Assombrissement global?

Premièrement, si on essayait d’estimer la sensibilité climatique uniquement à partir des réponses du 20ème siècle, nous aurions besoin de connaître un certain nombre de choses très exactement : principalement la magnitude de tous les forçages significatifs. Néanmoins, les incertitudes, en particulier sur les différents effets des aérosols empêchent une détermination exacte fondée sur la seule période instrumentale. Bien qu’il soit vrai que, toutes choses égales par ailleurs, une amélioration du chiffrage du refroidissement associé aux aérosols devrait conduire à une amélioration de l’estimation de la sensibilité climatique, les barres d’erreur sont trop grandes pour que cela prenne de l’importance. L’estimation de 3 +/-1 °C (pour le doublement du CO2) fondée sur des paléo-données et des études sur modèles reste valable, même après cette émission.

Deuxièmement, est-ce qu’une réévaluation de l’effet des aérosols implique que les projections pour 2100 doivent être pires que celles déjà suggérées ? Si la sensibilité du climat reste entre les bornes considérées dans le TRE (Troisième Rapport d’Evaluation) du GIEC, (ce qui, je l’affirme, est encore le cas), la réponse est non.

Le scénario le plus extrême développé dans le TRE (A1F1) considère déjà une forte réduction du forçage par les aérosols du type sulfate ce qui fait que le changement de température est presque exclusivement fonction du forçage par les gaz à effet de serre. Ce changement n’est donc pas affecté par une réévaluation des effet indirects des aérosols.

Le ’doublement’ suggéré de la vitesse de réchauffement dans le futur comparé même au scénario le plus extrême développé par le GIEC est donc hautement exagéré. Les conséquences supposées comme l’assèchement du bassin de l’Amazone, la fonte du Groenland, et l’établissement d’un climat nord-africain en Grande-Bretagne sont de simples extrapolations construites à partir de ces exagérations.

Ces conclusions sont-elles le reflet fidèle de ce que les scientifiques cités dans l’émission voulaient dire ? – Cela n’est pas connu. Néanmoins, si ces notions extrêmes peuvent faire le succès d’une émission de télévision, elles n’en desservent pas moins la science.

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25 comments on this post.
  1. Ramon Guirado:

    Please I need some help, Where can I find a graph with data of concentrations of the different gasses in the history of the planet?

  2. Grant:

    If “The most extreme scenario postulated in TAR” is almost solely dependent on GHG emissions, why would the introduction of aerosol effects not change the results?

    [Response: If the scenario postulates that GHG are going to be much higher, and that aerosol changes are going to be small, than the bulk of the response will be due to the GHG. Therefore revisions to the magnitude of the effects of the small component are not going to make a significant difference – gavin]

  3. Linus Senhen:

    Noted Climatologist James K. Glassman of the American Enterprise Institute recently wrote an article for a southern California newspaper about global warming being caused by changes in the sun. Specifically, he states “Research indicates, more and more, that recent warming at the surface of the Earth is mainly influenced by cyclical changes at the surface of the sun …” I am not in the field but I was not aware of any recent research about the sun affecting recent warming. What gives? At any rate, this made me think of the Milankovitch theories of Earth’s orbital attitude. As I remember, they were connected to fluctuations in Earth’s climate and were pretty well accepted after WWII. They were all in cycles of 1000’s of years and consisted of wobble around the axis, change in obliquity to the orbit and precession effect. Has any work been done to improve on the theories or on their effect on climate?

