Two statements agreed by the IPCC

“Here are two statements that are completely agreed on by the IPCC. It is crucial to be aware of these facts and of their implications.
1. A doubling of CO2, by itself, contributes only about 1C to greenhouse warming. All models project more warming, because, within models, there are positive feedbacks from water vapor and clouds, and these feedbacks are considered by the IPCC to be uncertain.
2. If one assumes all warming over the past century is due to anthropogenic greenhouse forcing, then the derived sensitivity of the climate to a doubling of CO2is less than 1C. The higher sensitivity of existing models is made consistent with observed warming by invoking unknown additional negative forcings from aerosols and solar variability as arbitrary adjustments.
Given the above, the notion that alarming warming is ‘settled science’ should be offensive to any sentient individual, though to be sure, the above is hardly emphasized by the IPCC.”
(Richard Lindzen, from here. H/T WattsUp)

25 thoughts on “Two statements agreed by the IPCC

  1. The first statement is technically correct, but leaves out very important information. The second statement is incorrect at nearly every level.

    The first statement neglects to say that while there is uncertainty, AR4 claims that equilibrium climate sensitivity to a doubling of CO2 is “likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.” Further studies since AR4 have continued to add weight to 3ºC.

    The second statement begins with a false assumption “all warming over the past century is due to anthropogenic greenhouse forcing”. No serious climate scientist claims this. Second, it elides the well-known lag between GHG levels and atmospheric temperatures (which means effectively ignoring the rising ocean temperatures, which is where the vast majority of warming initially goes). And this leads to the second sentence being inaccurate. It is made further off the mark since although aerosol forcings are still uncertain, they are not unknown. And solar variability is carefully studied and even further from being unknown.

  2. No effort is made to offer citations for where these claims are said to be made by the IPCC.

    Sam, there is a big difference between debates about the conduct of scientific enquiry (as represented in your previous few posts and for which I have some sympathy), and the deliberate falsehoods (or often, half-truths) spread by Lindzen. I have looked at the first ten slides of his presentation and apart from the first one, every one has had serious problems, or at least deliberately obscures relevant information.

  3. Hi Byron, there is some interesting stuff here.
    i) Lindzen is a pretty well qualified sceptic – so in order to disagree with his arguments, we (I!) have to give some other equally qualified person’s perspective a greater weight. Which I think confirms what I was arguing for the other day, that we can’t get away from judging these arguments on the specifics, in so far as we can;
    ii) the second statement begins with a conditional ‘if’;
    iii) the substance here is about how accurate the models are, which I think is a very interesting question. Are there any sites you’d send me to which defend the models and their applicability? This might be a good issue to concentrate on as a beginning, not least because the principles are fairly well understood by laymen (ie not the advanced mathematics _within_ the model, but the status of a model as such).

  4. i) No need to cite other authorities in this case, since my criticisms of Lindzen can be checked by anyone willing to read IPCC AR4. He is not yet putting forward an argument in these two claims, they are simply the basis from which he tries to move forward by claiming common ground with AR4. If anything, it is Lindzen who is appealing to authority here, not me! And I am simply saying that his appeal in this case is disingenuous, since he does not cite where in AR4 he bases these claims, and yet he also omits highly relevant information to make his claims more useful.

    ii) It does, but my point is that it is thereby rendered entirely hypothetical, since it is not a condition that anyone with a basic understanding of greenhouse theory would accept. “If one assumes that Mary Magdalene was appointed by Christ as the first bishop of Rome, then we can confidently say that not all the popes were male.”

    Lindzen’s hypothetical ignores the very careful investigation of natural climate forcings, conflates analysis of early and late 20thC when quite different conditions obtained under each, and apparently conflates transient and equilibrium sensitivity (or at least fails to specify which is meant). In short, he is setting up a straw man. Again, this doesn’t require appeal to other authorities, simply an honest reading of Lindzen’s claim against those whose views he is claiming to present. It would be possible to go through and give chapter and page references in AR4 for each of these.

