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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.


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. 251
    Stephen Berg says:

    Re: #239,

    “You say “My thoughts are that ….” Science is not based on ‘thoughts’.”

    Of course, it is. A scientist must observe the results of his/her study and think about what it means. Without this, no conclusions can be made. Science is not based on beliefs, however.

    “If you have evidence that hurricanes have increased in number and intensity in correspondence to CO2 emiissions, please provide a reference.”

    CO2 emissions cannot be THE cause of this increase. However, the overwhelming evidence is that global warming (at least partly a result of increased greenhouse gas-emitting fossil fuel burning) is also resulting in a warming of the oceans, increasing the energy (latent heat) necessary for hurricane formation. The Trenberth, Webster, and Emanuel studies have, by and large, concluded this.

    “What the IPCC TAR report says (Table 1 of ‘The Scientific Basis’) is:

    ‘Increase in tropical cyclone peak wind intensities: Not observed in the few analyses available’

    ‘Increase in tropical cyclone mean and peak precipitation intensities: Insufficient data for assessment'”

    I’d like to emphasize “few analyses available” and “insufficient data for assessment.” As a result of the likely increase in hurricane numbers (according to NOAA forecasts for the next decade at least), studies like Trenberth’s, Webster’s, and Emanuel’s have been undertaken.

    If the likely scenario is that climate change will (at least partly) cause greater numbers of TCs, TSs, and TDs to occur, it is essential that we try to reduce climate change as much as we can so the impacts of these TCs, etc. are minimised. If we do not, then more people will die, more property will be damaged, and greater destabilisation will occur. Governments which do not take these actions will be regarded as negligent to their duties (i.e. to protect their people).

  2. 252
    Pat Neuman says:

    Ending comments in 250 remind me of a simplification, assumed by many meteorologists, politicians, and the general public, which has been very damaging in the prolonging of a do nothing attitude toward dealing with fossil fuel emissions driven rapid global warming.

    They assume a person knowledgeable in meteorology is knowledgeable about climate change. Climate deals with the atmosphere, but also hydrology, geology, biology, oceans, and the study of Earth’s past. That’s much more than knowledge in modeling and prediction of day-to-day weather. Also, many in the U.S. that are part of the AMS or AASC are meteorologists and/or experts in weather records, not climatologist in the climate change sense. Unfortunately, the media, the policy makers and the general public fail to understand that, and unknowingly accept their skeptical views about global warming as equal to those professional much more qualified in the disciplines needed to understand past climate change, and global warming.

  3. 253
    PJK says:

    Seems to me that there are two basic facts:

    It is 100% certain that the GW has an impact on each and every single hurricane. No two storms are the same, Even slight changes in the actual weather situation produce a different storm.

    It is also 100% certain that the exact impacts can not be “proven” in the sense that the general public expects, i.e. in this discussion. As you note, it is the same argumentation as in the general climate disussion, all over again.

    My limited contacts with the hurricane research community have made me believe that a hurricane comes in two quite distinct parts:

    Its strength and destructive power depends on its thermodynamic history. The discussion concerning tracks of individual storms crossing specific warm currents in the Gulf is very interesting in this respect. This could be a means of investigating this aspect in a reliable way.

    The path of a storm is driven by the general meteorological process around it. Thermodynamic fields within a radius of a thousand miles steer directly the course it takes. The storm has no internal and independent will in this respect, it is rather like a ball in an old pinball game.

    There was also a response on my previous input concerning the Africa contribution of storm generation. Greening of Africa is indeed predicted by some climate models. This will impact the amount of dust over the Atlantic. This in turn will, then, have an impact on the storms’ early stages of development. Weather modification to suppress hail from thunderstorms is based on the observation that some storms do not have enough condensation nuclei – adding more nuclei changes the convection process as many more and much smaller hailstones are formed at a lower altitude.