    [Response: Noted climatologist? Hmm… However, see our posts on one potential mechanism for solar forcing of climate. Solar changes are quite uncertain prior to the satellite era, but the best estimates only show a modest role for solar forcing over the last century (see Foukal 2004, Lean et al, 2002 or the new NAS study. Milankovitch forcings are well accepted, and are understood to be the driving forces controlling the glacial-interglacial cycles, see Loutre and Berger (2000) for one of the latest discussions of this and the relevance to future climate change. – gavin]

    [Response 2: Presumably JKG, and this article or like it. To be fair to him, does doesn’t claim or possess any climatolgy qualifications… or was the “Noted” bit sarcastic? – William]

  4. dave:

    Peter Cox (Hadley Centre) played a big part in that BBC presentation. Also, drying out of the Amazon by 2050 is one modeling result of the HADCM3 coupled climate model. Cox seems to be straightforward in saying that reduced aerosol effects (cooling) will result in greater warming (from GHGs) and that the cooling effect now is stronger than normally supposed. Here’s Cox:

    We’ve got two competing effects really, that we’ve got the greenhouse effect, which has tended to warm up the climate. But then we’ve got this other effect that’s much stronger than we thought, which is a cooling effect that comes from particles in the atmosphere. And they’re competing with one another.

    I do not mean to stir things up, but are you saying the folks at the Hadley Centre have gone off the deep end?

    [Response: Not at all. The quote you give is fine as it stands. It’s well accepted that aerosol cooling (by both direct and indirect effects) has balanced greenhouse gas warming to some extent. My complaint concerns the putative conclusions that the narrator makes concerning climate sensitivity and the implications for the high end IPCC scenarios. Peter Cox does not explicitly make those points, though he does appear to assume a similarly high rate of warming is possible. If Peter is reading, I’d love a clarification of what he meant. – gavin]

  5. Mark Bahner:

    Gavin Schmidt writes, “The suggested ‘doubling’ of the rate of warming in the future compared to even the most extreme scenario developed by IPCC is thus highly exaggerated. Supposed consequences such as the drying up of the Amazon Basin, melting of Greenland, and a North African climate regime coming to the UK, are simply extrapolations built upon these exaggerations. Whether these conclusions are actually a fair summary of what the scientists quoted in the program wanted to say is unknown.”

    I think it’s pretty well known. Just look at the BBC transcript to which you hyperlinked:

    DR PETER COX: “If we don’t do anything by about twenty thirty we could have a global warming of exceeding two degrees, and at that point it’s believed the Greenland ice sheet would start to melt in a way that you wouldn’t be able to stop it once it started it, it would melt. Take a long time to melt but ultimately it would lead to a sea level rise of seven or eight metres.”

    DR PETER COX: “2040 it could be four degrees warmer, the climate change could have led to big drying particularly in the Amazon Basin, that would make the forest unsustainable, we’d expect the forest to catch fire probably, turn into savannah and maybe ultimately even desert if it gets really really dry as our model suggests.”

    DR PETER COX: (Referring to the United Kingdom): “We’re talking about a change from er a lush, moist climate, environment like this, to a North African climate in just a few decades or a hundred years.”

    DR PETER COX: “You can imagine ten degree warming in the UK in a hundred years is catastrophic. Ten degree warming in a hot country already makes it essentially uninhabitable.”

    [Response: Without access to the full interview, you don’t know the context in which Dr. Cox was speaking. He may have been asked ‘Let’s just imagine that climate sensitivity was twice as big as you thought, what would that imply for the UK under the top-end IPCC scenario?’ . Absent further information, you cannot conclude that he was saying these impacts were likely or even plausible. ‘Mugging’ of scientists in such ways is not unknown. My personal dealings with Dr. Cox have lead me to respect his judgement and expertise, and so I am inclined to give him the benefit of the doubt on this one. – gavin]

  6. Deltoid:

    Global Dimming

    David Tiley has an has an
    interesting summary of a BBC program on Global Dimming. It seems
    that, over the past 40 years, while the amount of sunlight reaching
    the top of the atmosphere has not changed, the amount of sunlight
    reaching the surface…

  7. Tom Rees:

    I thought the basic idea was that, if sulphate cooling is greater than we thought, then GHG warming must be greater than we thought, and therefore climate sensitivity must be greater than previously thought. The 10 deg C rise was presented as a ‘warning’ rather than a ‘prediction’. i.e. something that could possibly happen (including carbon cycle feedbacks) but probably won’t.