    Furthermore, even if we accept his dubious hypothetical antecedent, the consequent does not seem to follow. Pre-industrial levels were about 290 ppm. We’re currently at around 390ppm and have seen approximately 0.7ºC warming (these are all approximations, but will do to demonstrate whether we’re in the right ballpark). That is, we are about 100/290 of the way into a doubling, or a little under a third. During that period, temperatures have risen by about 0.7ºC, which would mean that the climate sensitivity on this counterfactual hypothetical would be over 2ºC.

    Finally, his claim that aerosol and solar forcings are “unknown” and so their inclusion in modelling is “arbitrary” is a misrepresentation of the state of knowledge put forward in AR4 (let alone the greater knowledge gained about both phenomena since then). Once more, this requires no appeal to authority on my part, it is simply a matter of reading what AR4 says about these things.

    The latter is obviously much simplified.

    Lindzen is probably the best-qualified outlier on climate and one of the very few with serious publications. His hypothesis that negative feedbacks overwhelm positive ones (particularly involving clouds) leading to low sensitivity has been taken seriously and studied at length, but the data simply doesn’t support his position on this and the debate has moved on.

    These statements in his slideshow are both misleading in what they imply through omission and inaccurate in some of the things they claim.

  5. You’re not giving me a chance to read your links 😉

    i) in essence you’re putting the authority of IPCC against that of Lindzen – which works for those who accept the authority of the IPCC and not for those who don’t;
    ii) I don’t that that analogy works; the point of Lindzen’s ‘if’ is to maximise the agw contention (ie if the warming isn’t all due to CO2 then there is much less forcing from CO2. I hear your point about the lag.)
    iii) I’ll pursue the links.

  6. Byron – rummaging around about climate models, and wondered what you make of this assertion (from here: ) “Climate models do play a major role in climate science, but sometimes that role is over-emphasized. Hansen lists climate models third in his sources of understanding of climate change, after (1) paleoclimate and (2) observations of changes in the present and recent past” (ie that paleoclimate is more important than the modelling)

  7. Byron, I think you need to stop handwaving and actually read what is being said. You seem to be following the old conundrum ‘If a man talks in the forest and there is no woman to hear him, is he still wrong?’, insofar as you seem to believe that beacuse it is Richard Lindzen, it MUST be wrong and MUST be rebutted and refuted and negated immediately.

    There is nothing particularly contentious about what he is saying, he is simply paraphrasing a summary of the contents of AR4, and doing so reasonably well.

    In the idealised situation that the climate response to a doubling of atmospheric CO2 consisted of a uniform temperature change only, with no feedbacks operating (but allowing for the enhanced radiative cooling resulting from the temperature increase), the global warming from GCMs would be around 1.2°C (Hansen et al., 1984; Bony et al., 2006). The water vapour feedback, operating alone on top of this, would at least double the response.

  8. i) No. When I said that my criticisms of Lindzen can be checked by anyone willing to read IPCC AR4, I did not mean that IPCC disproves Lindzen’s arguments. I was just saying that Lindzen is not accurately representing the IPCC. This is not saying anything directly about the authority of either, simply noting that Lindzen is not being fair in his account of the IPCC position, which can easily be checked by reading the document he claims to be representing in his statements.

    ii) First, it matters that Lindzen has apparently overstated his (highly hypothetical) condition. Second, the current understanding (as I understand it) is that natural forcings alone would have lead to a slight cooling trend since the 1970s, so that anthropogenic forcings may contribute more than 100% of the observed rises.

    iii) Regarding models: that post you linked to was very helpful. In summary: models are important, contested, complex, improving, but far from perfect, especially at the regional level and for factors other than temperature (e.g. precipitation). These problems only increase the more climate actually changes due to the likelihood of new factors arising (e.g. the release of methane clathrates). They should not be oversold (but nor ignored).

    So Lindzen’s “less than 1ºC” is actually 1.2ºC, but only if we ignore water vapour and all other feedbacks? Not only does Lindzen not mention such a caveat in his slide, but implies that the difference in sensitivity arises from arbitrary changes made to account for the unknown effects of aerosols and solar variation. I’m still at odds to see how this is an adequate characterisation of AR4 WG1.

