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Say a little prior for me: more on climate change

Four out of the last 15 posts on this blog have been related to climate change, which is probably a higher ratio than Andrew would like. But lots of people keep responding to them, so the principle “give the people what they want” suggests that another one won’t hurt too much. So, here it is. If you haven’t read the other posts, take a look at Andrew’s thoughts about forming scientific attitudes, and my thoughts on Climategate and my suggestions for characterizing beliefs. And definitely read the comments on those, too, many of which are excellent.

I want to get a graphic “above the fold”, so here’s the plot I’ll be talking about.
WarmingProbDists.png

I have a PhD in physics, and although much of what I do at work is statistical data analysis and some engineering-type modeling I still have a physicists’ outlook in general. I don’t think a really simple physical model can tell us exactly what the doubling the amount of carbon dioxide in the atmosphere will do…but I do think it can tell us a lot. So, let’s take a look.

Carbon dioxide absorbs and re-radiates infrared radiation. It can’t help but do so. I’d stake my life on it, and I mean that sincerely. Doubling or tripling the amount of carbon dioxide in the atmosphere will definitely affect the radiation balance of the earth. Nobody credible would (or does) disagree with that. The whole debate comes down to: what is the magnitude of the effect. For simplicity, I’ll just talk about the “climate sensitivity”, which is usually defined as the change in global-average temperature if the carbon dioxide concentration is doubled from its pre-industrial level and is held indefinitely at that higher concentration.

I do not claim to be more than a mediocre physicist, but even I can easily follow the calculation that doubling the amount of CO2 in the atmosphere, IF NOTHING ELSE CHANGED, would increase the temperature by between 1C and 2C. (There’s an excellent discussion in John Harte’s “Consider a Spherical Cow”, or at least in the 1988 edition). The reason for that factor-of-two range is the crudity of the specific calculation I have looked at, it’s not inherent: a more careful calculation can give a much more precise estimate for what would happen if the CO2 concentration were doubled and nothing else changed. I don’t think Hal Lewis or Freeman Dyson or any other credible physicist would disagree substantially with the estimate.

But nobody thinks that nothing else would change. One thing that many people agree will change is the amount of water vapor in the atmosphere: if the earth and air warm slightly, the air can hold slightly more water vapor, and the evaporation rate will be slightly higher at a given humidity. Since water vapor is a greenhouse gas — an even more potent one than CO2, per molecule — this would amplify the warming over what you would get from CO2 alone. So, a simple model is: more CO2 -> some warming -> more water vapor -> more warming. This is called “feedback”, but note that this isn’t a feedback loop — it’s not like the arrow comes around to “therefore, more CO2” — so this doesn’t predict runaway warming. It just predicts a climate sensitivity that is somewhat more than 1.5C per doubling of CO2….if the only thing that were to change, in addition to the CO2 level, were the level of water vapor.

It’s worth noting that at this level of abstraction — not worrying about the spatial distribution of heat or water vapor, but just drawing a sphere outside the earth’s atmosphere and looking at how much energy flows in (from the sun) and how much flows out (from reflected sunlight and from heat radiated from the earth and atmosphere) — there are only a few parameters that are needed to describe the system: what is the solar intensity; what is the average temperature of the earth; what is the earth’s albedo; and a few parameters to describe the atmosphere. This simple calculation is good for some purposes, but it’s limited in what it can tell you. If you want to calculate what will happen to a car if it is in a collision, you can determine things like the post-collision speed of the car with a very simple model that involves only a few parameters…but if you want to know what will happen to a person in the car, that’s a whole different level of complexity.

Anyway: the model that considers only CO2 and water vapor suggests substantial climate sensitivity. So is it “game over”? No, because (1) the amount of water vapor to expect is hard to estimate, and (2) other feedbacks could work the opposite direction from the water vapor one. You only have a few choices, though, if you want to find negative feedbacks. You have to find either a mechanism that increases the albedo, or one that substantially decreases the heat-trapping capacity of the atmosphere even though it has extra CO2 and water vapor in it. I don’t think anybody seriously thinks there is anything to be found in the latter category, so most of the controversy is in the albedo.

For example, if we’ve got extra water vapor in the atmosphere, maybe that will lead to more cloudiness, which would increase the albedo. For people who think climate sensitivity is over-estimated by the IPCC, clouds are almost always one of the things they emphasize, since there aren’t many other ways ways of increasing the albedo. If the cloudiness effect happens and is big enough, the effect of more clouds could even surpass the effect of the water vapor, and the climate sensitivity (as defined) could even be a bit lower than the effect of CO2 alone, though it would still be positive. (Note that we would have a more humid, cloudier earth, which would imply changes is weather and precipitation patterns, so one might say that the climate is sensitive to CO2…but the global average temperature change would be low, and that’s the definition of climate sensitivity that I am talking about).

There are ways of testing the hypothesis that higher temperatures will lead to more cloudiness. For instance you can look at the level of cloudiness when it is cool compared to when it is warm — in winter versus summer, for instance — to help gauge how much cloudier it will be, if any, if the atmosphere warms a little. Or you can try to use physics to model cloud formation and destruction…but now you’re trying to model what happens to the driver inside the crashing car, rather than just what happens to the car; it’s a lot more complicated and nobody really trusts these models. (This is the sort of thing Freeman Dyson has in mind when he says cloudiness is only calculated through fudge factors in the models). The uncertainties in the effect of clouds (negative feedback) and the amount of atmospheric water vapor (positive feedback) are two of the biggest uncertainties in predicting climate sensitivity. There are other feedbacks in both directions, such as desertification (negative), and melting of ice and snow (positive), but these are not as important as water vapor and clouds on a global scale, though they can be very important for local climate.