    Clarification from Peter Cox would be excellent though. Perhaps you can get him to blog something?


    cheer up, it was only mass hysteria
    Tim Lambert has picked up on the global dimming story with his usual readable rigour. He points to a sceptical post on RealClimate which brings a needed breath of sanity to the story. In the process he also pointed to…

  9. Brian S.:

    Thought I recognized James Glassman’s name: he’s the author of “Dow 36,000”. I see he’s transferring his financial prediction accuracy to climatology. More info on him is available at:

  10. Ken:

    If there were no CO2 in the atmosphere, average surface temperatures of the Earth would be -15 deg or lower. Although only one air molecule in 2500 is CO2, this is sufficient to raise the temperature to +15 deg. How does one reconcile the -30 deg C sensitivity for a -100% decrease in CO2 with the estimated +3 deg C sensitivity for a +100% CO2 increase? I would think that the sensitivities for +/-100% CO2 change would be of comparable magnitude.

    Regarding the “Faustian bargain”, has anyone looked at the theoretical feasibility of introducing some kind of biologically benign, synthetic aerosol into the atmosphere to counteract GHG warming? (Clearly, this approach should only be considered to prevent catastropic warming effects, such as melting the Greenland ice sheet or destabilized methane hydrates.)

    [Response:: Firstly, the response to CO2 is non-linear. Secondly, I think the ~30 oC value you are quoting is for the *total* greenhouse effect, not just CO2. Lastly, I haven’t heard of the aerosol idea, but others (e.g. iron fertilisation of the oceans have been semi-seriously proposed (and in the case of iron, some experimental work done) – William]

  11. David:

    Can any of the experts lurking here refer me to any updates to the anthropogenic direct effect aerosol radiative forcing digram – Figure 6.8 -(Figure 401) in the TAR?

    I am familar with the Liepert et al. ground based work which shows fairly consistent reductions in surface solar radiation upto the 1990s (and the apparent linkage of this to the evaporation paradox).

    The question is whether this increase in aersol forcing is continuing, or if cleaner burning technologies have reversed the trend?

    Many Thanks,


  12. Pat Neuman:

    The Arctic is warming more rapidly than IPCC scientists
    had anticipated in 2001.

    What is the amount of additional greenhouse gases now
    expected from more rapid permafrost thawing in the Arctic?
    What about feedbacks from methane burps in warmer waters?

    These questions should be addressed before one concludes,
    as gavin did above, that:

    > The suggested ‘doubling’ of the rate of warming in
    > the future compared to even the most extreme scenario
    > developed by IPCC is thus highly exaggerated.
    > … while these extreme notions might make good
    > television, they do a dis-service to the science.

  13. Beate Liepert:

    A comment and/or answer to David’s question. New studies on ground based observations extending to 2000 are currently in the publication process. In General without details, your suspicion seems correct that there is reversing tendency in direct aerosol forcing in highly industrialized areas. But these papers will be out soon.


  14. Ferdinand Engelbeen:

    While particulate emissions over the Western world decreased since the mid-1970’s, the emissions in developing countries increased rapidely in the past halve century. The global trend in general leveled.

    It is difficult to find exact results of the most comprehensive investigation on aerosols, the INDOEX, over SE Asia on the open internet, but there is one interesting investigation done in the Indian Ocean, where a highly aerosol contaminated area in the NH was compared to a far less contaminated one in the SH. See: If aerosols act as cloud seeds (and prolong the cloud’s lifetime), there should be different trends between the two regions. Which is not the case.

    Sulphate aerosols give more reflection of sunlight, thus should have a cooling effect. Soot aerosols absorb more sunlight and increase tropospheric warming. Both have a dimming effect. But where is the balance between the two? And what is the ultimate effect on surface temperature?