    I also attended a conference some 15 years ago where a NHC staff scientist told each and everybody: “bolt them walls to them concrete foundations of your new house, or face disaster when the storm strikes”, with numerous air photos and on-site damage reports to support his admonishment. He also showed detailed results from flooding models on various parts of the Gulf coast. Maybe they did not say specifically that the “levees will be overflown”, though, but a potential flooding disaster in New Orleans was certainly mentioned.

  4. 254
    Gerald Machnee says:

    RE #250 & #252 – The question remains: What is Dr. Nye knowledgeable in?
    I asked how long has he been forecasting hurricances. In addition what is his training in climatology? What is being assumed about him?
    The problem is not being skeptical about global warming causing hurricanes, but how much to attribute to it. Nobody has been able to come up with a definite percentage since the question of how severe a hurricane becomes is a very complex process.

  5. 255


    Fully agree, but basic matter of fact world weather reporting can be dissiminated once in a while by meteorologists. Its the larger picture that most people don’t see which is the problem, if the public is not informed regularly about the larger world, they won’t request the experts you mentionned. We rather see met guys focused on their little corners of the world, but I am certain that most of them know about important world records, which should be passed along to the public in general as often as they come.

    I keep looking at that BBC sea temperature graph lacking meaning traction and comparing it with the PDI graph above and find them incompatible, as if both were displays are from different regions of the world.

  6. 256
    Pat Neuman says:

    Re 199, on PDI: … “So it’s the wind speed *cubed*, integrated over the surface area covered by the hurricane” … “over the entire area covered by the storm.”

    I would like to see data on surface area covered for the lifetime of tropical storms and hurricanes.

  7. 257
    Dan Allan says:

    Re #227

    I hope Realclimate takes the opportunity to respond directly to William Gray’s implication that this year’s activity does not represent an anomaly. He is obviously someone who is widely respected, and his assertion that this is part of a normal oscillation is a little hard to fathom. We have had the strongest July storm on record this year, the second strongest August storm on record, and the third strongest September storm. All in the same year. We will also likely have the largest number of named storms ever. This is after last year, which was again one of the busiest hurricane seasons ever. So someone please tell me what past year is similar to this one?

    There seems to be a rather monumental lack of scientific curiosity among those who do not find this a little interesting.

    Nor is “natural oscillation” anything approaching a scientific explanation. What is this oscillation? What causes it? Somebody please explain.

    Finally, to Michael’s J.s’ point regarding Bush’s statement that “nobody could have anticipated the levees to break”.

    Errr…sorry. Forget the Washington Post editorial. Worry about the levees breaking and whether they would hold was the subject of Weather Channel documentary over the summer, an extended series of articles in the New Orleans media drawing on many scientific findings, extensive discussion on CNN in the days leading to landfall, coverage also on every other major news network in the days leading to landfall. In fact, just about the entire country, except apparently the president and one dreadfully mistaken researcher at LSU, worried a great deal that the levees would break. And the president’s comment that nobody could have anticipated it is truly one of the most shocking I have ever heard uttered by a public official in this country.

    [Response: See our response to comment #263. -mike]

  8. 258
    betina wolfowicz says:

    does anybody know why is a hurricane called “nature’s safety valve?”

  9. 259
    Steve Bloom says:

    Re #257 (Dan Allan): The article (in paragraph four) was pretty specific in rejecting Gray’s position, albeit without naming him: “…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.”

  10. 260


    One example of a natural oscillation is a grandfarthers clock pendulum. In this case the pendulum slowly swings once a year , consider the South Atlantic all time temperature average as the vertical, pendulum at rest, with yearly temperatures , swinging to the left (colder), or to the right ( warmer) , last year the pendulum was 0.4 degrees to the right, it seems to be slowly swinging upwards to the right this year. (Hope this helps)….

  11. 261

    #184 “would the Northern Pacific ocean warm more?”

    Sorry I missed that , The Atlantic and Arctic Ocean have a lot more in common than with the Pacific,
    you must look at the map and see that they would be considered as one ocean if not named otherwise, while Behring strait severely restricts the Pacific from mixing with the Arctic Ocean. Given this South Atlantic Oscillation fixation by cycle theorists, Is it a wonder why the Pacific region is equally warmer then?