Putting it all together, obviously just from my point of view: “zero” or a very low number (or a negative number) is an idiot’s estimate for the likely value of climate sensitivity. Such a value _could_ happen, if feedback effects worked out just exactly right, but there’s no reason to expect them to work out just exactly right. The default estimate — the one it should take substantial evidence to move you away from, if you follow the physics — is somewhere in the range of 0.5C to 6C for a doubling of CO2. If someone wants to claim the number is outside that range, well, I agree that it could be but you will have to have some really good evidence, or think of a big effect that has been overlooked somehow. If we think of it from a Bayesian statistics standpoint, then on physical grounds I have a prior estimate that puts most of the probability between 0.5C and 6C for climate sensitivity. (In an earlier draft of this paragraph, I had a range from 1C to 5C instead. I wouldn’t want to be forced to defend to the death every feature of the prior.)

Finally, we get to the graphic. Each of these probability distributions is supposed to summarize the belief of a different person. In blue, we have an “anthropogenic climate change denier.” This is someone who just doesn’t believe that doubling of atmospheric CO2 could have any substantial impact on the global mean temperature. I don’t know if any such people think the effect could be negative, but maybe they do; if they don’t, then just move all of that negative probability into the low positive range somewhere. At any rate, these people are convinced that there is just the right amount of negative feedback to cancel out the known effect of CO2 and the expected effect of water vapor.

In orange, we have a certain type of skeptic. This person thinks low values of climate sensitivity are quite likely. This hypothetical skeptic thinks “experts” (as represented by the IPCC, I’ll get to them, below) are very overcertain in their estimates. But the skeptic also thinks the experts are much more likely to overestimate the sensitivity than to underestimate it.

In red, we have the “consensus” estimate of the Intergovernmental Panel on Climate Change (IPCC). Actually, it’s possible that in their reports they give a statistical distribution, in which case it certainly won’t exactly match the one shown: I based the red curve purely on the IPCC statement that the climate sensitivity is unlikely to be less than 2C, and that values over 4.5C can’t be excluded but don’t provide a very good fit to data.

The purple dotted curve represents what I am calling my “prior distribution,” explained above. I think that if I knew only the physics calculations discussed above, plus the fact that very high climate sensitivity probably isn’t consistent with data from the past 50 years, the dotted purple curve would represent my distribution. It’s hard to return myself to that “state of innocence” though, as Andrew calls it: in fact, I didn’t think hard about that “prior distribution” until I was already aware of a lot of predictions, data, controversy, and so on.

One thing worth noting: the IPCC curve is only narrower than the “Phil prior” by about a factor of 1.5. One way to interpret this is that the IPCC folks don’t actually trust their models very much: all that modeling, and they can only narrow the distribution a little bit compared to what a beginning climate modeler might get from a really simple model. This is worth noting because a standard critique of the climate modeling community is that they “believe their models” too much. And I happen to agree with the critique! The modelers only believe their models a bit, but even that bit is too much, I think. Still, it is clear that the modelers (or at least the IPCC consensus process) are allowing that there is lots of uncertainty.

And, finally, the solid purple line…that’s about where I am now. I’ve given weight to the experts (as represented by the IPCC), but knowing the general truth that everyone, experts and non-experts alike, tends to be overcertain, I have a wider distribution than the IPCC, so I’ve got a non-negligible bit of probability down there at very low values of climate sensitivity. I even agree with the “skeptical” view that the experts are more likely to be overestimating the sensitivity than to be underestimating it, although I think both are possible.

But I think the hypothetical “skeptic” curve puts way too much probability on very low values — not as bad as the “denier”, but still, this is someone who is unjustifiably convinced that negative feedbacks will come close to counteracting the effects of CO2.

(By the way, none of the lines are supposed to go below zero, or even go to zero, at 6C, but the drawing software I used has done some funny stuff there and it doesn’t seem worth fixing. Oh, and each of the curves is intended to have the same integral — unity — but since this is just a by-hand sketch, they probably don’t).

Above, I’ve opened my soul, as it were, to discuss why I believe what I believe. Part of my belief, actually a substantial part, is informed by a very simple physical model that I believe is useful in spite of its simplicity, that shifts my prior well away from 0 as a reasonable estimate of climate sensitivity. What if you don’t have the physics background to evaluate such a model for yourself? Then, you’re more or less forced to choose who you care to believe: deniers, skeptics, “experts,” journalists, bloggers, friends…
In a comment on Andrew’s entry about forming attitudes on scientific issues I said this:

When it comes to anthropogenic climate change, if someone wants to allocate some probability to the chance that the skeptics have it right, I think that’s a very reasonable thing to do. Make it 90% mainstream, 10% skeptics, or even 75% mainstream, 25% skeptics if you are are heavily inclined towards the skeptical camp. But there are people out there who are 90-10 the other way! If you are an expert climate modeler and you think your colleagues have the science wrong, that’s one thing. If you’re just some schmoe who only knows what he reads in the papers, and you choose to assign a 90% or 95% probability to the conclusions of the small band of skeptics…where does that come from? Do you really think the experts in a field get it wrong 90% or 95% of the time?

I think I’ll leave it there.

36 Comments

  1. Fred says:

    Some of us are less concerned with the development of fuzzy priors than with the basic data on which science is supposedly founded. We worry about garbage in, garbage out.

    With respect to AWG, there are basic questions about the data that need to be answered. How does one really measure global temperature? Are the choices that the CRU Team made defensible? Do they introduce bias in the measurement of modern temperatures?