    According to the investigation:
    “There is a strong increasing trend in sea surface temperature over the northern Indian Ocean during the 1952-96 time period” and “Soot was a sizeable fraction of the aerosol mix and caused substantial absorption of solar radiation. Satheesh and Ramanathan [2000] infer from satellite and surface measurements that aerosol heating in the lower atmosphere over the northern Indian Ocean at local noon is 1-3 K/day, an increase of 50-100% over aerosol-free solar heating.”

    Thus the balance is that soot has more positive effect than the combined direct and indirect negative effects of sulphate (and other) aerosols. It seems that at least the regional effect of aerosols in S.E.-Asia is warming, not cooling… Thus any reduction there would have a cooling effect.

    Further dimming also occurs at places where there is no or a minor trend in aerosols: Antarctica, 9% dimming – no trend in aerosols. Australia, continuous increasing dimming – minute change in aerosols and no in cloud cover. Still a lot of research to do…

  15. dave:

    This is strange…

    I thought I would just go ahead and e-mail Peter Cox to get his thoughts here. If you do a Google search on “Peter Cox climate”, the first link you currently get is

    Peter Cox’s Position Available (dated October 22, 2004). What follows is a job description. Perhaps he is indisposed at the moment for whatever reason.

    Anyway, here’s some Hadley Centre research (Chris Jones and Peter Cox) issued prior to his statements in the BBC interview (an AGU news release, May 2003): New Climate Model Predicts Greater 21st Century Warming. I believe this news release gives his statements some context. Global dimming is not mentioned. The research article has the title Strong carbon cycle feedbacks in a climate model with interactive CO2 and sulphate aerosols and appeared in Geophys. Res. Lett., 30(9), 1479. I can not find the original on the web.

  16. Mark Bahner:

    Responding to my quotes of Peter Cox in comment #5, Gavin Schmidt writes, “Without access to the full interview, you don’t know the context in which Dr. Cox was speaking. He may have been asked ‘Let’s just imagine that climate sensitivity was twice as big as you thought, what would that imply for the UK under the top-end IPCC scenario?’ . Absent further information, you cannot conclude that he was saying these impacts were likely or even plausible. ‘Mugging’ of scientists in such ways is not unknown.”

    First of all, I cut/pasted four paragraphs. Nearly 200 words. Unless the BBC deliberately cut out his caveats, he certainly was quoted enough to provide his own context.

    But more importantly, if Peter Cox was misquoted or quoted out of context, why wouldn’t the Hadley Centre Met Office issue some sort of statement on their website?

    They certainly don’t appear adverse to commenting on public events, e.g. their nifty promotional piece for “The Day After Tomorrow”:

    I especially love the movie poster with Big Ben. Top notch science, there.

    [Response: I think you’re being unreasonable. The site actually says: The timescales depicted may be unrealistic, but some of the science behind the movie is real enough.. And if you follow the immeadiately following link on the page, you get a discussion of the gulf stream, and its possible shutdown, and a clear statement that modelling results don’t show this (just a 20% slowdown), and that the probablility of a shutdown is “low”. I would have written the lead somewhat differently (to emphasise more clearly how unscientific the movie is), but they clearly aren’t supporting the movie – William]

  17. dave:

    I am a bit put off by Mark Bahner calling Peter Cox on the carpet after I originally mentioned him in comment #4. Bahner, on his BLOG, says

    The IPCC Third Assessment Report’s (TAR’s) projections for methane atmospheric concentrations, carbon dioxide emissions and atmospheric concentrations, and resultant temperature increases constitute the greatest fraud in the history of environmental science.

    I just want to make sure that I am not associated with that position. I have no doubts about the fact that climate change is happening and that the warming trend is real and unusual, given the GHG/aerosol forcings. I am also interested in the apparently more extreme climate modelling results Hadley Centre is getting, but I’ll have to be “sold” on those.

