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Hurricane Spin

Filed under: — group @ 24 April 2007

Michael Mann and Gavin Schmidt

A recent paper by Vecchi and Soden (preprint) published in the journal Geophysical Research Letters has been widely touted in the news (and some egregiously bad editorials), and the blogosphere as suggesting that increased vertical wind shear associated with tropical circulation changes may offset any tendencies for increased hurricane activity in the tropical Atlantic due to warming oceans. Some have even gone so far as to state that this study proves that recent trends in hurricane activity are part of a natural cycle. Most of this is just ‘spin’ (pun intended), but as usual, the real story is a little more nuanced.

We have commented on the connections between hurricanes and climate change frequently in the past (see e.g. here, here, here, and here). The bottom line conclusion has consistently remained that, while our knowledge of likely future changes in hurricanes or tropical cyclones (TCs) remains an uncertain area of science, the observed relationship between increased intensity of TCs and rising ocean temperatures appears to be robust (Figure 1). There is nothing in this latest article that changes that.

Figure 1. Measure of total power dissipated annually by tropical cyclones in the North Atlantic (the power dissipation index “PDI”) compared to Aug-Oct tropical North Atlantic SST (from Emanuel, 2005; data)

The Vecchi and Soden (V+S) study suggests that increased ‘vertical wind shear’ in the tropical Atlantic might overcome this effect. Wind shear is related to the rate at which different layers in the atmosphere move – zero shear means that the layers all move together, large shear means that the upper layers are moving very differently to those below – and is inimical to hurricane formation and intensification. The well-known impact of El Niño on reducing Atlantic hurricane activity is in fact due to increased shear from the associated atmospheric circulation changes. The V+S results come from analysing the results of 18 different model simulations that were done for the IPCC AR4 and which now provide a superlative database for assessing what models do and do not project. It’s important to be clear that these models do not resolve hurricane processes and that the analysis is related to the large scale ‘background’ environment in which hurricanes form. Nonetheless the idea of looking at these simulations to see what happens to that large scale environment, as V+S have done, is certainly interesting and worthwhile.

V+S find that the IPCC AR4 models produce an decrease in shear near the equator and an increase in the subtropics. Over the ‘Main Development Region’ for Atlantic hurricanes, the results are mixed and, to our eyes at least (see Figure 2), don’t provide a compelling argument for hurricane activity reductions. However, the conclusions rest heavily on something that is not robust at present; the prediction of mean changes in the Walker circulation. As we have discussed in some detail, this latter issue rests upon considerations that take us to the heart of where the models are currently at their weakest–getting marine stratocumulus clouds right, producing a realistic intertropical convergence zone in the tropical Pacific, producing realistic Kelvin wave behavior in the tropical Pacific ocean–things that are all critical for an accurate representation of the Bjerknes feedbacks which are, in turn, so central to the mean state and variability of the Walker circulation (and El Niño). It is conceivable that the various simulations in the AR4 ensemble analyzed by V+S are at the same time mostly in agreement, and yet wrong, in what they predict for future Walker circulation changes. The prediction of increased wind shear in the tropical Atlantic is no better than the underlying predictions in the models of Walker circulation changes.

Figure 2. Average of 18 models changes in Genesis Potential Index (GPI) of hurricanes for 2081-2100 (IPCC A1B scenario). GPI includes the effects of heating and wind shear. (Update: figure updated to fig 4d from the paper.)

Furthermore, the fact is (as shown in Figure 1) that hurricane intensity has increased in recent decades as SST has risen (at least in the North Atlantic for which trends are most reliable) and this prediction is based on fairly fundamental and robust thermodynamic arguments explored by Emanuel and others for decades now. Emanuel (2005) makes a compelling case that the warming ocean temperatures (and associated changes in atmospheric temperature and humidity profiles) are behind the increased TC intensity in the Atlantic. Independent analyses, such as those described in the Santer et al PNAS article, show that this warming is inconsistent with natural variability, i.e. it is likely only explainable in terms of anthropogenic forcing. That would seem to close the loop on the argument that anthropogenic forcing is likely behind a substantial component of the observed increased intensity of Atlantic TCs. So the observational evidence thus far is not in favour of increased shear preventing this increase in intensity.

This view is echoed by Kerry Emanuel in comments on the paper in the Washington Post, where he suggests that the impact of wind shear changes relative to warming SSTs in the real world, as diagnosed from trends observed thus far, may be overstated by the V+S study:

Emanuel, who was not involved in this research, said he published a study last year that calculated that increasing the potential intensity of a storm via warming by 10 percent increases hurricane power by 65 percent, whereas increasing shear by 10 percent decreases hurricane power by only 12 percent.

In the same WaPo article, Chris Landsea discusses these projections into the future as if they had relevance to the attribution of past change. While this mistake is often made, it is nonetheless incorrect. Attribution can only be done using simulations and observation of the period in question.

Finally, a cautionary note seems warranted. Suppose that the V+S findings are in fact correct, and that increased wind shear will play a substantial role in future changes in TC behavior. This could be a mixed blessing. Wind shear in the tropical Atlantic will remain highly variable from year to year, changing at the whim of individual El Niño and La Niña events which influence the Walker Circulation. Temperature trends, on the other hand, are far more steady over time, and every simulation examined by the Vecchi and Soden predicts substantial warming in the main development region for TCs in the tropical Atlantic in the decades ahead. While increases in wind shear could offset the impact of tropical temperatures in some — maybe even the majority — of storm seasons, one might worry about what happens during those seasons where there is anomalously low shear (e.g., a very strong La Niña event). The warm ocean will still be sitting there, waiting to produce tropical cyclones and Hurricanes–and the prospects for destructive Hurricane activity during those seasons could be especially grim. In short, the V+S results could presage a future where there is increased interannual variability in TC behavior, and where the worst Hurricane seasons are considerably more destructive than today.

The findings of V+S represent an important contribution to the ongoing scientific discourse on the issue of climate change impacts on tropical cyclones, and the study should spur additional work looking at the complicated issues involved in greater detail. It remains the case that the modeling of Hurricane-climate change interactions is still at a relatively primitive stage and this study is very unlikely to be the last word. We will of course follow the future developments closely.