    Would one's opinion of the science behind AGW claims change if one knew that the number of thermometers used to produce readings for the GHCN dropped dramatically in the 1990's or that a larger fraction of the cuts came in colder areas than warmer ones? Would it change if one knew that data adjustments look as if they were biased in favor of increasing temperatures?

    Would one's opinion change if one knew that missing temperature data for GHCN stations jumped after 1990? Many of the missing temperatures were from rural stations. Their values were filled in using temperature from urban centers.

    Would one's opinion change if one knew that official weather stations in the US are often located next to air conditioner vents and barbecues? Would it change if one knew that the single station apparently used to produce "the" temperature for the Antarctic Peninsula appears to be Rothera Point, which has the highest temperatures of the 27 stations from which Antarctic temperature data are available?

    Would it change if the homogenization techniques applied to the "raw" data appeared to decrease temperatures from increase temperatures byabout 0.25 degrees C from 1900 to 2006 and then decrease temperatures by about 0.10 degrees C from 1990 to 2006?

  2. Nick N says:

    I'm anxiously awaiting the next step of your discussion – if the predicted (via model) average global temp increase does occur, what are the effects and to what extent do those effects justify present-day efforts to decrease CO2 emissions (from a global warming perspective only – ignore other reasons for wanting fewer CO2 emissions)?

    I fall firmly in the skeptic camp. I'd be more 'Phil-like' if the modelers were more open with their methods of adjusting the raw temps and their models (source code) were all available online for inspection. Until then, I don't feel that I can trust them. They may be experts, but they're experts playing with poorly documented models (in CRUs case) with a large incentive ($$$) to get results that keep the funding coming.

  3. Manoel Galdino says:

    Wonderful text!!
    Thanks very much for this piece of text!!

  4. mengbomin.wordpress. says:

    I have to wonder what Fred and Nick N have to say about Phil's starting point on this issue: carbon dioxide has the physical property of absorbing and re-emitting radiation in the infrared spectrum, which leads to, all else being equal, a warming effect if atmospheric concentrations double and at the next level of abstraction, this would lead to increased water vapor concentrations, which due to its properties would likely create more temperature forcing.

    As Phil, pointed out, the Earth is a complex system and thus we know that those two factors (carbon dioxide levels and their subsequent effect on water vapor levels) are not the only ones that impact climate by a long shot, but the fact that they have a significant effect, it would seem that the burden of proof would be on a skeptic who took the view of the situation as modeled above.

    Now, I know that you haven't specifically endorsed the probability curve that Phil suggested as a skeptical view of the situation, but I wonder what evidence would suggest that feedback effects have such a significant chance of completely canceling out the effects of carbon dioxide and water vapor concentrations. The possibility of biased data gathering and the presence of possible conflicts of interest on the part of the researchers involved do not change the underlying fact that increased levels of carbon dioxide would have some effect on Earth's thermal dynamics.

    Such possibilities warrant investigation, but they do not warrant ignoring the underlying dynamic that is purportedly producing a warming effect. So in saying that you are a skeptic, are you indeed following the view point of the "skeptic" proposed by Phil, or are you taking some other viewpoint?

    As a non-expert, I don't pretend to know the dynamics of the climatology community, but it does seem to me that allegations that the majority of scientists in the community are either crooked (essentially the suggestion posed by Nick N) or incompetent (essentially the suggestion posed by Fred) require a significant amount of evidence before using them a significant factor in determining one's own opinion with regard to the likelihood of warming and that even if such possibilities properly should play a significant role in determining the probability of the effects of increasing carbon dioxide concentrations, it would still be unclear that 0 degrees Celcius change in temperature would be the correct temperature to bias one's probabilities toward.

  5. A. Zarkov says:

    Four out of 15 is not so much considering the how important the subject has become. We have the Copenhagen conference along with the email and documents leak. I'm glad to see a focus on the climate sensitivity factor since that lies at the heart AGW. If this factor is really on the low side, then most (if any) future warming will be natural and unavoidable unless we engage in environmental engineering.

    I agree we can get pretty far with a zero-dimensional energy balance approach. Simple models often produce better results than highly complex models, but they don't lead to the funding from Washington. The book "A Climate Modeling Primer" by McGuffie takes the reader on a grand tour starting with the basic physics and ending with complex models. The author provides good discussions on what more complex modeling gets you. I note that these days economists are not as attracted to the highly complex models with hundreds if not thousands of equations we saw them using in the 1960s and 1970s. They learned through bitter experience the pitfalls with that approach. I'm afraid the climate modelers have yet to achieve that wisdom, but to repeat, simplicity threatens funding and funding trumps everything.

    I agree that cloud cover is the most important and largest negative feedback. But there are other factors that can reduce temperature. Something caused the cooling from the 1940s to the 1970s while atmospheric CO2 concentration continued to increase. The alarmists claim that industrial activity and volcanic eruptions produced enough sulphate aerosols to cause the cooling, and after the Clean Air Act, the aerosols went away causing global temperature to resume its upward trend. I'm not familiar with the literature on this, but it sounds plausible. But let's recall the TTAPS model for Nuclear Winter tried to model the cooling effect from aerosols, but more realistic models reduced the magnitude considerably. Let's also not forget that Sagan et al like Michael Mann refused to release data and codes. Anyone see a pattern here? It turned out that TTAPS made a lot of arbitrary assumptions, and got catastrophic predictions. Now we don't know for sure that the TTAPS people simply fiddled with the parameters until they got some "interesting results" and then called a news conference– just like Pons and Fleishman did with cold fusion. But it must have been tempting.