    I will say, that humans have screwed up the atmosphere so much with these direct climate forcings and their indirect effects, as mentioned in the original post, that we don’t know if we’re coming or going sometimes with respect to climate. Global dimming, indeed. It looks like the climate effects over time will be quite damaging. We just don’t know the full extent of it and how or when it will happen (abrupt, non-linear changes).

    And that disturbs me very much.

    As Wallace Broeker says: “Climate is an angry beast and we are poking at it with sticks.”

    Perhaps some posts on paleoclimate are in order. To tell some of these people that the Holocene is basically an anomaly and that climate can change (at least regionally) very fast and has done so often in the Pleistocene. Especially if you poke it with sticks.

  18. Ajax Bucky:

    It won’t be possible to just publish accurate scientific information about anthropogenic climate forcing and leave it at that, and expect the general public to respond in a prudent and rational way.
    Especially, and I say this to you Mark Bahner, when for their whole lives they’ve been subjected to the most virulent forms of hucksterism and conscienceless deception the human mind is capable of.
    This debate is not happening in a Socratic community of scholars, it’s happening in the crowded alleys of a black market where slaves and stolen art are sold right next to fresh produce and brand new bicycles.
    As has been hinted at here there is resistance to factual debate on climate change that amounts to intrigue – as Sir David King said, he was “being followed around the world by people in the pay of vested-interest groups that want to cast doubt on the science of climate change”.
    It’s the height of folly to infer that this is not the case, to pretend that the men whose selfish decisions were and are responsible for this – whatever it is, disaster, dilemma, looming extinction, transitional threshold – are not still running the world, politically and economically. They are. But only because the majority, the great majority, of decent people, in the U.S. especially, have been and are being kept in the dark about what’s happening.
    Speaking to those decent people in plain and honest language under such circumstances, while easily derided, is a brave and responsible thing to do.
    Responding to someone of King’s stature and proven integrity with scorn isn’t a mistake, it’s an act of cowardice and duplicity.

    [Comment: I’ve allowed this through but its pushing the boundaries. This is a science site, please remember that – William]

  19. O. Linde:

    Things almost never happen as expected. The degree of incorporation of an unknown number of gases and particles (not only CO2, methane, methyl bromide,CFC substitutes, and the other known as GHG) is not known, and we would need maybe more than a century to have an estimate; at the same time, new compounds would be incorporating in the atmosphere. We created something that never existed before. We cannot talk of “air” as we study in Chemical texts. We are living in a bottle of a kind of garbag-air. Thermodynamics governs us, we are not governing it, nor can we know what are billions of souls doing, what are industries throwing to our lungs and nature´s lungs. less can we change the minds of educated and poor, illiterate people, in case we had the Truth.

    I find Pat Neuman, Dave and some others are right. We have to be cautious. Since the middle 1980´s I was reading climate changes could lead to something not pleasant, sea level rise, and all the etc., then in the 1990´s, the projections were not too dramatic, but serious, I felt the estimates were low, and things would get worse than predicted.

    Then in 1998 some of us felt the future was going to be a kind of disaster if not thinking seriously about how to behave. For more than two decades, some of us are used to find that the predictions are almost always wrong. Obviously… How on Earth can we know about millions of parameters interacting? Computer models can only spit what computers are fed. I feel Wallace Broeker knows the beast and is right.

    We have to continue doing things the best way possible, because we know almost nothing still.

    I will paste a bit from a post by Lee A. Arnold, somewhere in Prometheus (brilliantly written) :

    *** “Perhaps concern over “uncertainty” in complex, adaptive, open systems should be investigated by inductive generalization from observations of the dynamics of a wide range of such systems: ecosystems, social systems, computer systems, immune systems, economic systems… It is curious that the following things are never admitted as “facts about the world,” but here goes: the observer would note of all of these systems that they undergo oscillations within apparent parameters and occasionally flip into new regimes; they often demonstrate novel emergence; and that increased forcing, whether of native elements or exotic ones, increases the rates of oscillation and catastrophic shifts, sometimes after a quieter period of sub-threshold build-up. The observer would also see that these events are not tractable to analytic prediction beforehand, due to any or several of various regular functions: including definition, modeling, measurement, calculation, experiment control, and repeated verification. Yet it will remain a fact that, even though you can’t predict any exact occurrence or its timing, all complex systems will show these general dynamics.