    Reading all of AR4 WG1 gives support for my comment about the rejection of Lindzen’s work on clouds as negative feedback:
    “The mean and standard deviation of climate sensitivity estimates derived from current GCMs are larger (3.2°C ± 0.7°C) essentially because the GCMs all predict a positive cloud feedback (Figure 8.14) but strongly disagree on its magnitude.”

    I don’t think that Lindzen has to be wrong simply because he is Lindzen. As I said, he is probably the most qualified critic of current understandings and has had important input into stimulating and contributing to a scientific understanding of the role of clouds in climate, even if many of his specific arguments in that field are no longer seen as likely to be true. But this presentation is quite different from his published work. And I still think that they are a misleading presentation of what is “completely agreed upon by the IPCC”.

  9. I am not closed to criticism of the methods or culture of scientific discourse. Like all areas of human society, such discourse is not immune from unhealthy patterns of deliberate manipulation and inadvertent blindness. I am sure we have plenty more to learn about the earth’s rhythms and responses to human activity. I am aware that oversimplification is an endemic problem across much of the online discussion of climate change (on all sides of the discussion, including many of my own contributions). I have no monolithic faith in the IPCC and am open to hearing informed criticism of their conclusions (including, for instance, the question of whether they have accepted unlikely assumptions about available fossil fuel reserves). However, I am still persuaded that the vast body of evidence gives us very strong reasons for taking the threat of dangerous anthropogenic climate change extremely seriously. And from a growing amount of reading, I am generally fairly underwhelmed by most of the most common contrarian arguments.

    So I’d like to ask a question to help clarify this discussion. Sam, do you agree that there is a significant body of hard-core deniers who are not open to evidence? I am not saying that every contrarian belongs to this group (nor that pathological behaviour is confined to contrarians), simply that there are powerful economic, ideological and social forces leading many to a place of epistemic closure on this matter. Would you agree?

  10. “Embraces” may be a little strong. He makes some polite opening comments and then makes a case for a real world of facts that is independent of political commitments, and argues for science-based policy. And then he offers the advice that if his audience want to oppose centralist policies (as he does), then it is best to embrace the science and propose free market solutions, rather than stoking an us vs them paranoia that closes down discussion. “What is missing from this debate are constructive solutions from the political faction that is represented in this room.”

    I would read his speech as an attempt to rebuild some political bridges by a scientist who is a fan of the free market but who is disappointed by the retreat of the right from the policy debate into the ghetto of anti-science paranoia. Not exactly a ringing endorsement of the rest of the gathering, but you’re right that it’s more than complete dismissal. He is trying to find what is best and emphasize that, affirming common ground to rebuild trust. These are all commendable goals.

    I just wonder if he was paid his $1,000 honorarium. 😉

  11. @Byron: “Furthermore, even if we accept his dubious hypothetical antecedent, the consequent does not seem to follow. Pre-industrial levels were about 290 ppm. We’re currently at around 390ppm and have seen approximately 0.7ºC warming (these are all approximations, but will do to demonstrate whether we’re in the right ballpark). That is, we are about 100/290 of the way into a doubling, or a little under a third. During that period, temperatures have risen by about 0.7ºC, which would mean that the climate sensitivity on this counterfactual hypothetical would be over 2ºC.”

    Your calculations aren’t quite right. 100/290 is more than a third (and actually your 290ppm is above the usual 280ppm I’ve seen quoted.) You calculations should yield 0.7 times 290/100 =2.0, so lower but not by much. So I reckon that when Lindzen says that the “derived sensitivity of the climate to a doubling of CO2 is less than 1C”, he should have said 2 degrees rather than 1 degree. But I agree with you that he is referring to a calculation along these lines.