    I second the motion made by #256â¦.

    Iâve joined the BBC graph with the PDI above, and see some semblance getting really confused by the 80âs until 2004. This is likely the graph inspiring the famous 10 to 20 year forecast of increased hurricane activity. Likely of the South Atlantic, perhaps with data from the same geographical area as with the PDI. (A lot of may beâs thanks to the BBC).

    The biggest question that comes to mind, aside from those already mentioned, is why the PDI shoots up almost exponentially way above all previous PDIâs with warmer sea surfaces?

    Yearly Average temperatures of combined Hurricane paths may not offer such confusion, and may reveal a different graph more comparable to the PDI.

  12. 262
    Gerald Machnee says:

    Re #257 – The North Atlantic Oscillation is explained in the following site as well as others:

    {So someone please tell me what past year is similar to this one?}
    On the following site there is a lot of data on hurricanes over the last hundred years or more. One of the things you will find is that there were a lot of hurricanes(including some very severe ones) in the early 1940’s and also early in the century.
    Re named storms – Hurricanes have only been named since about 1950, so that statement can be confusing.
    Another confusing statistic is numbers of hurricanes or storms. Some count only those making landfall in the USA, some count storms(which may not be of hurricane strength), and others count all storms and hurricanes in the Atlantic whether or not they made landfall.
    Soms scientists have only analyzed the last 30 or 40 years so that analysis will show an increase. If you did a study for the period 1940 to 1975 you would likely show a significant decrease in Hurricanes.
    The NOAA site covers over a hundred years.
    On the following site you will find some Q & A on numbers:
    The year 1933 was the most active.
    The years 1886 and 1887 were very active.

  13. 263
    Dan Allan says:

    re #260,

    I didn’t mean to suggest that there are not natural oscillations, in climate and of course in many other fields. But I keep hearing very vague statements about “a natural cycle” of hurricane frequency. Yet (a) nobody seems to agree on the exact period of this “cycle”, and (b) nobody seems to be presenting a theory that explains this particular cycle. This makes me very suspicious that the signal for a natural multi-decadal cycle of hurricanes is very strong at all. Finally, I can’t help wondering if we are inferring a “cycle” from a 1 or 1.5 periods. There is obviously a very big difference between “natural variability” and “a natural cycle”. Variability is obvious and quite dramatic in this case. Whether it is cyclical – where is the evidence?

    [Response: There is actually some solid science behind the oft-made claim that there is indeed a multidecadal cycle in Atlantic climate. This includes evidence from instrumental measurements, paleoclimate proxy data, and control simulations of coupled ocean-atmosphere models–see our glossary item on the “Atlantic Multidecadal Oscillation” (AMO). Unfortunately here, as in many cases, the actual science has been overstated, misinterpreted, or selectively used in the public discourse. As we discussed in the current article, the amplitude of this internal oscillation is modest and cannot alone explain the warming trend established, for example, by Emanuel. Moreoever, there is no convincing evidence that long-term changes in Hurricane intensity can be attributed to this cycle. It is therefore as (if not more) irresponsible to tie the unusual number of intense hurricanes this present season to a “natural cycle” as it is to tie it to global warming. Unfortunately, this has not stopped some public commentators. -mike]

  14. 264
    BdG says:

    An argument is being made that the global warming based prediction that storms would increase in frequency is false because the National Hurricane Center’s records show a reduction in storms.

    I checked the National Hurricane Center’s web site (, in particular, and found that there has indeed been a decrease… in the number of Atlantic cyclones THAT HAVE ACTUALLY ACHIEVED LANDFALL.

    That key phrase is omitted from the popular argument. The upshot is that while there have been fewer storms that have struck the US, nothing can be concluded regarding any changes (relative to the time period pre-1960 when there were no weather satellites) in total numbers or even ferocity. Maybe there were fewer storms, maybe there were more. Maybe they were weaker, maybe they were nastier. Frankly, nobody knows.