    At that point, we opt for the Precautionary Principle, as your grandma already knew…………………………..

    It is a curious defect of mentality that economic predictions of experts are held to be sacrosanct–by the same crowd that dumps on the climate scientists! But surely economics is far less determinative. Surely our economic system will rise to the challenge of working in almost any climate policy. Indeed, the climate debate is not exactly a “conflict over values,” unless one of them is “greed.” It has been registered time and again that, aside from a few serious climate scientists who make useful points in the real debate, all of the anti-warming people are paid industry hacks.

    The idea that “economics trumps climate” should be exactly reversed, on our best and most comprehensive understanding of the processes involved. The human race will do just fine, although some people may have to find another job. “***End of Arnold´s excerpt

  20. Tom Rees:

    This may be what Peter Cox was referring to (maybe the Horizon journalists mistook celsius for fahrenheit – wouldn’t be the first time…): Strong carbon cycle feedbacks in a climate model with interactive CO2 and sulphate aerosols

    By the end of the 21st century, the authors state, the increase in carbon dioxide and decrease of sulphates will cause a substantially higher global warming of 5.5 degrees Celsius [9.9 degrees Fahrenheit] compared with 4 degrees Celsius [7 degrees Fahrenheit] when these interactions are neglected.

  21. JC:

    In response to Ferdinand Engelbreen’s reply:

    Sulphate aerosols give more reflection of sunlight, thus should have a cooling effect. Soot aerosols absorb more sunlight and increase tropospheric warming. Both have a dimming effect. But where is the balance between the two? And what is the ultimate effect on surface temperature?

    Here’s your answer:
    ‘Scientists Say Antarctica Is Cooling, Not Warming’

  22. Tom Rees:

    A more recent paper by Jones & Cox: Perhaps someone here could give their opinion of it?

    Both low Q_SO4/low climate sensitivity and high Q_SO4/high climate sensitivity combinations are consistent with the historical climate record. But they lead to very different future climates when the sulphate cooling drops off: if high Q_SO4/high sensitivity is the case then the strong aerosol cooling of the present day does indeed imply a hot future.
    (Note that this warming is extra to the warming just caused by the removal of the sulphate cooling – previous work by Jones et al (2003, GRL) has shown that reduction of sulphate cooling into the 21st century causes temperatures to rise more steeply than in the absence of the cooling in the first place. The warming mentioned here is due to the fact that higher climate sensitivity is required to explain the observed warming in the presence of the sulphate cooling forcing).

  23. David Sington:

    I am the producer of the BBC Horizon Global Dimming, so I’d like to respond to Gavin Schmidt’s article, and also some of his further comments.

    Firstly, I want to refute the notion that Peter Cox, or any other scientist taking part in this or in any other of the films I have made, was “mugged” with trick questions and made to seem to say things he does not believe. Such a practice would be clearly contrary to BBC Guidelines, and would obviously be entirely unethical. Dr Schmidt’s suggestion is, not to put too fine a point on it, a serious libel (tantamount to accusing a scientist of falsifying his or her data). As a matter of fact, Peter Cox was shown the entire transcript of the film – that is both interview extracts and linking commentary – before transmission and asked to comment. This is my normal practice, designed precisely to avoid the situation of inadvertently misrepresenting a contributor’s views or making scientific errors. Peter pointed out a few slips, which we corrected. Overall, he thought the film “very good”. The finished programme, of course, reflects his and the other contributors’ views fairly and accurately. I am sure that on reflection Dr Schmidt will wish to withdraw his factually incorrect and damaging allegation.