    However as you yourself say about Lindzen’s hypothesis that all warming over the past century is due to anthropogenic greenhouse forcing, “No serious climate scientist claims this.” If this is true, then – and I have seen this point made on I think Climate Skeptic (I can look it up if necessary) – the calculation becomes more like say 0.5 times a number less than one over one third, equals around 1.4C. BUT, if you then take into account that the greenhouse effect isn’t linear in CO2 concentration (I think it’s supposed to be proportional to log), so goes the argument, you end up closer to 1C.

  12. I think this debate about sensitivity to a CO2 doubling is *incredibly important*. As I understand it, climate sensitivity is what all the (real) debate is about. I think I am coming at the whole issue from a very similar angle to Sam’s. I have noticed the following with respect to the kind of argument that the two of you have been discussing on this kind of calculation, but I don’t know how general this really is: The sceptics quite often come up with the kind of calculation of sensitivity to a CO2 doubling based on the last hundred years or so just referred to. I have seen this on a number of sites. I have not seen something similar of the pro-AGW sites. (But this may simply be insufficient sampling). What you find on the latter typically is a discussion of the results of models usually, but also non-model based paleo studies going back often tens of thousand, or more, years. The studies pretty much uniformly come up with a higher sensitivity than 1C, i.e., they indicate quite strong positive feedbacks. (I say uniformly, I presume the sites are not being selective in the studies they locate.)

    To someone like myself, the relatively straightforward argument based on the demonstrable CO2 increase over the last 150 years or so seems somehow more convincing than the much more esoteric models and studies of the past. So it’s funny that the AGW sites don’t so obviously meet this one head on. (e.g., the skepticalscience site doesn’t discuss it at all on their page dealing with sensitivity, despite some excellent but esoteric discussions on the page.)

    I guess that I am just suspicious about these paleo studies, having like Sam ploughed through Montford’s rather –to a layman– convincing demolition of some of the more recent paleo studies. Whatever one thinks, clearly an awful lot of assumptions have to go into these studies. Also, disentangling cause and effect between temperature and CO2 concentrations in the past seems like a real problem, so I am inclined to put little weight on these studies (though, if they really all come up with the number 3C maybe that is something real).

    And the truth is, I gather, that the scientists really do not know what effects clouds have as far as feedback is concerned, so the models have to be taken with a pinch of salt.

    Byron’s point that the Lindzen type calculation—however it is done– does not capture the long-run sensitivity because of lags in the sytem (oceans mainly I guess) is well taken though, and I notice this point does not appear on the sceptic presentations of the calculation. I don’t know how important it is though for say the next hundred years.

  13. @Roger: You are right that climate sensitivity is very important in estimating just how bad our situation is. And you are also correct to point out that my calculations were a little rushed. My apologies. I should have said a little under a third and ended up at around 2ºC (depending on where. But Lindzen’s answer is half that. Perhaps he is using something a little more complex? If so, that is not clear.

    As you point out, the sensitivity is also on a logarithmic scale and pre-industrial levels were indeed around 280, though by a century ago (the timeframe Lindzen references) they were close to 300.

    To someone like myself, the relatively straightforward argument based on the demonstrable CO2 increase over the last 150 years or so seems somehow more convincing than the much more esoteric models and studies of the past. So it’s funny that the AGW sites don’t so obviously meet this one head on.
    As you note, the reason climatologists do not simply look at the last 150 years and compare how much CO2 increased to how much atmospheric temperature increased is due to the lag between increasing energy input into the earth system and increasing atmospheric temperatures. This lag is caused largely by the oceans, which is where the vast majority of the energy initially goes. Also, the effect of other forcings, both natural and anthropogenic, mean that a “straightforward” calculation is much too simplistic.

    And the truth is, I gather, that the scientists really do not know what effects clouds have as far as feedback is concerned, so the models have to be taken with a pinch of salt.
    This is an overstatement of where things currently stand. There is still debate on the precise effects of clouds, though Lindzen’s claim that they are a negative feedback has been rejected by pretty much everyone but Lindzen.