    Another possible cooling mechanism is cosmic ray flux– see Nir J. Shaviv, "On Climate Response to Changes in the Cosmic Ray Flux and Radiative Budget", JGR-Space, vol. 110, A08105, and see Harrison, R.G. and Stephenson, D.B. 2006. "Empirical evidence for a nonlinear effect of galactic cosmic rays on clouds." Proceedings of the Royal Society A: 10.1098/rspa.2005.1628.

    Cosmic ray flux is not in the GCM models used by the IPCC. A number of papers have appeared claiming to refute Shaviv, but a careful tracking of the literature show he has answered their criticisms. What I did not know until today is Shaviv claims an editor rejected his paper without forwarding his detailed responses to comment made by reviewers. This is peer review? See here– http://www.sciencebits.com/node/211.

    So where are we on global climate change? We have some basic physics that predicts some warming caused by CO2, but a lot of positive and negative feedbacks that could amplify and attenuate temperature increases. We have computer models we can't trust for a variety of reasons. We have temperature station data that might have been corrupted by arbitrary "adjustments" to produce a warming trend. We have the north polar ice area decreasing, while the south polar ice area is constant or increasing. Next year an earth satellite will launch that should give us good measurements of polar ice thickness using radar. Let's hope that data doesn't get corrupted. We have some alternate theories to explain temperature increases such as cosmic ray flux. All this adds up to a confused and uncertain picture. The science is hardly "settled."

    Finally the public is not buying AGW. Anyone with common sense can see that the big funding governments have poured into climate science has corrupted it. Until this whole thing gets an independent review from trustworthy people, it will not enjoy general acceptance. You can look for that at the ballot box next year.

  6. Tom Rees says:

    Models and theory are not the only way to investigate climate sensitivity. It can also be done empirically, e.g.: http://www.sciencedaily.com/releases/2009/12/0912

  7. ben farrer says:

    would it be at all interesting here to think about a role for elicited priors in studying global warming as a political issue? Firstly I think with enough interviews it would hypothetically be possible to recreate this graph but with priors for different respondents, possibly self-identifying 'skeptics' or other sub-groups. Also it seems like most of the debate here is perhaps less on the dimension of 'how many degrees of temperature' but more on what should inform answers to that question. So wouldn't it be possible to get not only elicited priors on temperature chance, but also on what sources on information (climate change modelling, IPCC pronouncements, natural disasters in national headlines, partisan politics, etc etc) people had prior inclinations to turn to during their updating process?

  8. Nick N says:

    mengbomin – "I have to wonder what Fred and Nick N have to say about Phil's starting point on this issue: carbon dioxide has the physical property of absorbing and re-emitting radiation in the infrared spectrum, which leads to, all else being equal, a warming effect if atmospheric concentrations double and at the next level of abstraction, this would lead to increased water vapor concentrations, which due to its properties would likely create more temperature forcing."

    It's a good starting point except that no one knows if increased water vapor concentrations are a positive or negative feedback. As Phil stated, "The uncertainties in the effect of clouds (negative feedback) and the amount of atmospheric water vapor (positive feedback) are two of the biggest uncertainties in predicting climate sensitivity." I'm sold on CO2 causing warming by itself, but it's not by itself.

    As for the modelers being 'crooked' I think the better term is 'AGW religious fundamentalists' who truly believe in AGW but are so committed to their models and theory that they are unable to give an unbiased look to all the data – the adjusted temps for Antarctica is an interesting case study. How much evidence would you require before you thought the malfeasance was enough to cast doubt on a model? Perhaps my limit is lower than yours, but it's been reached.

    I think that A. Zarkov put it well, "So where are we on global climate change? We have some basic physics that predicts some warming caused by CO2, but a lot of positive and negative feedbacks that could amplify and attenuate temperature increases. We have computer models we can't trust for a variety of reasons. We have temperature station data that might have been corrupted by arbitrary "adjustments" to produce a warming trend."

  9. ed_finnerty says:

    The figure that is often used in the 'skeptic community' is at most 1C of warming for a doubling of CO2 (ceterus paribus) – can't we nail this figure down more – it seems to be a very important thing to get right if we want an informed discussion

  10. Jonathan says:

    Nice work, Phil. But I think you've left out an important issue which animates at least some skeptics' viewpoint. (And it fits with Andrew's last post as well.) You ask the question about the effect of doubling holding all else constant. But all else won't be constant. Whatever you feel about the Medieval warming period or anything else in this debate, non-CO2-based climate changes have dwarfed CO2-based ones, and we have only the vague sorts of glimmerings of what those are all about. A lot of them are local, of course, but it's unclear to me to me what the implications of that are. If we're due for a new Ice Age (and I note I have no evidence on this one way or the other) then an extra couple of degrees might be very helpful in some places and very bad in others. But nobody really cares about the partial derivative of CO2, they care about the aggregate effect, and your model, since it can't explain aggregate temperature as a function of CO2, can't tell us whether we need warming or cooling. You're giving us ceteris paribus, and we need mutatis mutandis.

  11. Phil says:

    I don't know how Andrew finds the time to do this blog. It's taken me something like 3 hours each for my last few entries — including reading up on some things — plus the time to read the comments, and respond to them…man, it's like a full-time job. So I had decided to just let the comments run on this entry, without responding. But I can't resist.

    Fred: I am a big fan of the simple declarative sentence. I do not like the Michael Moore style of argument: "would it change your argument if such-and-such," "why am I supposed to believe so-and-so…" Have the courage of your convictions. Say what you believe, and let people demonstrate that you are wrong if you are wrong.

    As for what you say, or rather insinuate, about the global temperature record: if you don't trust the thermometers, look at sea level. Only two things matter, on the timescale we're looking at: how much water is in the oceans, and how warm is it. Sea level rise means the average ocean temperature is rising.

