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Did global warming cause Hurricane Katrina?

Posted on 20 October 2007 by John Cook

One of the criticisms directed towards Al Gore's An Inconvenient Truth is that he claims Hurricane Katrina was caused by global warming. Gore never actually says this and it's useful to look at his exact words as it brings up two broader questions - does global warming cause more frequent hurricanes and does it cause more intense hurricanes?

Does global warming affect hurricane frequency?

In July 2007, Greg Holland of the National Center for Atmospheric Research and Peter Webster of Georgia Tech published a paper Heightened Tropical Cyclone Activity in the North Atlantic: Natural Variability or Climate Trend? They note an increase in the number of observed hurricanes in the North Atlantic over the past century, concluding "increasing cyclone numbers has lead to a distinct trend in the number of major hurricanes and one that is clearly associated with greenhouse warming".

However, this was refuted by Chris Landsea's Counting Atlantic Tropical Cyclones Back to 1900 stating "improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical cyclones". In other words, the reason we're recording more hurricanes is due to our improved ability to observe them thanks to aircraft, radar and satellites.

In addition, a 2007 paper by Gabe Vecchi, Increased tropical Atlantic wind shear in model projections of global warming looked at the results of 18 climate models and concluded increased wind shear from rising sea surface temperatures make it more difficult for hurricanes to form and grow. So at this stage, the jury is still out on whether global warming causes more frequent hurricanes.

Does global warming affect hurricane intensity?

Kerry Emanuel wrote the definitive paper on hurricane intensity Increasing destructiveness of tropical cyclones over the past 30 years. Emanuel found a close correlation between hurricane intensity and sea surface temperature. He also suggests wind shear has much less effect on hurricane intensity than increased sea temperatures. This is confirmed by the observed data:

So the empirical evidence linking global warming and hurricane intensity is robust.

What Al Gore said about hurricanes

Here are Gore's words according to a transcript of An Inconvenient Truth by Greg Hoke:

Now I'm going to show you, recently released, the actual ocean temperature. Of course when the oceans get warmer, that causes stronger storms. We have seen in the last couple of years, a lot of big hurricanes. Hurricanes Jean, Francis and Ivan were among them. In the same year we had that string of big hurricanes; we also set an all time record for tornadoes in the United States. Japan again didn't get as much attention in our news media, but they set an all time record for typhoons. The previous record was seven. Here are all ten of the ones they had in 2004.

The science textbooks that have to be re-written because they say it is impossible to have a hurricane in the South Atlantic. It was the same year that the first one that ever hit Brazil. The summer of 2005 is one for the books. The first one was Emily that socked into Yucatan. Then Hurricane Dennis came along and it did a lot of damage, including to the oil industry. This is the largest oil platform in the world after Dennis went through. This one was driven into the bridge at Mobile.

And then of course came Katrina. It is worth remembering that when it hit Florida it was a Category 1, but it killed a lot of people and caused billions of dollars worth of damage. And then, what happened? Before it hit New Orleans, it went over warmer water. As the water temperature increases, the wind velocity increases and the moisture content increases. And you'll see Hurricane Katrina form over Florida. And then as it comes into the Gulf over warm water it becomes stronger and stronger and stronger. Look at that Hurricane's eye. And of course the consequences were so horrendous; there are no words to describe it.

How in god's name could that happen here? There had been warnings that hurricanes would get stronger. There were warnings that this hurricane, days before it hit, would breach the levies and cause the kind of damage that it ultimately did cause. And one question that we, as a people, need to decide is how we react when we hear warnings from the leading scientists in the world.

While not explicitly claiming a connection between hurricane frequency and global warming, Gore does allude to hurricane formation & numbers which gains him some demerit points. However, the general thrust of Al Gore's words are correct in that warmer water causes stronger hurricanes. He clearly doesn't say that Hurricane Katrina was caused by global warming but that warmer waters intensified it. All in all, he scores a pass mark with the way he handles hurricanes with a few marks off for not being clearer on hurricane frequency.