100 Responses to “Hurricane Spin”

  1. 51
    Peter Webster says:

    I had hoped to supply some diagnostics on the Vecchi-Soden paper for RealClimate but Carlos Hoyos and I are off on a campaign to Bangladesh for the next couple of weeks so they will have to wait. But the reason that we have decided to do some diagnostics comes from a number of questions we had about the manuscript:

    (i) V-S look at the IPPC model results for the next century. But absent from the analysis is a comparison of the shear that the models produced during the runs for the last century with the shear that appears in reanalysis products. Simply, how well do the models replicate the shear in the areas discussed in the text? How well do the models replicate the variance in the shear?

    (ii) V-S plot the vertical shear for two 20-year periods. The later period shows a small increase of about 2 m/s through the troposphere. How large is this change relative to what has occurred during the last 50 years due to either natural or SST increases? Furthermore, if one looks at shear variability during the last 50 years, is a change of this magnitude in the two regions sufficient to impact hurricane genesis? I feel that some benchmark is needed.

    I must say that I am a little amused by the fervor of Chris Landsea with respect to the paper. It seems that there is almost a moral imperative to state that there will be no changes in hurricane characteristics in the future. But it is interesting that Landsea has chided us in the past for concentrating on the trend in SST stating that one should not just look at one variable. I guess that is OK unless the variable is vertical wind shear!

  2. 52
    Joel Shore says:

    Re #14: Thanks for the heads-up on Dr. Trenberth’s talk here in Rochester. As a result of seeing your post, I attended most of it. I thought he did an excellent job.

    (To broaden this out to the rest of the readers here, I’ll just add that it is definitely worthwhile to see Trenberth if he comes to your neck of the woods.)

  3. 53
    Mike Donald says:

    Sounds like a good idea Tonya. Think I’ll call myself “Deliverance” on it.

  4. 54
    Tonya says:

    Thanks Deech, Barton, Mike, and the rest of you with your wise words of advise!

    Deech, I took a look at the thread at Fredrick News, and must say, I admire your patience and way with words. I’m starting to think there is a psycological phenomenon here like denial after a break-up or a death in the family. Maybe people can’t deal with the guilt? Perhaps this should be covered in a different venue. (Maybe a research proposal for a psycologist or social scientist)

    I’m an educator, and I think as a society we are doing very poorly in instilling a basic respect/faith in the scientific method. I’ve talked to many supposedly learned people and have been shocked to hear their opinion of GW as “hyped” and “overblown”. People also think the scientists are unscrupulously trying to secure funding by making a big deal out of a “non-issue”. Most profs I know though, don’t do it for the $$$.

  5. 55
    Timothy Chase says:

    I have a few questions related to this topic…

    Someone had asked earlier whether, given greater windshear, if this is sufficient to reduce hurricanes or at least the strength of hurricanes, this might result in more active tornado seasons. The answer to this was given a little earlier – although indirectly. Windshear increases the production of thunderstorms, and more powerful thunderstorms will tend to spawn tornadoes. Thus during seasons of greater windshear (e.g., el Ninos) one should expect tornadoes. However, this shouldn’t compensate for the absence of hurricanes during a given season since hurricanes will produce heat exchange at a much greater scale.

    Is this correct?

    Likewise, as I understand it, el Nino years which diminish the number and intensity of hurricanes are also associated with drought in the US, but monsoons (and I presume cyclones) in Asia. Likewise, la Ninas tend to produce droughts in Asia. So I would expect the the cyclones in Asia and droughts in the US during el Nino years and droughts in Asia during la Nina years to be more frequent, severe and over a wider area (including but not limited to a wider range of latitudes) than they are currently as this century progresses.

    Does this seem reasonable?

    Now one of the points made earlier by Gavin was that the ocean is able to retain heat, and thus if a given season fails to produce hurricanes one season, then the next season that there is less windshear should see more powerful hurricanes due to the increased heat reserve. However, as I understand it, we are also expecting to see hurricanes develop in more temperate zones, and in the absence of heat exchange due to tropical hurricanes, I would expect to see hurricanes in more temperate latitudes for those years.

    Is this something we might tend to see?

    I also noticed someone refering to a “conveyer belt” potentially developing between the cooler, more arctic regions and the tropics. I would assume that this means the jetstream is likely to take on a more north-south direction than it has at present, since due the greater temperature differential it will be able to compensate for (or “overcome”) the rotation due to the earth’s rotation.

    Is this correct?

    If so, will the see-saw with dry and wet alternating between east and west breakdown? And if so, is this more difficult to model given the fact that we haven’t any experience with that sort of thing?

  6. 56
    Ray Ladbury says:

    Tonya, there are indeed psychological phenomena going on here–many of them. One issue is calle asymmetry of information–a phenomenon where one party knows much more than another about an issue of interest to both of them. Economists have done lots of research into this phenomena when it comes to markets (informed seller vs. uninformed buyer and vice versa) and have found that a sale is very unlikely to occur under these conditions. Another phenomenon is the fact that people simply do not like to be told that they must change behavior–particularly to avoid some adverse consequence. Indeed, some people (mostly those with a Y chromosome) seem to think it enhances their “coolness” to blatantly disregard risk–e.g. smoking, driving, overeating… all come to mind. There also seems to be a phenomenon I have seen with regard to relativity and evolution. I used to be an editor at a physics trade magazine, and cannot count the number of times I had to deal with people who thought they had disproved Einstein’s theory of relativity. Some were hog-assed loco, while others were normal folks who became obsessed with this quest–despite having no mathematical background or ability. It was as if there was an equation: “Einstein = Genius, therefore if I can prove I’m greater than Einstein, I must really be smart.” There also appears to be some of this at work among anti-evolutionists–christian or no. Some people seem to have this idea that disproving climate change will prove they’re smart and get them on Oprah. In reality, it’s more likely to get them on Jerry Springer, but that doesn’t seem to matter.
    Finally, as with everything else, despite the fact that every reasonable Republican politician/activist/donor of any stature or integrity–and the President besides–have all said we’re changing the climate, and despite the fact there really is room at the table for conservative/business friendly solutions to climate change, the subject has become a red vs. blue issue. And that suits the denialists just fine–at least until the consequences of our inaction catch up with us. But I suppose they expect to have bought up all the high ground by then anyway.