    Nick: You refer to "present-day efforts" to reduce greenhouse gas emissions. I'm not aware of any such efforts on a large scale. There's a lot of talk, but off the cuff I would estimate the difference between what is actually being released worldwide, and what would be released if there were no efforts at all, is less than 1%. We'll have to see if anything substantial actually gets done in the next ten years. I'm guessing not.

    You also say you're "firmly in the skeptic camp." As Meng points out, you (and Fred) don't say why you think zero, or a very low number, is the right way to be "skeptical." Meng says "The possibility of biased data gathering and the presence of possible conflicts of interest on the part of the researchers involved do not change the underlying fact that increased levels of carbon dioxide would have some effect on Earth's thermal dynamics." Right!

    Meng: well said.

    Zarkov: Long entry, well-written as always, but… what's your point, when it comes to climate? What does your probability distribution for climate sensitivity look like? You seem to be suggesting that cosmic rays will increase cloudiness just enough to counteract the effects of greenhouse gases…but even though you _seem_ to be suggesting that, I'm sure you don't believe it. What are you saying?

    Tom: unfortunately, that "empirical" estimate that you mention is based on a model: "We found that, given the concentrations of carbon dioxide prevailing three million years ago, the model originally predicted a significantly smaller temperature increase than that indicated by the reconstructions. This led us to review what was missing from the model."

  12. Nick N says:

    "don't say why you think zero, or a very low number, is the right way to be "skeptical." and "some effect on Earth's thermal dynamics…"

    Agreed. The dispute is the amount of change DUE TO CO2 because of all the unknowns / all the things that won't remain the same. 1 deg C warmer is an effect and falls safely within the skeptic range you put up. It's even in the 'Phil Current' pdf and, although it's hard to tell, may even be within your 95% CI.

    "You refer to "present-day efforts" to reduce greenhouse gas emissions. I'm not aware of any such efforts on a large scale…".

    FYI, I make no claim that the efforts will be effective. The Kyoto Treaty was a large scale effort (completed) and the meeting in Copenhagen is an effort (attempted). On a smaller scale, the cap-and-trade system in Europe (completed) and a similar proposal in the US (attempted) are efforts.

  13. manuelg says:

    Thank you for publishing your probability density for climate sensitivity, and the precise reasoning behind it. This is, practically, the _only_ way to communicate one's considered beliefs for this subject – if one is truly interested in communicating and not just indulging in motivated obscurantism or the art of controversy.

    I wish I had the chops to draw one myself. I don't, so I rely on the experts currently publishing articles. I find it hysterical to shriek about the corrupting influence of funding – I am not holding my breath waiting for the appearance of researchers comprised solely of incorruptible energy, freed of the need for money because they draw sustenance from the empty ether. Oh, please. All humans have their self-serving motivations, and reasonable people deal it with accordingly and with due measure. And, scientists never claimed to not be human.

    I wish I had the chops to draw the _other_ curve with regards to carbon dioxide – loss of tonnage of fished protein/nutrition due to ocean acidification. Global temperature and ocean acidification both have a large expected impact on human civilization.

  14. manuelg says:

    > The Kyoto Treaty was a large scale effort…

    Surely the intervention can only earn the description of "large" if *some* authority judges it probable to effect the desired change. Otherwise, I could call the Dubai Towers a "large scale effort" to bridge the span to the surface of the moon.

    It can be honestly asserted that no large scale efforts have been made to reduce greenhouse gas emissions.

  15. [quote]When it comes to anthropogenic climate change, if someone wants to allocate some probability to the chance that the skeptics have it right, I think that's a very reasonable thing to do. Make it 90% mainstream, 10% skeptics, or even 75% mainstream, 25% skeptics if you are are heavily inclined towards the skeptical camp. But there are people out there who are 90-10 the other way! If you are an expert climate modeler and you think your colleagues have the science wrong, that's one thing. If you're just some schmoe who only knows what he reads in the papers, and you choose to assign a 90% or 95% probability to the conclusions of the small band of skeptics…where does that come from? [b] Do you really think the experts in a field get it wrong 90% or 95% of the time?[][quote]

    Nassim Nicholas Taleb is the only voice I heard talking about the impending financial doom before the crisis. Thousands of bankers in SoCal, with billions of dollars of money on the line, made wrong decisions. Likewise, Ronald Fisher and Joseph Berkson both famously denied the harmful effects of smoking. Their contributions to statistics are immense. And, yet, further evidence has proven them dead wrong.

    Most climate scientists have vested interest in research funding for climate science. As such, I expect them to develop models and report findings that further those interests. Thus, not only have experts been wrong in the past, but climate science experts have great incentive to promote a position, regardless of the evidence.

  16. Nick N says:

    manuleg,

    I was asked about large scale efforts, not largely effective efforts or large impact efforts. The scale refers to the number of entities coordinating the effort. If, like you say, scale should relate to the impact or expected impact then I agree with you – nothing large has been done.

    I question the need to do anything large and expensive, but that's been partially discussed already. If you can find something with large CO2 reducing impacts with minimal cost, then I think you'll have yourself a win-win project.

  17. Tom Rees says:

    "Tom: unfortunately, that "empirical" estimate that you mention is based on a model"

    Not quite: they used the empirical estimate to validate the model (and show that it was wrong, because it didn't include longer-term feedbacks).

    But the real point is that it's just a recent example (last week) of an ongoing body of research to estimate climate sensitivity empirically. As you would hope, there is cross talk with the climate modelling community because it gives a vital 'reality check' on the models.