  15. 265
    Michael Jankowski says:


    I simply think his comment was poorly worded and/or taken out-of-context, and I don’t quite see why it was included in the report in question other than for badgering.

    The vast majority of reports/predictions I have seen suggested the levees would not fail, even under a Category 5 with a similar path to Katrina. I think this is in-line with what he said, or what he intended by what he said. Maybe “nobody” was a poor choice of words, but I don’t think it was meant to be taken literally or should have been taken literally. Along those same lines, I can say that “everyone” knows that a structural failure can occur in any object at any time, especially a levee in a rain/flood/hurricane event.

    I hear rumblings of local and state officials neglecting levee project funding, improper and unaccounted spending by the Louisiana Office of Homeland Security and Emergency Preparedness (along with federal indictments), etc.

    There’s a lot of finger-pointing and blame to go around, but not a lot of serious talk about solutions.

  16. 266
    Gerald Machnee says:

    Re # 263 – the hurricanes do not follow a “natural” cycle. However the temperatures in the Atlantic where the hurricanes form have a cycle which is about 30-50 years in length. When the temperatures are warmer there are more hurricanes and likely more stronger ones. The temperatures and hurricanes were up in the 1940’s then down in the 1970’s. At this time the temperatures are higher and we are seeing more activity. This has been predicted to continue for several more years. Re # 264 – while more hurricanes were predicted for this year, it is really difficult to predict how many will make landfall. The sites I noted in #262 give numbers of hurricanes in various years. Thr following site has year by year totals:
    The BBC site gave average Atlantic temperatures.

  17. 267
    Dan Allan says:

    re #266 – Thanks.

    This is the closest I have heard to something specific. But it still raises some questions:

    (1) how many of these 30-50 year periods have we observed and measured, that we can be confident in this cycle? And (2) do this year’s SSTs appear to be within range of a high SST period, or are they anomalously high, even for a warm period?

    [Response: See our response to comment #263. -mike]

  18. 268
    Gerald Machnee says:

    Re #267 – re your first question – an interesting article appeared in the St. petersberg times in September:

    Re (1) – Fishermen had tracked the temperatures. Then scientists checked tree rings and found that the cycle went back hundreds or a thousand or more years.
    Re (2) – According to the hurricane forecasters, we are somewhere in the middle of the (warm) cycle. Of course there could be variations in the number of events, e.g. we know that winter will be colder, but the daily temperatures could vary by 20 degrees and we do not know exactly how many storms will develop to very severe ones.

  19. 269
    Siobhan Sullivan says:

    one problems with blaming increased hurricane activity on global warming is that weather patterns tend to last up to twenty years before they change. There appears to be little doubt that warmer waters fuel hurricanes to catagory 5 status, if teperatures in general
    continue to increase then the intensity of storms will increase.

  20. 270
    Dan Allan says:

    Mike and Gerald –
    Thanks for the responses to 263. Makes sense. I probably should have read the article more carefully before shooting off.

    Nonetheless, one point still stands – there is a great deal of overstated attribution of this season’s hurricanes to multi-decadal cycles.

    I heard one expert on TV (I think it was Max Mayfield) actually say that “this was just a return to normal.” DIfficult to see how what is likely to be the busiest / most severe season ever (if one uses a simple metric such as multiplying each named storm times maximum category, and then summing) could be seen as “normal”. Maybe it is truly attributable to mult-decadal cycle, or 99% attributable, but the lack of scientific curiosity is a little strange.

  21. 271


    Nonsense going unchallenged is a very bad thing.

    Lets accept, very temporarily, that there was a weak but identifiable average temperature oscillation of the Sourth Atlantic, today it is no longer the same.. Why? If a grandfather clock pendulum cable is shortened or lengthened, the frequency of the said grandfathers clock changes . A physical change has occured, no longer the same effect can happen. And so goes the South Atlantic temperature oscillation. The air above, especially CO2 forcing is not the same, there is no such thing as returning to “normal” , we don’t live on a planet having identical closed system parameters through time, everything changes especially now a days, and once upon cycles, real or not, get transforned. I think it is simply wishful thinking to assume that every thing can return to normal when the very shape of the Earth’s coastal shores and atmosphere has changed. The concept of regular meso-cycles can apply for a distant planet devoid of inhabitants, like Mars or Venus. Perhaps they can be found there.