  7. 57
    JPK says:

    The extent of ENSO associated wind shear can be overstated. If the Bermuda High builds into the eastern half of the US and remained anchored there through the last summer months, TSs could easily translate westwards, but farther north of thier usual track. The Gulf of Mexico would be spared, but the Tidewater States through the New England Corriegdor would not. There have been instances where the Bermuda High built itself all the way into Central Canada. Under these benign conditions aloft, high level wind shear would be neligiable.

    Once these storms get caught up in the Westerlies and evolve from warm core to cold core storms, the UK could get hammered. Mother Nature always has the last word.

  8. 58
    Jerry Alexander says:

    Nigel Williams #42 – The problem with your analysis of the flooding of Aisan Islands is that the latest studies indicate wasteage from Greenland ice sheets have yeided a diversity of values. Sea-level rise is the equivalent of 1.0 mm/year to a sea-level fall equivalent of 0.15 mm/year. This is based upon the variability exsisting in the ice sheets. Remember, most of the Aisan Islands are volcano tops and are subject to sinking. (16 March 2007 issue of Science)

  9. 59
    Richard Ordway says:

    15. [I’m trying to make sense of two apparently contradictory bits of model output: one which says that warming is likely to produce more El Nino-type events, another which implies that either frequency or intensity or both are likely to increase under the same scenario.]

    I still don’t understand your question…but here goes anyway:

    As I understand it, if (for me to push it to an extreme), El Nino increases to a “permanent” state because of (human) AGW- global warmingclimate change, climate distortion, etc. , you will still have the potential for a few hurricanes even in an El- Nino environment.

    Remember, even in a strong, El Nino, you usually still get humicanes even today. El-Nino only *reduces* the Atlantic hurricane activity…not eliminates it. (I believe this is trustable information).

    So if it becomes a permanent El Nino situation, you will still have the chaotic weather effects of when the “hurricane formation destroying” wind shear misses the formating hurricane due to the shifting jet stream…and Voila…

    Now, when that hurricane finally starts with all that possible extra available pent-up energy… it might make Katrina look like a current thunderstorm in the arctic… which the Inuits didn’t have a name for until a few years ago.

    2) Now for future increased “hurricane activity” in an AGW world: I have read data, that indicates that the frequency and intensity of *Pacific hurricanes* (called typhoons) might very well increase no matter what Atlantic hurricanes do. Might this be what you are talking about and be the confusion?

    3)If you are talking about models contradicting each other on this issue…well, no one says “this” issue is settled…or this post would not be here…right? Now, if these models are wrong…it *does not* mean that the scientific community has low confidence levels in the general trends that future climate models indicate which model larger area.

    Remember, for these “hurricane models”, you are trying to deal with a much higher resolution (or more regional effects which are more weakly modeled so far) than “climate models” which deal with easier-to-model larger affects…ie. if you add CO2, you get more average surface warming, etc. This is immensely simplified but gets the general point.

  10. 60
    Steve Reynolds says:

    Craig Allen> Do you see people running around in the streets wailing and doing crazy things?

    Saying we should limit our TP use to one sheet seems rather crazy.

    Craig Allen>…its unlikely that next year will be as dry as this I guess (heck what are the odds of it being as dry or drier than a record dry year after all!).

    I assume you are taking the side that next year will be drier than this year in Australia. I will accept your bet at even odds. For $100?

  11. 61
    Ron Taylor says:

    I spent some time on the climatebrains website. Deech, you are a real hero if you really plan to take that on. I could see nothing serious going on there. It consisted mainly of people weighing in with opinions based on their personal desires and philosophy of life, with little reference to data or even science. Much paranoia in the air. Rather pitiful efforts to attack AIT and the hockey stick, the latter dredging up stuff long since dealt with by the NAS, etc. They will happily talk to each other while the real world moves on. My deep gratitude to you good RC folk for this quality source.

  12. 62
    Tonya says:

    Re #56 Ray, very interesting. Climate may be easier to analyze than peoples’ beliefs and behaviors. What worries me is that the local weatherman as well as the radio show hosts “Rick and Bubba” all with ties to R. Murdoch seem to be egging people on around here to be vehemently closed minded about this issue…makes me wonder exactly how much denial is innate and how much is learned.

  13. 63
    Ike Solem says:

    RE#60, Steve – I’d suggest taking a look at the World Meteorological Organizations 2006 State of the Climate Report (4MB pdf), in particular the global maps on pgs 6 and 7. Record droughts in many areas of the world, the loss of arctic sea ice – what you see is an increasing trend that is superimposed on annual variablity (no bets on what happens next year, but the five-to-ten year average in global temperatures, sea surface temperatures, ocean heat content – those will increase – and ice sheet volumes, tropical glacier volumes, sea ice extent will decrease. Those trends are pretty robust.)

    Notice also that regions experiencing drought or flooding also tend to have a 10% decrease in agricultural productivity – no CO2 fertilization effect is possible if no water is available. It’s not just agricultural yields that are affected – rates of carbon storage by the terrestrial biosphere will probably be reduced, meaning that CO2 will accumulate at a faster rate in the atmosphere, in contrast to claims made by (and the latest front group for the fossil fuel lobby, “”, home of Idso, Balling, Baliunas, Singer, Michaels, etc.).

    Here’s a recent quote from an icecap link to an interview with Lindzen:
    Q Is there any scenario in which global warming could be beneficial for the planet?
    A Of course. Canada looks like it will benefit considerably if it were to happen. And it might very well happen — but it won’t be due to man.
    Q I read that you bet one of your colleagues that the Earth will actually be colder 20 years from now?
    A I haven’t bet on it, but I figure the odds are about 50-50.

    What does he base his ‘figuring’ on?

    There’s also this exchange:
    Q Are you suggesting that scientists manipulate their findings to get in on the gravy train?
    A You have to differentiate the interests of different groups. In the scientific community, your interest is for your field to be recognized so that it will have priority in government funding.
    Q So you are not accusing your scientific colleagues of corruption?

    And Lindzen complains about ‘personal attacks on his integrity’. Amazing, especially when you consider that this is the same Lindzen who charges oil and coal interests $2,500 a day for his consulting services.