  18. efrique says:

    I think this is a good discussion, but to my mind it misses something crucial.

    The discussion treats the degree of belief in the extent of AGW as an endpoint of the analysis, but it isn't, since the next stage is what, if anything to do about this, if we believe it at all.

    This requires us to examine the costs of acting versus not acting (and within "acting", considering the costs of various possible actions weighed up against the potential effect – negative costs – associated with the potential reduction in AGW).

    I'm probably somewhere between your "skeptic" and your "IPCC" distributions in terms on my belief about what's going on… but once one considers the costs associated with the actions that result, my conclusion is "we must act, and act fairly radically, if we can find any actions that will substantially mitigate the anticipated problems".

    Indeed, my understanding of the relative sizes of the costs indicate to me that even if I was MORE skeptical than your skeptical distribution, I should come to the same conclusion.

    I think it's highly unlikely that my house will burn down (I am a major skeptic about it being a likely outcome), but I also think it would be crazy not to insure against that possibility, because the impact on me and my children would be so large.

  19. jonathan says:

    I have tried to use a cruder version of this line of argument to people but they can't get it through their heads that the issue isn't the reality of climate change but what one should do rationally given the chances. I'd say the odds of these people understanding a more complex argument are zero.

    A short version of the argument is that x has some chance of happening. If x is a catastrophe, what do you do? If x is bad but hurts only people in the poor world, what do you do? Etc. People tend to focus on the really bad possibilities in these guessing games but, again, they can't get to that point because they're hung up on arguing about the reality and can't understand you can assign a probability to that being true or false and then to the level of possible effects. That drives one to making rational decisions that balance concerns, with the understanding then that each voice has a different stake in the outcome – as in China's stake is not ours is not Bangladeshes is not an Alaskan (like Ms. Palin) or Texan who exports oil & gas, etc.

  20. A. Zarkov says:

    Phil writes, "Zarkov: …, but… what's your point, when it comes to climate? What does your probability distribution for climate sensitivity look like?"

    For a Bayesian analysis I would choose a non-informative prior for the climate sensitivity factor.

    "You seem to be suggesting that cosmic rays will increase cloudiness just enough to counteract the effects of greenhouse gases…but even though you _seem_ to be suggesting that, I'm sure you don't believe it. What are you saying?"

    No I don't think that cosmic rays would exactly cancel the effect of greenhouse gases, but they and other factors could reduce it. Le Chatelier's Principle tells us that large complex systems tend to have negative feedbacks that act to restore equilibrium. Now where that equilibrium lies we don't know. Do we have some kind of fundamental argument, a kind of Bell's Theorem that tells us there are no hidden variables that will act hold down the global temperature so the increase must be at least 3C for a doubling of CO2? I think not. Why couldn't the increase be 1C?

  21. BCC says:

    Nick,

    You want source code?

    Climate models:

    http://www.giss.nasa.gov/tools/modelE/
    http://www.ccsm.ucar.edu/models/ccsm3.0/
    http://mitgcm.org/

    GISS temperature calculations:

    http://clearclimatecode.org/

    And here's a review paper with a bunch of climate sensitivity distributions (see p 738):

    http://www.iac.ethz.ch/people/knuttir/papers/knut

  22. Nick N says:

    Ah, thanks for the link to clearclimatecode.org. I wasn't aware of them before.

  23. Phil says:

    BCC,
    That Knutti and Hegerl article is really great! I wish I had known about it before: it would have saved me a lot of time in preparing this blog entry, for one thing.

    Zarkov,
    I take it back, you _do_ think that if the atmospheric concentration of CO2 increases, this might cause an increase in cosmic radiation! Dare I ask, do you have a proposed mechanism?

    I agree with you, the increase per CO2 doubling could be 1C. That's why it's in my "current" probability distribution. (Did you not look at the figure? I encourage you to do so). I just think that a figure that low is pretty unlikely.

    As for your state of belief, I wasn't asking about your _prior_ distribution, I'm asking where you are now.

    Finally, your feedback argument is confused, presumably because you don't understand the distinction between "feedback" as the word is used in the climate debate, and "feedback" as people usually use the word in the context of a "feedback loop." Most climate scientists agree that for small perturbations in CO2, perhaps even including a doubling, there are mechanisms to return the system to equilibrium. If we stopped emitting CO2 now, the climate would likely return to something like its pre-industrial state within a couple of hundred years. The question is, though, what will happen if we keep on increasing the concentration of greenhouse gases, and then hold it at a higher level? It will eventually reach a dynamic equilibrium, yes…but it won't be the same equilibrium that we would have without the extra greenhouse gases. Or, rather, there's no reason to believe it will be. And yet, some of you are convinced that it will be! This is what I can't figure out.

    Efrique, Jonathan, and others:
    I'm deliberately avoiding all discussion of consequences of global warming, and actions to take to prevent or mitigate it. Maybe someday I'll post about that. I certainly have my opinions about those issues, but I don't really see why this blog's readers would care about my opinions — they're so values-driven, and different people have different values. The topic of assessing climate sensitivity seems like a better match for this blog.

    Cody:
    Boy, talk about Taleb-worship! There were lots of people other than Taleb warning about the financial collapse. See this video for example. But also, my parents took a lot of money out of the market in 2007 and 2008, convinced it was overheated. And so on. But: are you saying that because a lot of economics experts were wrong, all experts are likely to be wrong? Are we better off believing non-experts about _everything_?

    Ed:
    As you may have noticed, I've been trying to get the skeptics in these comments to give us some quantitative information about their state of belief on climate sensitivity, and some explanation of why they have those beliefs. Most of them won't do it. I agree, I wish they would.