  14. Byron, Just to refine slightly the non-linear response: If you use a 0.5ºC increase attributable to CO2 over the last century, your 100/290 on the CO2 increase, and a logarithmic relationship between CO2 concentration and forcing (if this is the right terminology), I worked out you get a 1.13ºC sensitivity to CO2 doubling. Whether this is the argument Lindzen has in mind, I have no idea. If we use the 0.7ºC figure for warming attributable to CO2, you’ll get about a 1.6ºC sensitivity.

    I agree there are many other things going on which complicate this simplistic approach. There is also the lag; but I would have thought that the change we have observed over the last century might still be a good predictor of the current century since the same lagginess will be present. Your point is that the long-run equilibrium will have a higher sensitivity. This is surely correct but if we’re thinking about the next 90 years then it may not be so relevant.

    I also have to say that from a brief reading, the IPCC AR4 was not clear on this matter.

  15. To continue this now moribund thread based on Sam’s quote from Lindzen just a little longer, I did a little more digging for my own curiosity’s sake to see if I could get to the source of what he says. However it was without too much success, and it rather confirmed my impression that the pro-AGW sites don’t go in for attempting simple explainations of estimates of climate sensitivity based on the last 100 years or so. The only coherent discussions I found were but apparently this is a sceptical site (again) and on wikipedia ( which has a nice paragraph based on a Brookings publication (using numbers from the IPCC AR4). The latter does possibly explain the Lindzen quote: if one adds in all estimates of anthropogenic greenhouse gasses, not just CO2, we find that by IPCC estimates there is a radiative forcing of 2.6 W/m2 and this needs to be compared with a forcing of 3.7 W/m2 for a doubling of CO2. That is, rather than the increase in CO2 of around one third that we were discussing (leading to +0.7ºC), the relevant calculation for all GHGs is around 2.6/3.7 = 0.7. This then apparently yields a sensitivity of 1.1ºC.

    The issues are however that there are still many uncertainties due to other forcings, both positive and negative (especially aerosols), so the calculation remains tricky. Aerosols in particular reduce the 2.6W/m2 figure and thus increase sensitivity. Both articles suggest however that sensitivity is likely to be at the lower end of IPCC forecasts (they don’t seem to factor in the non-linearities which would further reduce the estimate, but the Wikipedia conclusion of 2.1ºC does include the ocean effect).

  16. Here is Hansen explaining how the estimates of climate sensitivity are reached.

    Includes this line: “Estimates of climate sensitivity based on the last 100 years of climate change
    are practically worthless, because we do not know the net climate forcing.”

  17. Thanks for the link. I still find it highly surprising that the recent data – when we can actually measure instrumentally much of what’s going on -are deemed to be so useless that we have to go back instead (many) thousands of years. If there is this much difficulty in disentangling current forcings, one might wonder how any sort of prediction can be made for the future (prediction is hard, as we know, especially about…). I repeat the point; reading Montford’s account of temperature reconstructions over just the last 1000 or so years makes pretty clear how many essentially abitrary/dubious assumptions have to be introduced to make any headway. I imagine this only gets worse with longer time periods, but maybe proper progress can be made if the temperature and forcing changes are much larger.

  18. Thanks for the references. The Hansen one is useful but a bit alarming: “If the planet gets too warm, the water vapor feedback can cause a runaway greenhouse effect. The ocean boils into the atmosphere and life is extinguished.[…]
    [H]ow large a forcing must be maintained to cause runaway global warming? Our model blows up before the oceans boil, but it suggests that perhaps runaway conditions could occur with added forcing as small as 10-20 W/m2.” (Slide 23).
    I think it helps having a christian outlook when reading some of this stuff!

    I am working through some of the others. I tend to take those articles more seriously which are capable of articulating the basic conclusions in a way that is graspable by an intelligent (hopefully) layman- of the ones I have read this is not typical.

    Consistency of conclusions about sensitivity is a strong point, although it might merely reflect that all studies are being consistent in following the same inappropriate methodology, a real issue when basing conclusions on proxy studies (see e.g., here or here for two recent perspectives on this issue).

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