  22. 272
    Gerald Machnee says:

    Re #270 – I think what Max Mayfield meant is that the numbers are “normal” or “average” for a high SST cycle. Most of the last 10 years have had a high number of storms, but 2 of the years were low. There was also about 10 years with no landfalls. So having a couple of landfalls this year is not out of the ordinary. A normal or average is the sum of events over a number of years divided by the number of years. So you can expect some to be higher and others to be lower. So a bit above the average is close to normal. They have said that we are in a higher occurrence cycle.
    Similarly normal temperatures are obtained by adding up the (highs, lows) and dividing by thr number of years. Some countries use 30 years for doing averages. We do not see a “normal” temperature very often. This is what makes the study so interesting and exciting.

  23. 273
    Ike Solem says:

    I don’t understand why the larger issue of net equator-to-pole heat transfer, of which hurricanes can be seen as just one component, is not being brought up here. Rather then asking if global warming is responsible for hurricane frequencty/intensity increases, isn’t the more important (and more accessable) question, is global warming responsible for increased sea surface temperatures, including (importantly for hurricanes) depth profiles? I’d like to know what the predictions for equator to pole heat transfer changes are in global warming models…

    [Response: SSTs are indeed studied, Barnett et al. “Penetration of Human-Induced Warming into the World’s Oceans” (Science, Vol 309, Issue 5732, 284-287, 8 July 2005), say that the observed warming of the oceans cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences. Note, BTW, that std theory would suggest a *diminished* pole-to-eq temperature gradient under GW, since the poles warm faster (at equilibrium) and the North Pole warms faster in transient runs too – William]

  24. 274
    Dan Allan says:

    Re 272:

    Gerald, even if I accept your interpretation of Mayfield’s language, that he was referring to the last several years of activity rather than just this year, I would still have to quibble with what he said. A “return to normal” clearly implies that the low-hurricane years that preceded the last several were “abnormal”. But using your same method of averaging over several years, wouldn’t you find that these years were “normal” as well? By implying that this period is normal, and the previous quiet period is not, he is effectively preparing us for a future of high hurricane incidence, while avoiding any possible attribution to AGW.

  25. 275
    J. Sperry says:

    Re #270

    I heard one expert on TV (I think it was Max Mayfield) actually say that “this was just a return to normal.” DIfficult to see how what is likely to be the busiest / most severe season ever (if one uses a simple metric such as multiplying each named storm times maximum category, and then summing) could be seen as “normal”. Maybe it is truly attributable to mult-decadal cycle, or 99% attributable, but the lack of scientific curiosity is a little strange.

    I did this analysis, since I’ve heard of this kind of index (though Iâ??m not sure how much climatologists use it) and already had most of the data available. Including all Atlantic hurricanes (I donâ??t like to limit data to only hurricanes making landfall, as some do), 2005 is currently close to or less than at least 10 years in the last century. Using this index, the average is around 11-14, and the following years are over an index of 24:
    2004, 1999, 1995,
    1969, 1961, 1955, 1950,
    1933, 1926, 1916 (these include storms recorded before naming began)

    Aside from the (intentional) grouping of years above, I canâ??t help but notice the back-to-back years for 2004-05. However, we still have a ways to go to reach the 1950 level (e.g., two Cat. 3 and a Cat. 1 would do it). There is also an Accumulated Cyclone Energy (ACE) index, which also shows a projected range for 2005 that is similar to 9 years since 1950. (Again, we have a ways to go to reach the levels of 2004, 1995, and 1950 using this index).

    Of course, these are all different from the PDI in Fig. 2 of the original article above. I wonder what RealClimateâ??s thought is on how these other indexes compare with the PDI in determining any trend in hurricane activity.