  14. 64
    Steve Horstmeyer says:

    RE: #27

    Substitute the word “change” for “shear” and the concept is clarified. Storm chasers are in general referring to the change of wind, both speed and direction, in the vertical.

    Vertical velocity shear means an increase of speed with height.
    Vertical directional shear means a change of direction with height.

    Both are important in thunderstorm formation and severity.

    Wind speed generally increases with height. If the change exceeds a certain value (that value is dependent on the environment and not a specific numerical value applicable to all situations) the thunderstorm updraft will be tilted and interaction between the updraft and downdraft will be minimized. Because the up and down drafts are in separate parts of the storm the evaporatively cooled downdraft will not cut off inflow of warm, moist air fueling the thunderstorm. Thunderstom life is longer and can become stronger in these cases.
    Another way vertical shear of velocity enhances thunderstorms is by evacuating air at high elevations. Large values of velocity shear in the vertical indicate a jet stream aloft which can enhance thunderstorm strength by transporting air down wind and strengthening the updraft.

    Change of wind direction with altitude, directional shear, can cause a thunderstorm to rotate, leading first to a mesocyclone then a possibly a tornado. Typically if winds “back” with altitude (SW >> S >> SE named by direction FROM WHICH the wind is blowing) positive rotation will occur in a thunderstorm. Positive rotation in the northern hemisphere is counterclockwise and is rotation in the same direction Earth rotates on its axis.
    The combination of strong directional shear and strong velocity shear is combined by meteorologists into a parameter called “helicity” which is used in tornado forecasting.

  15. 65
    Fergus Brown says:

    Re #59: Thank you Richard, that is pretty much as I understand it: semi-permanent El Nino = more Atlantic shear = more resistance to TC formation, which is otherwise enhanced by rising SSTs. Which appears to be one of the things which makes the sums difficult.

    Given, though, that TCs are only one issue amongst many climate impacts which are included in the WG1 SPM (hat tip to Eli for reminding me of the chart), and one of the less certain, shouldn’t we focus more on issues arising from hydrological changes and large circulation shifts (the Walker, the Hadley, etc…), than on an uncertain ‘metric’?

    Honest, I’m not as naive as all that, but sometimes I think it is important to ask simple questions, even if I am the only one who benefits from an answer.

  16. 66
    pat n says:

    Re: ENSO associated wind shear can be overstated …

    At NOAA’s NWS Climate Prediction Center conference in 2003 many presentations were on ENSO and wind shear but I don’t remember if any related wind sheer to hurricanes. My poster was the only one related to climate change out of several hundred. Mine was just ignored.

  17. 67
    Steve Reynolds says:

    Ike Solem >…no bets on what happens next year, but the five-to-ten year average in global temperatures, sea surface temperatures, ocean heat content – those will increase – and ice sheet volumes, tropical glacier volumes, sea ice extent will decrease. Those trends are pretty robust.

    How about a bet on whether average world food production will increase or decrease over the next 5 years?

  18. 68
    Pat says:

    Re 64: one interesting thing about tornadoes – when dry air at midlevels impinges on the side of the cloud and causes evaporation and sinking, this transports some of the horizontal momentum it had downward, so that at the edge of the gust front, there can be a tendency for the air to turn due to the downdraft’s momemtum being partly drawn from higher up. When the gust front gets under the updraft, it pulls up on the rotating air associated with the gust front, stretching it out, and causing it to spin faster… Both the source of rotation of the mesocyclone and the tornado ultimately come from environmental vertical shear, but a tornado can actually sometimes spin in the opposite direction.

  19. 69
    Craig Allen says:

    Re 60# and taking a punt on the weather:

    Steve, Australians have pretty much been making that bet for quite a few years in a row now. Now we are at the point where the estimates are that even if there is above average rainfall in the coming year, catchment runoff will not increase because soils are so parched and aquifers so low. The price we are now facing is the financial ruin of many in the farming community and the collapse of aquatic ecosystems along the Murray River.

    I was too harsh on the good people of Adelaide though, they are the most water-wise in the country, they only use 5% of the Murray River’s water, are implementing sensible and forward thinking strategies to reduce water use and to use it more efficiently, and the state of South Australia apparently agreed to stop over-allocating their share way earlier than other states (see here). Right now some very hard decisions are being made, including a total stop to agricultural irrigation, and the possibility of a near total ban on the outdoor use of water. The problem is that the other states use so much more and have been far less willing to cut back on allocations. And now bets are being called in, there isn’t enough left in the kitty, and so the environment along with many farmers will be short changed.

    I know this is all way off topic, but I think the current Australian situation is very instructive of the consequences of ignoring the best available science when it points to the possibility of an outcome that has very undesirable implications. Even if the science is somewhat uncertain – as seems to be the case with Hurricanes – it seems very foolish to me to gamble when the outcome of loosing your bet is potentially so bad.

    So Steve, if you like a bet, let me suggest this one, find an agricultural company in one of the Murray-Darling irrigation districts, and buy some s_hares. Why not get serious about it, put yourself in the shoes of an Australian farmer, m_ortgage you house and invest the lot. I suspect you could pick up s_hares very cheap right now because all of the hysteria whipped up by global warming alarmists. You could make a fortune!

  20. 70
    Pat says:

    Re 68 (myself) – of course, some tornados are not from supercells – or are not formed by the exact same mechanism; generally what is needed is an updraft over a region of partly horizontal shear such as that associated with a gust front – it could be a gust front from another storm… there are landspouts, gustnados, etc… these tend to be weaker then the supercell tornados, as far as I know (And I’m not entirely sure what defines a ‘landspout’; for that matter, if a supercell tornado moves over water, is it then a waterspout or is it still a supercell tornado… anyone?)


    Where could I find more information – preferably in a synopsis (as in a textbook) – of expected changes in the global circulation and tropical, extratropical, and mesoscale storm frequency, structure, evolution, timing, and tracks (I’ve found data sources and annual maps ( ) but I’m not sure I’d get around to deriving any trends from that any time soon) … and also, maps of trends or expectations of trends in surface pressure and winds?