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Comments 1 to 12:

  1. No Al Gore does not get a pass on Hurricane Katrina. He clearly implies in his book and never once qualifies it that Katrina was caused by global warming. Katrina was a Category THREE -- count it -- Three hurricane when it hit New Orleans.
    This is far less intense than many of the worst hurricanes that the Gulf has historically produced. It also hit a city that was an accident waiting to happen: the result of decades of poor infrastructure maintenance and bad urban planning. Those who suffered the most in New Orleans -- poor people of color --deserve better than to have a group of elite white men prey upon their suffering to further their own agenda.
    The scientist who argues that we are currently seeing more intense hurricanes due to global warming, Kerry Emanuel, also wrote: "it would be absurd to attribute Katrina to global warming." The storm wasn't what caused the disaster, but I imagine that FEMA, the city mayor, and La's governor are all thrilled to be exonnerated by you.
    Where on earth is the liberal conscience here. What Al Gore wrote in the book where Katrina is concerned (which I understand captures almost word for word the film) is propaganda pure and simple. This does nothing for the cause of global warming, and the people of New Orleans deserve better.
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  2. Well you could start here... Though there are many others. What many of them say is that warming has not increased intensity or frequency.

    Free, M., Bister, M. and Emanuel, K. 2004. Potential intensity of tropical cyclones: Comparison of results from radiosonde and reanalysis data. Journal of Climate 17: 1722-1727.

    However since CO2 is supposed to produce warming primarily in cold dry air, the models I am familiar with predict that the poles should see the most significant warming. As temperature differential between the tropics and the poles is a major driving force of cyclonic storms a decrease in the differential should lead to a decrease in storm intensity. Prior to Katrina the relatively low number of major hurricanes in recent years had been seen as evidence that the models were right. I have looked at a number of papers and while 15-20 years ago I would have said warming should increase system energy and therefore intensity, I think I was wrong back then. It seems pretty clear that frequency and intensity have not increased in the last 70 years, if GW theory predicts they should this would in fact be strong evidence against it.
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    Response: Thanks for the link - interesting that the 2004 paper is co-authored by K Emanuel who wrote the 2005 paper that is the primary evidence linking greater SST to hurricane intensity. There's more to this, I'll see what I can track down.
  3. The mere fact that he devotes a portion of his slideshow/movie to hurricanes would imply that he wants his audience to believe that GW and an increase in hurricane frequency/intensity go hand-in-hand. He gets no free pass for any aspect of that.
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  4. A basic explanation of hurricane formation can be found here:
    and the basic requirement for a hurricane to form is that the sea temperature must rise to around 27C. Since GW produces elevated sea temperatures, it increases the probability that they will occur. It does not, however mean they will be more intense - it may well be that we would experience greater numbers at lower intensities; that's a a lot of weather processes, we simply do not yet know enough to make meaningful predictions.
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  5. John,

    Your article correctly states that there is no observable link between GW and hurricane frequency.

    However, your inference that the ink between sea temperatures and hurricane intensity can be attributed to GW is seriously flawed, as explained in this article by William H. Gray, professor emeritus in the Department of Atmospheric Science at Colorado State University;-

    Hurricanes and Hot Air
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  6. That's incorrect Healthy Skeptic. Perhaps you are unfamiliar with the science on this issue. I've had a look at the newspaper interview (!) you are sourcing your info from and it's pretty poor stuff. Much of the "article" is assertions about the absence of an increase in the number of hurricanes...Gray concludes a section with "The hypothesis that increasing carbon dioxide in the atmosphere increases the number of hurricanes fails by an even wider margin when we compare two other multi-decade periods..."

    However it's clear from John Cook's top article that the issue isn't about the number of hurricanes but the number of high intensity (cat 4/5 hurricanes), and these have increased in line with raised sea surface temperatures (SST), as Gray acknowledges. So most of the "article" is uncontroversial.

    Unfortunately Gray's asertions about the source of raised SST doesn't accord with the scientific evidence, since:

    ONE: The evidence indicates that the global rise of SST has a major component of man-made warming due to greeenhouse gas emissions:

    e.g. Barnett, T. P. et al (2005) Penetration of human-induced warming into the world's oceans Science, 309, 284–287.

    Abstract:"A warming signal has penetrated into the world's oceans over the past 40 years. The signal is complex, with a vertical structure that varies widely by ocean; it 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. Changes in advection combine with surface forcing to give the overall warming pattern. The implications of this study suggest that society needs to seriously consider model predictions of future climate change."

    [TWO] The AMO (Atlantic Meridonal Overturning) that Gray speaks of is likely to have made only a small contribution to increased SST:

    e.g.: K. E. Trenberth and D. J. Shea (2006) Atlantic hurricanes and natural variability in 2005 Geophysical Research Letters, VOL. 33, L12704

    Abstract:"The 2005 North Atlantic hurricane season (1 June to 30 November) was the most active on record by several measures, surpassing the very active season of 2004 and causing an unprecedented level of damage. Sea surface temperatures (SSTs) in the tropical North Atlantic (TNA) region critical for hurricanes (10° to 20°N) were at record high levels in the extended summer (June to October) of 2005 at 0.9°C above the 1901–70 normal and were a major reason for the record hurricane season. Changes in TNA SSTs are associated with a pattern of natural variation known as the Atlantic Multi-decadal Oscillation (AMO). However, previous AMO indices are conflated with linear trends and a revised AMO index accounts for between 0 and 0.1°C of the 2005 SST anomaly. About 0.45°C of the SST anomaly is common to global SST and is thus linked to global warming and, based on regression, about 0.2°C stemmed from after-effects of the 2004–05 El Niño."