  26. 276
    Gerald Machnee says:

    Re # 272, 274 – If Mayfield meant that this year is a longtime normal, then I would probably disagree or say he is misquoted. The site from NOAA with FAQ gives numbers for over a hundred years and I believe there is an “average” calculation. The study by Emmanuel is limited in that it begins in a lower number and ends in years with a high number. He used satellite data which means he could not look at data in the 1940’s or at the end of the last century.

    [Response: I would suggest that you re-read the Emmanuel article. Nowhere in the article does he even mention satellite data. His analysis was based on historical wind observations which date back to the mid 1940s. The relationship between the PDI he calculates from these data, and SSTs over the appropriate season and region of the tropical Atlantic is strong enough that one would suspect it is likely to hold further back in time–the trend in the most recent decade is unmatched in this SST history back into the mid 1930s.

    Perhaps you are thinking of a more recent study in Science by Webster et al which comes to very similar conclusions regarding trends between SSTs and Hurricane intensities, based on a shorter, but entirely independent (yes–satellite-based) dataset. -mike]

  27. 277
    Gerald Machnee says:

    Re #276 – Yep – I was probably referring to Webster. Thanks.

  28. 278
    Pat Neuman says:

    From ABC World News Tonight…

    POINT BARROW, Alaska, Sept. 27, 2005 â?? This season has ushered in the warmest Arctic summer in 400 years. A NASA report to be released this week finds the polar ice pack has shrunk by nearly 30 percent since 1978, and new satellite photos show the melting is speeding up. …

    In responding to the general public:

    How significant is the rapid thaw in the Arctic for hurricane development?

  29. 279
    Dan Allan says:


    If you have a handy link where you got info on hurricane frequency / intensity by year, I’d be interested.



  30. 280
    Gerald Machnee says:

    Re # 279 – The following site has named storms, hurricanes, strong ones – level 3,4,5 and ACE which is a calculation of energy.
    Is this what you need? It has yearly totals to 2003.

  31. 281
    Deborah says:

    this is all very alarming. In 500 years from now the Earth would probably be inhabitable, it would be too hot. I think based on estimates we would be gaining 5-10 degrees per century, and when we pass the critical point, the increase in temperature will exponentiate. Does anyone know of an article that can clarify the temperature increase in the next 100-500 years?

  32. 282


    Someone above said there was no link between Polar Ice melting and stronger hurricanes, absurd as it may sound for environmental experts, it may be the prevailing vu from a great many people. They are not totally at fault for thinking likewise, because I have yet to read in the public discourse or from many media outlets, that this summer was the warmest in history for the Northern Hemisphere, at least from May till August, however many people hearing this would make the connection immediately by themselves: warmest summer = stronger hurricanes. I wonder if Mr Gray mentioned this during his Senate appearance? Movie stars personal lives still overwhelm the news channels, so there is plenty of time to report something serious. This is the greatest weather lack of reporting scandal of all time, with all these outlets, hundreds if not thousands of met media specialists, somehow oblivious to this fact. I am deeply puzzled by why this is not reported.

  33. 283
    Gerald Machnee says:

    Re # 282 – The answer is simple – that type of correlation is not science, but more like gossip. Have you checked all the past high hurricane seasons since 1850 to see if that is true?

  34. 284

    Re: 279

    Complementing the data mentioned in #280, I have links to both the raw NOAA Atlantic hurricane records and a MySQL import file of a slightly cleaned up version of that data here. The page includes descriptions of the cleaning and how to reproduce the Emanuel PDI calculation for the Atlantic from the data. As soon as I get a chance to hack trig functions into Tableau, I will update the page with Emanuel’s corrections.

    The ACE page sounds interesting and I will have a look at it. Does anyone know if it is derivable from the raw observational data? I would presume so, but I haven’t looked at the page in any detail yet.