    I had read on an ealier RealClimate post that small midlatitude storms might decrease in frequency while larger ones might be more common, and generally a poleward shift in storm tracks may occur (which suggests to me that the subtropical dry belts would also shift poleward). Is this right, and then, I’m wondering what the mechanism is by which these things happen.

  21. 71
    Pat says:

    Oh, well I did just find this…

  22. 72
    Steve Reynolds says:

    Craig Allen>…if you like a bet, let me suggest this one, find an agricultural company in one of the Murray-Darling irrigation districts, and buy some s_hares.

    That is an interesting idea. How much have such s_hares reduced their value since the start of the drought? That is a measure of how serious the market considers the situation.

  23. 73
    Ike Solem says:

    Steve, I posted these questions a few threads back and you replied: the answer to those questions is “unknown”. For your benefit, here it is again:

    Global ocean heat content has increased over the past three decades.

    The Greenland and West Antarctic Ice Sheets are losing mass.
    *Waiting on CryoSat 2 for the definitive answers.

    Drought in the Amazon, Africa, the American West and Northern China is due to anthropogenic climate change. (Let’s add Australia)

    Anthropogenic global warming is changing ocean circulation patterns.

    Burning fossil fuels has resulted in a moister atmosphere.

    Increases in hurricane intensity are linked to anthropogenic climate change.

    Sea level will rise faster than expected due to ice sheet dynamics.
    *probably TRUE

    Climate models produce realistic estimations of future climate change over the next century.
    *So Far, TRUE

    Halting the use of fossil fuels will result in the stabilization of atmospheric CO2 levels.
    -Uncertain due to carbon cycle feedback effects

    Climate change will have devastating effects on human civilization under business-as-usual scenarios.

    Energy choices made today will have a large effect on future climate change.

    Hope that helps!

  24. 74

    #63 Ike, Lindzen’s words speak for themselves, he is incapable of predicting anything:

    “Q- I read that you bet one of your colleagues that the Earth will actually be colder 20 years from now? A I haven’t bet on it, but I figure the odds are about 50-50.”

    Ridiculous words from a professor in Atmospheric Physics, sort of very demeaning for the entire field. I’ll put it this way, since he can’t predict anything, since he does not practice forecasting, he is no authority on temperature projections, just a certain professor from a great school, which is still great by leaving him speak his mind despite what would qualify as political ranting mascarading as some form of intelligence. The Canadian Journalist doing the interview was totally to the right side by the cliff at the edge of the flat Earth, she was not critical of Lindzen at all, bringing out nothing but an ideology devoid of scientific thought, a fun fest basking with a friend of mind. I have a great deal of respect and I consider those in authority in this field as capable of predicting the future to a certain degree of accuracy, this shows understanding in Atmospheric sciences, those who predict and fail have my salutations for trying , but not much more especially if they keep on the same path leading to failures. Its not money that matters it is rather a struggle to place back science where it belongs amongst thinkers and those who really work hard at it, true journalists expose explorers with a good track record from pretenders, they are needed now, more than ever.

  25. 75
    Will says:

    Mike and Gavin,

    Very interesting article. This website is a great service. I didn’t quite understand your statement about attribution, and was wondering if you could spell it out for me:

    “Attribution can only be done using simulations and observation of the period in question.”

  26. 76
    Timothy Chase says:

    RE #63

    Interviewer: I read that you bet one of your colleagues that the Earth will actually be colder 20 years from now?
    Lindzen: I haven’t bet on it, but I figure the odds are about 50-50.

    Solem: What does he base his ‘figuring’ on?

    Either it gets warmer or it gets cooler… His “null hypothesis” is that neither is more probable than the other, I suppose.

  27. 77

    Scientists —

    What peer-reviewed references are there for global warming causing drought in continental interiors and violent weather on coastlines? I got challenged on this recently and could only find some websites with news reports about studies, not the studies themselves.

  28. 78
    Ray Ladbury says:

    Re #76 and #74: I believe Lindzen’s hypothesis is that by placing a bet, he gives the impression he has confidence in his postion that anthropogenic contributions are no big deal. He gets to enhance prestiege for 20 years and only has to pay thereafter–a pretty good investment. As near as I can tell, everything Lindzen does is playing to the audience.

  29. 79

    Regarding your blog entry on the V+S paper, one statement needs clarification: “…hurricane intensity has increased in recent decades as SST has risen (at least in the North Atlantic for which trends are most reliable) and this prediction is based on fairly fundamental and robust thermodynamic arguments explored by Emanuel and others for decades now.”

    The fundamental and robust thermodynamic arguments, along with idealized modeling studies, predict nowhere near the observed increase in North Atlantic intensity. While the theory gets the sign right, the mismatch in the magnitude of the effect means that the existing theory does not provide a satisfactory explanation and there is still much theoretical/modeling work to be done to explain the observed relationship.

    [Response: Thanks for your comment. Indeed, and this is discussed towards the end of the Emanuel (2005) article. This is why we were careful to refer to “warming ocean temperatures (and associated changes in atmospheric temperature and humidity profiles)”. While the changes in e.g. PDI are highly correlated with SST changes, clearly SST is really only a proxy for a more complicated set of atmospheric and oceanic environmental variables with which it is correlated. One possibility that has been discussed by Emanuel and others is the fact that the warming of the upper ocean may be inhibiting the negative feedback associated with upwelling of colder sub-surface water due to wind stirring. That is just one possible positive feedback that would lead to enhancement relative to the simple thermodynamic ‘heat engine’ theories. But certainly there is much additional work to be done in this area. -mike]

  30. 80
    Steve Reynolds says:

    Ike> “I posted these questions a few threads back and you replied: the answer to those questions is “unknown”. For your benefit, here it is again:

    Global ocean heat content has increased over the past three decades.

    While the evidence for your answer to the first question has improved since then, relying on one recently published correction to a paper to proclaim ‘TRUTH’ is dangerous. You do not seem to believe Gavin:

    “New papers need to stand the test of time before they are uncritically accepted.”

    As for the rest of the questions, all I can say is that your standard for ‘TRUTH’ is extremely low.

  31. 81
    Timothy Chase says:

    In the inline response to #79, John wrote:

    One possibility that has been discussed by Emanuel and others is the fact that the warming of the upper ocean may be inhibiting the negative feedback associated with upwelling of colder sub-surface water due to wind stirring.

