that SST’s are proportional to intensity, Katrina at landfall being a Cat 4 vs Orphilia’s Cat 1. It is better to compare individual events, one on one. instead of bunching them up in a de-personalyzed lump of statistics which looses meaning traction.

    The path speed of Katrina seemed slower at times, especially before hitting the Florida coast.
    But, I am intrigued by two consecutive hurricanes at slowish speeds, in large measure with many other world wide weather systems stalled, stalling or slow, may I give the example of that Cyclone hanging for several weeks North-East of Boston this past May giving all this cool rain. The challenge for stat analysts, is to find data contradicting the Hurricane comparison cited above.


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