  35. 285

    Re #284

    I’m an idiot. ACE is almost the same as PDI except that it is a square, not a cube. Another fun graph to make ;-)

  36. 286
    Dan Allan says:

    Gerald, re #280 –

    didn’t find any data there for 2004 or 2005, but was able to show that 1994 thru 2003 was the most intense period (though not by a lot) since good data became available in 1944 (according to the site “ACE” method, which seemed reasonable. As 1994 was a very quite year and 2004 and 2005 very busy, I suspect that the ten year period 1996-2005 would be more obviously more intense than any previous 10 year period for which we have good data – maybe not dramatically more intense, but then, given the degree of GW experienced so far, I’m not sure we’d expect anything that dramatic yet. Maybe we lack enough data to draw a firm statistical conclusion or to confidently infer an the effects of GW – but the data is at least strongly suggestive to my lay-person’s eyes.

    – Dan

  37. 287
    Stephen Berg says:

    Re: #283,

    The correlation between warmer summers and stronger hurricane seasons makes some scientific sense, being that warmer summers have greater energy with which to work, which increases the strength of and leads to more frequent occurrence of such storms. Also, warmer summers increase (slightly) the SSTs of any water body which is affected by the higher temperatures, providing more fuel to the equation (leading to stronger and more frequent TDs, TSs, and TCs).

    It is scientific, not gossip, this correlation. Please stop sounding like Sen. Inhofe. You are and can do better than that.

  38. 288
    Gerald Machnee says:

    Re 285 –
    Try this site:

    just change the year at the end to check on 2004, etc.
    It is up to date for this year. looks nice

  39. 289


    Gossip???? You must be joking…. South Atlantic SST’s got boosted along with Lake Ontario’s surface lake temperature along with all the Arctic ice all time melt… Playing with statistics can’t possibly surpass these events actually happening now…. It is simple, it is warmer everywhere. If someone wants to complicate this image, then the motivation must be inspired by contrarian musings as usual, confusion is so much more better than a grasp of reality.

  40. 290
    Gerald Machnee says:

    Re #287 – The statement was made correlating melting ice and hurricanes for one year – THAT is not science. At the end I suggested checking from 1850 to the present for a correlation – that is a bit more scientific. You will note that 2002 was a warm year with little activity. There is more to hurricane formation than warm weather.

  41. 291
    Pat Neuman says:

    Re 282

    I think the greatest reporting scandal of all time is the lack of accurate information on anthropogenic enhanced global warming and it’s primary cause is fossil fuel burning.

    For example.

    A public comment was made this morning by meteorologist Jonathan Yuhas* on the release by NSIDC and NASA on: Sea Ice Decline Intensifies.

    Mr. Yuhas commented that he doesn’t know how they’re going to melt ice in total darkness in winter. The remark had a tone of skepticism toward the report and skepticism toward global warming.

    Mr. Yuhas has voiced his skepticism on global warming many times in the past. Should Mr. Yuhas and other weather-casters be blamed? Why are meteorologists and weather-casters so “in the dark” about global warming? Who is responsible for getting the facts on climate change to the meteorologists and weather-casters who broadcast the day to day information about weather and climate records? For many people, all they listen to from day to day are daily weather reports on TV or radio.

    * KARE11 in the Twin Cities.

  42. 292
    Gerald Machnee says:

    RE 3 291 – “accurate information on anthropogenic enhanced global warming”.
    The reason it is difficult to have “accurate” information is because it is impossible to measure it with instruments. What you have is theories and statistical association.

  43. 293
    Stephen Berg says:

    Re: #292,

    The accurate information is there. However, news station editors filter it out because it is seen by the elite to be subversive and will result in the pulling of advertising (and money) from the stations which include unbiased information (à la Mann, Bradley, and Hughes).

    Investigative reporters like Ross Gelbspan are often featured on these stations. However, the networks think they need to achieve balance by putting a skeptic (whose only purpose is to confuse or obfuscate the public) following a Gelbspan or one of MB&H. This is appalling, since it injects bias into their newscasts.

    No, Gerald, the information is there. It just isn’t getting out.