    The rate of global warming will result in reduced water density near the surface both directly as the result of surface layers expanding due to increased temperature and indirectly due to the fresh water from melts resulting in decreased salinity. Reduced density implies reduced mixing with a boundary which gradually descends over time – thus the effect should be more pronounced over time.

    Incidently, this will also increase the cost of global warming – as the surface layer will have a reduced capacity for absorbing oxygen due to its increased temperature, and less of this oxygen which is absorbed into the upper layers will reach lower ocean layers due to reduced mixing. In essence, if the degree to which hurricanes have strengthened is indicative of decreased mixing between upper and lower boundaries of the ocean, then it is also indicative of the threat of ocean hypoxia to sea life and to fish harvests in the years to come.

    When viewed alongside the threat of drought to agriculture due to decreased rainfall over land, this does not make for a pretty picture.

  32. 82
    Jeff Masters says:

    In paper published in 2006 in the Journal of the Meteorological Society of Japan titled, “Tropical Cyclone Climatology in a Global-Warming Climate as simulated in a 20 km-Mesh Global Atmospheric Model: Frequency and Wind Intensity Analyses”, Oouchi et al. used a high resolution GCM with 20 km grid resolution to look at the frequency of tropical cyclones late this century. They used an algorithm to count the number of tropical cyclones that formed in each ocean basin in the world of 2080-2099, when the assumed SSTs were 2.5 C higher than today’s (IPCC A1B scenario). Their results showed a substantial drop in the annual number of tropical cyclones observed in every ocean basin except the Atlantic, where a marked increase was seen. These results are in contradiction to the results of the Vecchi and Soden study, which pointed to a large increase in tropical cyclone activity (as seen in their GPI plot, Genesis Potential Index) in the Western Pacific, and lesser increases in the Southern Hemisphere oceans. Granted, the Oouchi et al. model had some problems simulating the present day number of tropical cyclones and their intensity, but one could argue that their results are more believable than those of the lower resolution AR4 models used in the Vecchi and Soden study. The bottom line is that we are still a long ways from being able to confidently say what the effect of global warming will be on tropical cyclone frequency.

    The Ouuchi et al. paper is available at

    Jeff Masters

  33. 83
    Steve Reynolds says:

    Timothy Chase> When viewed alongside the threat of drought to agriculture due to decreased rainfall over land, this does not make for a pretty picture.

    I will repeat my wager offer: Total world food production (including seafood) will increase over the next 5 years (and longer periods, but I do not want to wait to collect). Do you accept?

  34. 84
    Richard Ordway says:

    re. #77 Barton wrote: [What peer-reviewed references are there for global warming causing drought in continental interiors and violent weather on coastlines?]

    Here are some peer-reviewed references. You, or your librarian, can help find them at your local library or online if you have a subscription.

    Notice, importantly, that the following references are part of a *body* of evidence. Don’t trust any single study…it might or might not stand the test of time…or

    …or unfortunately…as I just saw two weeks ago a journal article that had obviously been paid for by the fossil fuel industry. The current editor is a well-known GW denier.

    It was in an obscure, but previously basically legit journal, that stated that suddenly, most accumulated peer-reviewed literature since 1842 about global warming is false…based on arguments long discredited in the peer-reviewed literature.

    So the fossil fuel industry is even buying out legit journals now to pander to their needs.

    But the following are long-established legit journals and part of a large body of evidence.

    Alley, R. B, J. Marotzke, W. D. Nordhaus, J. T. Overpeck, D. M. Peteet, R. A. Pielke Jr., R. T. Pierrehumbert, P. B. Rhines, T. F. Stocker, L. D. Talley, and J. M. Wallace, 2003. Abrupt climate change, Science, 299, 2005-2010.

    Cole, J.E., J.T. Overpeck and E.R. Cook, 2002. Multiyear La Niña events and persistent drought in the contiguous United States. Geophys. Res. Lett., 29, 10.1029/2001GL013561.

    Dai, A., K. E. Trenberth and T. R. Karl 1998: Global variations in droughts and wet spells: 1900â??1995. Geophys. Res. Lttrs., 25, 3367â??3370.

    Fye, F. K., D. W. Stahle and E. R. Cook, 2003: Paleoclimatic analogs to Twentieth-Century moisture regimes across the United States. Bull. Amer. Meteor. Soc., 84, 901-909.

    Giannini, A., R. Saravanan, and P. Chang, 2003. Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. Science, 302, 1027-1030.

    Haug, G. H., D. Gunther, L. C. Peterson, D. M. Sigman, K. A. Hughen, B. Aeschlimann, 2003: Climate and the collapse of the Maya civilization. Science, 299, 1731-1735.

    Hoerling, M., and A. Kumar, 2003: The perfect ocean for drought. Science, 299, 691-694.

    Hunt, B. G., and T. I. Elliott, 2002: Mexican megadrought. Clim. Dyn., 20, 1-12.

    IPCC (Intergovernmental Panel on Climate Change), Climate Change 2001. The scientific basis. Eds. J. T. Houghton, et al. Cambridge University Press, Cambridge, U.K. 881pp.

    Laird K.R., S.C. Fritz, K.A. Maasch and B.F. Cumming, 1996. Greater drought intensity and frequencybefore A.D. 1200 in the northern High Plains, U.S.A. Nature, 384, 552-554.

    Nicholson, S. E., B. Some, and B. Kone, 2000. An analysis of recent rainfall conditions in West Africa, including the rainy seasons of the 1997 El Niño and the 1998 La Niña years, J. Clim., 13, 2628-2640.

    National Research Council, 2002. Abrupt Climate Change: Inevitable Surprises, 182 pp., NationalAcademy Press, Washington, D.C.

    Stahle, D.W., E.R. Cook, M.K. Cleaveland, D. Therrell, D. Meko, H.D. Grissino-Mayer, E. Watson, and B.H. Luckman, 2000: Tree-ring data document 16thcentury megadrought over North America, EOS, Transactions, American Geophysical Union, 81 (12), 121,125.

    Stine, S., 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature, 369, 546-549.