    [THREE] Analysis of the dominant causative influence on high SST supports the conclusion that GW results in raised SST results in increased tropical storm intensity, rather than the alternative (AMO---> raised SST):

    e.g. Elsner JB (2006)Evidence in support of the climate change - Atlantic hurricane hypothesis Geophysical Research Letters 33 L16705

    Abstract: "The power of Atlantic tropical cyclones is rising rather dramatically and the increase is correlated with an increase in the late summer/early fall sea surface temperature over the North Atlantic. A debate concerns the nature of these increases with some studies attributing them to a natural climate fluctuation, known as the Atlantic Multidecadal Oscillation (AMO), and others suggesting climate change related to anthropogenic increases in radiative forcing from greenhouse-gases. Here tests for causality using the global mean near-surface air temperature (GT) and Atlantic sea surface temperature (SST) records during the Atlantic hurricane season are applied. Results show that GT is useful in predicting Atlantic SST, but not the other way around. Thus GT "causes" SST providing additional evidence in support of the climate change hypothesis. Results have serious implications for life and property throughout the Caribbean, Mexico, and portions of the United States."

    ...and so on. If we're really intested in understanding these issues, we really should be addressing the evidence from the science, and not unsubstantiated assertions from newspaper articles!
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  7. Chris,

    Let me get this straight? Are you suggesting that William H. Gray, professor emeritus in the Department of Atmospheric Science at Colorado State University is a liar?
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  8. As one of the leading researchers in atmospheric science and someone who has been issuing Atlantic basin seasonal hurricane forecasts for the past 24 years, I find Prof Gray's article compelling, particularly his concluding remarks'-

    "One reason may be that the advocates of warming tend to be climate modelers with little observational experience. Many of the modelers are not fully aware of how the real atmosphere and ocean function. They rely more on theory than on observation.

    The warming theorists -- most of whom, no doubt, earnestly believe that human activity has triggered nature's wrath -- have the ears of the news media. But there is another plausible explanation, supported by decades of physical observation. The spate of recent destructive hurricanes may have little or nothing to do with greenhouse gases and climate change, and everything to do with the Atlantic Ocean's currents."
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  9. HS you can "get this straight" by reading my post (rather than engaging in contrived indignation and attempted "argument" by hyperbole!).

    As I said in my post #13, "Gray's assertions about the source of raised SST doesn't accord with the scientific evidence", and I demonstrated the rather straightforward evidence on which my statement was based.

    You don't seem very interested in the science, and find unsupported assertions and vague insinuations about climate scientists in newspaper articles "compelling". That's fine.
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  10. No chris, unlike yourself I look at all the evidence, not just that offered up by the scientists embedded in the current paradigm.

    Why you keep bringing up newspaper articles all the time I have no idea. I respect Prof Gray's scientific opinion since he is one of the leading researchers in atmospheric science not because of something I have read in a newspaper! What credentials do you posess? (Besides being able to post a mass of vaguely-related papers and articles to prop up your a priori beliefs).
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  11. I didn't bring up newspaper articles, you did HS. You referred to a newspaper article (the Opinion section of the Wall Street Journal). It's right there in your post #12 above.

    The problem with getting your info from newspaper opinion articles is that the opinionator can say things that might not be a true representation of the scientific evidence. That seems to be the case with the newspaper piece you referred to.

    You didn't look at the evidence HS. You looked at an opinion piece in a newspaper. The science (see post #13) supports the interpretation that ocean warming is the result of enhanced greenhouse warming.
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  12. Chris,

    Again you seem to be implying that Prof. Gray's scientific opinion is somehow diminished, simply by having it reported in a media article rather than a scientific journal. If that is indeed the case, then you are more naiive than I thought you were.

    I think you are just clutching at straws here in trying to explain away the fact that a respected atmospheric scientist simply does not agree with your apparently 'inerrant' scientific paradigm.

    This trend is growing stronger and stronger in scientific circles... get over it and move on. It's how science works.
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