  24. anon says:

    "…I've been trying to get the skeptics in these comments to give us some quantitative information about their state of belief on climate sensitivity, and some explanation of why they have those beliefs."

    But then they'd have to be transparent about their methods. ;)

  25. Nick N says:

    Phil, if you're referring to me, then let me make it clear.

    1) I think my pdf would be the skeptic your provide, with the peak around .5 instead of 0.
    2) I think that negative feedback loops are more likely than the models presume (http://www.drroyspencer.com/2009/12/cloud-feedback-presentation-for-fall-2009-agu-meeting/ and http://www.atmos.washington.edu/~breth/CPT-public… – pages 7 to 8).
    3) I think that the warming trend is driven by far more than just CO2 (i.e. land use / deforestation) and thus CO2 has a smaller role than modeled (http://www.ileaps.org/).
    4) I don't trust the data that's input into the models – both the accuracy of specific stations and modelers selectively omitting stations. (http://wattsupwiththat.com/2009/12/08/the-smoking-gun-at-darwin-zero/ and http://noconsensus.wordpress.com/2009/12/13/ghcn-… for examples)
    5) Summarized: we don't know enough about climate to really model it and I think the modelers are kidding themselves that they have it figured out. Being conservative by nature, I say keep on figuring it out, but don't expect me to feel like there's a crisis in the making.
    6) Yes, there are rebuttals to all the points I list above, but I find them generally unpersuasive at this point – that's just my bias.
    7) I found this particularly interesting regarding measured temps compared to IPCC projections, they're outside the 95% CI – The money quote by the author in comments

    "There are always a bunch of assumption associated with any statistical test. But in this case, assuming the assumptions associated with the analysis apply, then the fact that the trend of 0.2C/decade lies outside the uncertainty intervals means that that trend is probably not consistent with what we are seeing in the earth’s data.

    That is: The rate of warming on the honest to goodness earth is probably less than 0.2 C/decade.

    There are a number of caveats associated with the assumptions, but before we go there: If the assumptions are true, then this result is saying the trend of 0.2C/decade is probably wrong." (http://rankexploits.com/musings/2009/gisstemp-for-november-0-68-c/)

  26. mozzie says:

    Of course, CO2 is only part of the problem, and temperature rises are only one symptom.

    Carbon (as soot), methane, CFCs, SO2, also figure. And heat energy can have effects beyond temperature. One possible heat sink is phase change – such the melting of ice (or increases humidity).

    There seems to be some confusion in the "rise in temperature = higher sea levels" equation – these may be associated phenomena but are probably not causally linked.

  27. A. Zarkov says:

    Phil writes,

    "I take it back, you _do_ think that if the atmospheric concentration of CO2 increases, this might cause an increase in cosmic radiation! Dare I ask, do you have a proposed mechanism?"

    Not at all. You are confusing the climate sensitivity factor with the estimate of the climate sensitivity factor. Obviously CO2 and cosmic rays are independent. However if cosmic rays affect the cloud cover, and cloud cover affects average surface temperature, then then cosmic rays belong in any model used to estimate the climate sensitivity factor. The GCMs used by the IPCC to estimate the climate sensitivity factor don't include cosmic rays. If the cosmic ray theory is true then the IPCC is wrong.

    "As for your state of belief, I wasn't asking about your _prior_ distribution, I'm asking where you are now."

    The title of the post is "Say a little prior for me." I gave you a prior I would use. I can't give you a posterior for a calculation I haven't done yet.

    "The question is, though, what will happen if we keep on increasing the concentration of greenhouse gases, and then hold it at a higher level? It will eventually reach a dynamic equilibrium, yes…but it won't be the same equilibrium that we would have without the extra greenhouse gases."

    You seem to be implying that we can treat the earth's climate system as a linear system and use all those linear system concepts to make judgments about equilibrium etc. I'm not sure it's even linear for small perturbations, assuming a CO2 doubling qualifies as small.

    Here is the larger point. I don't think we can arrive at definitive conclusions about the future surface temperature on the basis pure physical reasoning. We need some kind of model that's calibrated to data. If we can't trust the data, we can't trust the predictions. It's also possible that predictions are impossible. We know that even if we knew all the physics we can't do weather prediction for more than about a week because of sensitive dependence on initial conditions. Perhaps someone has proved that the uncertainties associated with temperature prediction don't grow explosively, but I haven't seen it. If you know of a proof than please point me to it.

    Yogi Berra said it best: "It's tough to make predictions, especially about the future."

  28. Phil says:

    Nick N,
    With respect to your point (1), if you've got something like 30% of your probability above 2C, as might be implied by taking the Skeptic curve and shifting the peak about 0.5C to the right, then, OK, that's not crazy. Welcome to the very small club of non-crazy people who are willing to state their probability distribution function! The next international meeting will be held in the elevator of my building (the last one was in a phone booth); hope you can make it.
    Point (2), the second citation: you are a rare skeptic (among those I have seen), in that you trust current models to provide insight into cloud dynamics! I'm not a "skeptic," in the sense we are using the term, and even I don't trust those models! (Well, not much. I realize people are working hard on them, and they are getting better). It's also interesting…and, I must say, odd…that you accept the CLIVAR discussion of cloud feedback, at least well enough to cite it, but you don't accept the resulting estimate of climate sensitivity, which corresponds to something like 2C per doubling of CO2. But that's a three-year-old citation anyway, and this is an area where there has been a lot of work, so presumably better numbers are available now.
    As for point (3), the IPCC has lots of land use change experts. I'll take their judgment over yours, while allowing that they might be wrong (which is one of the reasons my distribution is wider than the IPCC's).
    Point (4), Antarctic temperature data and trends are readily available, e.g. at http://www.antarctica.ac.uk/met/gjma/ so I don't know what that website you cite is complaining about w.r.t. a dearth of data or difficulties in getting the data for lots of Antarctic sites. I have no idea how or whether these data get used in climate estimates.
    Point (5) I think it makes sense to be "conservative" but I completely disagree with the implication that "no change" or "very little change" is the "conservative" estimate for the effect of doubling carbon dioxide. For reasons I outlined in my post, you have to take it on faith that some special things will happen, to get to those low numbers. I agree that those special things _might_ happen, but I don't see where one gets the expectation that they probably will.
    Point (7), http://rankexploits.com/musings/2009/gisstemp-for… you're joking, right?