    What about the substance of the programme, and Dr Schmidt’s criticisms of it? Here, it is important to make a distinction between criticisms of the science itself, and criticisms of the way it is presented. The latter first: essentially the complaint is that we present uncertain science as certain. In support of this a small paragraph from the transcript is extracted from its context. If Dr Schmidt had actually seen the film (which I doubt) he would realize that this is part of a short bridging sequence which acts as a “trailer” of material which will be treated more fully later in the film (the immediately following sequence explains the “indirect aerosol effect”, in fact). It thus states things rather baldly. Even so, the extract does not really support the interpretation put upon it – we quite clearly say that dimming “may have been behind” the Sahel drought – not “was the cause of”. And later in the film, when we deal with the topic in more detail, the commentary says that the Ethiopian famine was “partly caused” by the Sahel drought (because of course the proximate causes were political) and states only that “there’s now evidence” that Global Dimming was to blame for the drought. Dr Rotstayn says that “what our model is suggesting is that these droughts in the Sahel in the 1970s and the 1980s may have been caused by pollution from Europe and North America” and the commentary goes on to say “if his model is correct . . .” So the science is certainly not presented without caveats, and nowhere do we simply state that global dimming is unquestionably the cause of the Sahel drought.

    Dr Schmidt would probably count himself a “global dimming sceptic”, citing doubts about the sunlight and evaporation data (“birds drinking from evaporation pans” etc.). There is a certain irony here – very similar doubts about the global temperature record (heat island effects and so forth) were the stock in trade of global warming sceptics for many years. Such scepticism is far from illegitimate, but it is my observation that the scientists who deal most directly with the data have usually thought of and allowed for all the obvious problems which their critics trot out. I find persuasive the fact that two independent data sets (sunlight and evaporation records) point to the same conclusion – a point the film makes clearly, I think.

    Finally, we come to the other substantive issue raised by Dr Schmidt – whether the phenomenon of global dimming has implications for climate sensitivity. (This is presented in the film as a worrying possibility, not a fact.) The argument is simple. The sensitivity of the climate to radiative forcing is unknown. Different models give widely different sensitivities. So what is the real sensitivity of today’s climate? In trying to answer this question, some scientists (such as Dr Schmidt) refer to the palaeoclimate record. But it is far from obvious that the climate’s sensitivity to GHG forcing must be the same today as it was at the glacial maximum, when conditions were very different. Another approach is to look at the 20th-century record, where the temperature rise is about 0.6K. To a first approximation the climate sensitivity is a ratio of this warming to the net radiative forcing. We know fairly well that the positive forcing from increased GHGs is about 2.4 W/m2. But of course what matters is the net forcing, positive from GHGs, negative (broadly speaking) from aerosols. The forcing due to the latter is much less well constrained, and Anderson et al (Science 2003) estimate the range as 0.0 to -4.4 W/m2. So the climate sensitivity is itself very sensitive to changes in the negative forcing from aerosols. In a forthcoming paper Andreae, Jones and Cox calculate the relationship – they show that were there no aerosol effect at all, the implied sensitivity would be just 1.3K for a doubling of CO2 – well below the IPCC-TAR range of 2.0-5.1K. But if the aerosols are contributing a net reduction of just -1.7W/m2 (a value entirely consistent with the evidence of solar dimming), then that implies a climate sensitivity of 10K. So the phenomenon of Global Dimming is certainly relevant to the question of the climate sensitivity. This is the science behind Peter Cox’s interview and why he says that “If it turns out that the cooling is stronger than we thought . . . that means the climate’s more sensitive to carbon dioxide than we originally thought, and it means our models may be under sensitive to carbon dioxide.”