  44. 294
  45. 295
    Gerald Machnee says:

    Re # 294 – What the links you listed show are mainly temperature trends and changes. They do not calculate the percentage change that may be due to AGW. It still remains a theory based on statistical association that has been programmed into computer models and is used to forecast a future warming. The third reference has a lot of reprint from the IPCC which will likely be challenged for their next report.
    So if the temperature at New York was 68 deg F, you cannot tell me what part is due to AGW, even though you think some of it is. That is what I meant by accurate information (Calculation)
    You cannot tell me whether the temperatures will increase or decrease next year. If AGW was that significant or measureable you would expect temperatures to increase every year or you could calculate the change.

  46. 296
    Tanja says:

    Doesn’t hurricane Catarina indicate a connection between warming and hurricanes? The only one to ever hit the Southern Atlantic (Brazil).
    Apparently theoretically no hurricanes should form in this area.

  47. 297
    Gerald Machnee says:

    Re # 296 – Why would Catarina indicate a connection? It is more historically than theoretically that there have been none or few recorded events. There were two tropical storms in the area. That is an area with a lack of recording sites. When Catarina occurred was cooler than usual- so would that indicate that global warming was responsible? Tropical storms do form in the southern hemisphere – when conditions are right.

  48. 298
    Dan Allan says:

    Re #295 –


    A couple of things. First, it is possible to be accurate or inaccurate in the reporting of a theory. No? A perfect example is in your own email. You state: “If AGW was that significant or measureable you would expect temperatures to increase every year or you could calculate the change.” This is an example of a false statement about the theory of AGW. Within the year-to-year timescale, natural variability caused by many factors other than AGW can cause temporary decreases in temperate. This is forecast and expected by all global climate models.

  49. 299
    Stephen Berg says:

    Re: #297,

    It indicates a connection, since, over the last few centuries (since people have been inhabiting the coast of Brazil in rather large numbers), no TCs had ever been recorded.

    This is likely due to SSTs being sufficiently warm for the formation of these storms when previously, the SSTs inhibited TC/TS/TD formation. This would lead most scientists to conclude that the oceanic warming needed for storm development is likely due to human-induced climate change.

    As for “a lack of recording sites,” sure there may not have been many meterological stations at that time. However, there are PEOPLE who can record these events in diaries, newspapers, journals, logbooks, etc. A met-station is not the only way to collect data, Gerald.

    Also, as for:

    “When Catarina occurred was cooler than usual- so would that indicate that global warming was responsible? Tropical storms do form in the southern hemisphere – when conditions are right.”

    Maybe atmospheric temperatures were cooler than normal. However, SSTs were likely not!

    Also, TSs “do form in the southern hemisphere – when conditions are right.” However, they are so unlikely over the area Tanja (#296) had referred to (if not theoretically impossible, at least in the past) due to the cooler-than-necessary SSTs (i.e. less than 26 C). Now that SSTs have risen in this area (at least partly a result of human-induced climate change), these storms will likely occur with greater frequency (and intensity down the road).

  50. 300
    Pat Neuman says:

    Re # 294


    I think the text and links that follow address some of the elementary questions about global warming that you may still have. …
    How much has the amount of CO2 in the atmosphere changed in history?
    The figure below show results of CO2 measurements of air trapped in ice cores taken at the Law Dome site in Antarctica, along with present day measurements at the CMDL Mauna Loa Observatory in Hawaii. CO2 amounts have increased about 35% in the last 200 years.

    El Niño is a natural phenomenon that has been occurring throughout the centuries, though not always with the same regularity; it now occurs about every two to seven years. El Niño is the strong warming of the equatorial Pacific ocean. Its effects are felt worldwide, which demonstrates the interconnected nature of the Earthâ??s climate. … Scientists are concerned that the accumulation of greenhouse gases in our atmosphere may inject enough heat into the Pacific Ocean to make El Niño events more frequent and fierce.

    Declining Arctic Sea Ice:

    Earlier Midwest Spring Snowmelt Runoff

    Global Land Air Temperature ten yr mov avgs