    Trenberth, K. E., 1998: Atmospheric moisture residence times and cycling: Implications for rainfall rates with climate change. Climatic Change, 39, 667â??694.

    Trenberth, K. E., and C. J. Guillemot, 1996: Physical processes involved in the 1988 drought and 1993 floods in North America. J. Climate, 9, 1288â??1298.

    Trenberth, K. E., A. Dai, R. M. Rasmussen and D. B. Parsons, 2003: The changing character of precipitation. Bull. Amer. Meteor. Soc., 84, 1205â??1217.

    Trenberth, K., J. Overpeck and S. Solomon, 2004: Exploring drought and its implications for the future. Eos, 85, No. 3, 20 Jan. 2004, p27.

    Verschuren, D., K.R. Laird, and B.F. Cumming, 2000. Rainfall and drought in equatorial east Africa during the past 1,100 years, Nature, 403, 410-414. Woodhouse C.A and J.T. Overpeck, 1998: 2000 years of drought variability in the central United StatesBull. Amer. Meteor. Soc., 79, 2693-2714.

  35. 85
    S. Molnar says:

    Sorry to be late to the party (someone told me to get a life, so I did, but only for a few days), but I don’t understand the reference to “warming by 10 percent” in the Emanuel citation. To my layman’s way of thinking, warming by 10 percent represents about 28 K, which is a preposterous number. What am I missing?

    Also, don’t scoff at those who discuss bovine anal flati: raypierre still owes me an answer on whether I should be eating cow cheese or goat cheese to reduce GHGs.

  36. 86
    David B. Benson says:

    Hurricane forecaster William Gray states that global warming is due to ocean currents, not carbon dioxide, and the earth will likely cool in 5 to 10 years.

    That’s the gist of an AP story just realeased.

    My jaw dropped upon reading this…

    [Response: I guess the writer hadn’t read our Gray and Muddy Thinking… :( -mike]

  37. 87
    Deech56 says:

    RE #54 (Tonya) Thank you for the kind words. We are veering off a bit OT, but the subject of hurricanes lends itself to interesting scientific and AGW discussions. You wrote, “I’m an educator, and I think as a society we are doing very poorly in instilling a basic respect/faith in the scientific method.” and that’s why I take up the pro-science arguments (not just AGW) whenever I can. I have this naive thought that I can get people to be thinking about how scientists use the scientific method to make conclusions in the face of uncertainty (did I mention I’m a biologist?). It’s important to remember that what’s out there is out there, and all that scientists can do is observe, gather data and try to learn what is happening.

    This is a fascinating topic and there’s a lot to chew on. Judging from the correlations shown in Emanuel, when SSTs rise, some thing’s got to give.

    Question for our RC friends: From reading the news and the comments section, Australia seems to be going through a climate crisis. Would Australian climate be worthy of a post?

  38. 88

    Then it will take 5 years before Dr Gray realizes his mistake, I respect the man for at least placing a forecast based on his own conclusions, but he doesn’t believe in AGW at all, the two, the forecast of Ocean cooling and AGW are irreconcilable. Perhaps if the Ocean does not cool, he will change his mind. That is the benefit in placing at least a projection, if he happens to be right within no other circumstance at play (volcanoes, solar constant change etc) we will listen to him more. If the ocean warms as it should, time to take a good hard look at the theory which is correct.

  39. 89

    Re #85 — God bless you, Mr. Ordway. Thanks.

  40. 90
    Ike Solem says:

    Steve, the trend in global ocean temperatures would be unaffected by Lyman et al’s “Recent cooling of the upper oceans” even if it was correct, and a correction doesn’t change that either. You should go back and read Planetary Energy Imbalance at RC. Scroll down and look at the figure “Global Ocean Heat Content Change” – the black line is the observations from 1993-2003. Or just click here for the image.

    Of course, I did say three decades…so here’s the record from 1957 onwards. Hope that settles the issue.

    Regarding the chances of another severe hurricane season, here’s the latest ENSO report:

    Regarding betting, try getting an insurance company to sell you a hurricane policy in the hurricane belt: Hurricane futures spread risk, but aren�t inspiring insurers to embrace S.C.

  41. 91
    Grant McSorley says:

    Is it true that vertical shear has been decreasing in the last decade? I’ve seen a couple people refer to Hoyos, C.D., et al., 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. SciencExpress, March 16, 2006 in making this argument, but I don’t have access to the paper.

    If shear is decreasing, how does this fit with the model predictions that it should increase with global warming?


    [Response: Indeed (e.g. Figure 1c of their Science article) Hoyos et al demonstrate that there is a modest decrease in Jun-Oct wind shear for the tropical North Atlantic over the period 1970-2004. One could argue that decadal variability may mask any anthropogenic trend over such a relatively short time interval. But on the face of it, the observed changes do run counter to the prediction of increased tropical Atlantic wind shear in response to anthropogenic climate change. This once again underscores, however, the key distinction between predictions of future changes, and the detection of model-predicted changes in observations to date, two very different things as discussed in our article. -mike]

  42. 92
    Steve Bloom says:

    Re #s 62/87: I’ve had a look at the climatebrains blog, and my suggestion to Tonya would be to just monitor it and periodically drop in links to appropriate sources of reliable climate change information so that visitors have that option readily available to them. I completely agree with the point that debating the regulars there is otherwise a complete waste of time.

    Tonya, while I am no climate science expert, I do have a bit of a background in environmental issue organizing, and IMHO the bulk of your time would be better devoted to grassroots activities to educate the AL public. Certainly a locally-focused climate blog competing with climatebrains is a possibility, but if you go that route I would suggest trying to integrate it with whatever non-internet climate change education efforts are going on there. Maybe look into recruiting a non-denialist local forecaster (and/or academic weather researcher) to participate.

    Re #91: The paper is linked on Peter Webster’s pubs page.

  43. 93

    I see Chris Landsea is up to his usual spin.

    Any comments? Links?

  44. 94

    All I could find after a cursory search was this :

    It looks to me like most of William Gray’s work has been obsoleted now.

  45. 95
    Hank Roberts says:

    Asked a while back — does the prediction of increased vertical wind shear refer only to the hurricane-forming areas, where it would take them apart before they form? Or does it relate also to the tornado-forming areas, where it would enhance them?
    I recall someone (Mooney?) mentioning criticism of Gore’s one sentence mentioning tornados in his climate change book, and wondered if there’s any comment anywhere on this aspect of the predicted increase in wind shear.