    The film then goes on to explore what this might mean over the next century. This is presented as a worst-case scenario – what might be expected to happen if a) nothing is done to curb GHG emissions and b) the climate sensitivity is in the higher range Peter Cox and other leading scientists now believe possible. Of course, we are not saying that this is what will happen (to quote the film “this is not a prediction – it is a warning”) – but in assessing policy options (which is what in a democracy we are calling upon our fellow-citizens to do) a proper appreciation of the worst case is vital.

    This has been a long post. Can I just finish by saying that the Horizon film was seen by 3.5 million viewers (representing about 7% of the adult population of the UK) and that copies were requested by the Prime Minister’s office. The issues it discussed are being actively debated in Britain. I am now working on a new version of the film to be broadcast in the US on PBS’s NOVA strand. We hope to make this even better and would welcome comments from the scientific community.

    David Sington
    DOX Productions Ltd

    [Response: I’d like to thank David Sington for taking the time to respond to this article and the subsequent discussion. I am perfectly willing to trust his statement that Dr. Cox approved the program transcript. I will point out though that my previous comment was not an allegation against him or his program, merely an acknowledgement that absent any other information I could not rule out such an occurrence. A simple statement providing that information would have sufficed – threats to sue are neither appropriate as a first recourse nor useful in discussing scientific issues. (Note: no further discussion on this particular point will be entertained in this comment section).

    With respect to the presentation vs. the science, the programme transcript is available for public view and the highlighted section is an accurate quote. Caveats that appear later in the programme have to work very hard to counteract the strong impression thus set up. This was really my point – the scientist’s original papers are full of appropriate caveats which are not reflected in bald statements like ‘Global dimming is a killer’. The impression left on the viewer is not one of uncertainty.

    I have discussed the estimates of climate sensitivity at more length in connection to the results in another post. The bottom line is that uncertainties in the physics of aerosol effects (warming from black carbon, cooling from sulphates and nitrates, indirect effects on clouds, indirect effects on snow and ice albedo) and in the historical distributions, are really large (as acknowledged above). Therefore, unless those uncertainties are reduced, the constraints on climate sensitivity from the 20th Century will not be useful. This is the conclusion of many studies (Forest et al, 2001; Knutti et al 2001 etc.) that have not been able to rule out extremely high sensitivities using the observed 20th Century record. If indeed climate sensitivity is a significantly non-linear function of the base climate (as suggested above), then the whole concept is probably flawed. However, modelling studies suggest that this is not actually the case, and that useful constraints can be found in the paleoclimate record. As an aside, the radiative forcing by aerosols (in both long wave and solar radiation at the tropopause) is not the same as global dimming (which is a solar radiation effect at the surface) though they are related.

    I welcome further discussion and media interest in some of the difficult science invovled in understanding aerosol effects. If I were to offer any suggestions to the programme makers it would be that the uncertainty in these effects and their implications be made much clearer. – gavin]

  24. beate liepert:

    Dear David and readers,
    Again, I am one of the interviewed scientists and one of the leading experts on “global dimming” and its climate consequences and I initiated the meeting which started the entire debate (Liepert et. al. 2004). Gavin Schmidt is my collegue at NASA GISS and we have been discussing my research on “global dimming” for many years now. He has been very supportive and a great colleague. Graham Farhquar and Micheal Roderick (the other interviewed scientists) are frequent guests at our institutions. Furthermore during the research process for the documentary I repeatedly raised my concerns about linking the indirect effect and the Sahel drought without mentioning the study by Giannini et al (Science, 2004). Her study is regarded as the most convincing explanation. She provides good evidence that the Indian Ocean sea surface temperature was the driving factor behind the Sahel drought. My colleagues Rotstayn and Giannini correspond with each other. I respect and know them both quite well. Science might seem pretty boring to film makers in this respect. But that’s what it is.

  25. g-one:

    Global Dimming
    Last night ABC Four Corners aired a BBC – Science & Nature – Horizon documentary on Global Dimming. Also see this post on Global Dimming at Real Climate. I found the fact that the 9/11 grounding of air traffic lead…