    No single event of course is “caused by” climate change; perhaps a tally of this type of larger storm would be useful, though — do we expect an increase in “wedge tornados” along with a decrease in hurricanes?

    Or does the predicted wind shear increase only occur in specific areas, over oceans?

    Today’s news included:

    “Storm chaser Lance Ferguson followed the system which formed in the Oklahoma panhandle. “What was just amazing is that the storm system just kept regenerating, it just kept recycling and kept doing its thing … Mother Nature’s fury at its best,” he told CBS affiliate KWCH.

    “To see a tornado down on the ground for that long and to become that massive and that big, I havenâ��t seen anything like it before,” he said. “It just kept getting wider and wider and wider.”

    This one: The National Weather Service described the tornado as a “wedge,” an especially broad and tall formation. Frederick Kruse of the weather service’s Dodge City … – May 5, 2007

  46. 96
    Wacki says:

    A quick question if anyone is still reading 100 posts down. :-p In the NPR debate Lindzen talked about polar amplification reducing hurricanes. He said something of the tune “hurricanes are powered by the temperature difference between the pole and the equator and global warming will reduce that difference.” How much credibility is there to that argument? Is the polar amplification argument different than the windshear argument or are those two one in the same?

    [Response: The poleward temperature gradient in middle latitudes leads to the phenomenon of ‘baroclinic instability’, which governs the strength of mid-latitude cyclones (i.e. the winter storms that influence the extratropical regions of the world with their alternating cold fronts and warm fronts that come through every few days on the average). This is fundamentally different from the dynamical and thermodynamic factors which govern tropical cyclones (and Hurricanes, which are just strong versions therefore). In this case, the factors have a lot more to do with the vertical stability properties of the tropical atmosphere, and things such as atmospheric wind shear. We generally lecture on this key distinction in intermediate level undergraduate courses on weather and the atmosphere. It is possible that Richard Lindzen, who was in his day arguably one of the leading atmospheric dynamicists, simply doesn’t happen to know this. The other possibility is that he was intentionally misleading the audience to score some cheap rhetorical points. I’ll leave it to you to decide. -mike]

  47. 97
    John Oldshue says:

    I created the forum ( ) and wanted to say that the site is intended to be neutral so please come visit and voice your opinion if you can be nice and civil with your posts. My intent is to get the best minds from both sides together in a neutral setting and we are starting to accomplish that. If the forum seems one sided (which I don’t think it does) then invite more from whichever side you support to come in and voice opinions. I am going to do my best to keep it a nice and civil place. My hope is that a person that is neutral on the issue will find a place to hear intelligent debate. I will definetely try to get Dr. Mann and other from to write guest editorial which we will prominetly feature on our front page. We hope to launch the front page this week with editorial from both sides and a constantly updated headlines section. Please come visit and I will try to make it as nice and civil as I can. And for the record I am neutral and love hearing opinions from all sides.


    John Oldshue

  48. 98

    “And for the record I am neutral and love hearing opinions from all sides.”

    In other words, you are not a credible source of information.

    Thanks! Now we know where not to get our science.

  49. 99
    Pat says:

    Re – response to 96:

    I have been wondering about how the structure and behavior of baroclinic eddies would be affected – From the brief scan of IPCC WGI Chapter 9 (looking at a latitude-height zonal average temperature change plot), it seems polar amplification is limited to below 500 mb, with a reversal above there ;

    equatorward there would be increasing static stability (favoring longer wavelength eddies) while poleward of some point, static stability decreases (favoring smaller wavelength eddies), although moist convection, if I had to guess, would cancel out the increased static stability (as the moist adiabats would have lower lapse rates with higher temperature at constant relative humidity) while increasing tropopause height should favor longer wavelengths in general, I would think…

    back to the horizontal gradient, if the upper tropospheric thermal wind shear increase is greater than the decrease of the lower layer, then maybe the overall baroclinic instability would be stronger – but currently the upper level eddy circulations do not transport much heat poleward, so would the structure of cyclones change so that a deeper layer of air is involved in the thermal advection, compensating for a weaker temperature gradient? And then, would these storms move more slowly as the westerlies at midlevels would be weaker (in winter, anyway – brings up another point – summer effects vs winter effects?) – or would the steering level shift upward? Would they last longer, take longer/shorter to strengthen or weaken, move in slightly different directions etc… and what about the anticyclones – and how does all this affect the way the environments shape the mesoscale storms?

    (One idea I had was that decreased horizontal temperature gradient at one point might cause increased temperature gradient some distance downstream – the idea being that there would be less eddie circulation generated upstream, and this lack of heat transport might propogate downstream, allowing a greater temperature gradient to build up downstream, which would then give rise to stronger eddie circulations that would propogate even farther downstream… Would that work?)

    I could deduce some things about seasonal and longitudinal variations from the z(pressure) plots here:
    And I’ve seen other sources, but in case I don’t have the time for a while to go through it, it really would be nice to have a brief description of these things and how they’re related.

    PS do the transient cyclones and anticyclones, aside from steering winds, tend to drift toward the climatological cyclones (Icelandic low, etc.) and anticyclones (Bermuda High, etc.), respectively?

  50. 100
    ray ladbury says:

    Re: 97. John, I for one appreciate the invitation. A quick perusal of your site however was not encouraging. I found such topics as “The Global Warming Conspiracy,” in which it was suggested that the scientific community was prostituting itself for grant money. You can perhaps understand how that might not be well received by a scientist like myself (albeit a scientist not working in climate studies). I believe that such sentiments represent a profound misunderstanding and misrepresentation of scientists and of science. I for one am willing to believe that many who oppose the current scientific consensus may do because they are very concerned about the implications it could have for our economy. I to am concerned about this, but as a physicist, I understand enough of the science to see that it is quite cogent. I would strongly urge people who are truly skeptical to visit It is a tremendous resource, and people who approach the experts here respectfully and in a true spirit of inquiry tend to be well received. BTW, I tried to register for your site, but never got a key for it at my email. Thanks, Ray