  29. A. Zarkov says:

    More problems with temperature data. From a summary of Russian news …

    "On Tuesday, the Moscow-based Institute of Economic Analysis (IEA) issued a report claiming that the Hadley Center for Climate Change based at the headquarters of the British Meteorological Office in Exeter (Devon, England) had probably tampered with Russian-climate data."

    "The IEA believes that Russian meteorological-station data did not substantiate the anthropogenic global-warming theory.

    Analysts say Russian meteorological stations cover most of the country's territory, and that the Hadley Center had used data submitted by only 25% of such stations in its reports.

    Over 40% of Russian territory was not included in global-temperature calculations for some other reasons, rather than the lack of meteorological stations and observations."

    You can read the full story here– but be sure to scroll down as other stories appear. http://en.rian.ru/papers/20091216/157260660.html

    If it's obvious from first principles that temperature is increasing at such a rate as to lead to an eventual catastrophe, then why do we bother to collect data and make model predictions? Phil seems to be telling us that the first principles approach is so robust that we can ignore data tampering and modeling problems. In other words, a data-fee first principles approach establishes that warming exceeds 2C with probability >90%.

  30. Leo Martins says:

    I'll have no time to comment, but this is just to thank you all for for sticking to the science and not falling into the policy trap!

    BBC, thanks for the links.

    PS: my curve would look like the skeptic but lognormal.

  31. Nick N says:

    My Mk1 eyeball thinks 30% over 2C in your skeptic pdf shifted by .5 is being VERY generous – I'd give it a 10% (maybe 15%) of 2C or more.

    What's life without some humor? Boring!

    It's been fun, but I'm falling off the net for vacation. Happy holidays!

    Phil, FYI but the opening of your first climate post was, IMO, insulting. People can disagree about the magnitude of effects of CO2 on global temperatures and still be 'educated, intelligent people.'

  32. Eric Rasmusen says:

    Good post and comments! I like this blog.

    Jonathan's first comment is crucial. There has been tremendous variation in climate for reasons that are unknown except that we can rule out manmade carbon dioxide. Thus, while it is useful to do ceteris paribus priors for CO2, it's incomplete. We also need– and I'd like you to try it, as a new post— priors on what would happen to temperature if CO2 emissions halted at their current level. My own prior would center on 0 and thus have a negative component, which could result in net negative even once we add CO2 back in.

    Note, too, that the inexplicable 2000-2009 stalling out of global warming is a big problem for anyone who thinks CO2 is the main driver of climate change, and is particularly bad for anyone who thinks the effect is not concave.

  33. Phil says:

    We seem to have gotten into "garbage time" on this entry, to use an NBA term, so if I have anything new to say I'll put 'em in a new post. But I do want to give my reaction to a few comments.

    Zarkov says "If it's obvious from first principles that temperature is increasing at such a rate as to lead to an eventual catastrophe, then why do we bother to collect data and make model predictions? Phil seems to be telling us that the first principles approach is so robust that we can ignore data tampering and modeling problems. In other words, a data-free first principles approach establishes that warming exceeds 2C with probability >90%."

    For cryin' out loud, I GIVE MY FRICKIN' SPECIFIC PRIOR AND POSTERIOR IN THIS POST. The prior, which is based on a "first principles approach", has something like 30% of its probability below 2C. The posterior, which includes everything I now know and believe about climate change, STILL has a lot less than 90% of the probability above 2C. (Of course, this is just for a doubling of CO2, and I think we will do much more than just double it…but I'm not sure of that). So, Zarkov, the way you are characterizing my attitudes is false, and you obviously know that. Cut it out.

    Also, Zarkov, you clearly don't believe that a substantial portion of the probability should be above, say, 3C, so your refusal to summarize your beliefs in a statistical distribution is…I dunno, "cowardly" is the word that comes to mind. You're happy to criticize my distribution, not so happy to give your own, eh?

    Mozzie (and perhaps this applies to Zarkov, too): you say there's a reason for sea level rise other than increased ocean temperature. If what you were saying is that it can be hard to _measure_ sea level rise, e.g. because of local ground subsidence messing up gauge readings, influence of atmospheric pressure that has to be averaged over a long time, and so on, then I might agree with that. But if what you're saying is that the sea level can rise (on the timescale of decades) for reasons other than increasing average ocean temperature and increased input of water from melting land ice…well, that's news to me, and I'd be interested in hearing the mechanism.

    Nick N, the "skeptic" curve as drawn _already_ has more than 15% of its area over 2C, so when you say your curve would be a bit like that one but with the curve shifted to the right…well, whatever. It's a summary of your beliefs, you can have whatever distribution you want…but it sounds like you want something more like a slightly-right-shifted "denier" curve than a slightly-right-shifted "skeptic" curve.

    OK, that's it for me, as far as posting in this set of comments, although I'll still read new ones of course.

  34. Nick N says:

    Ah, foolish me. The Mk1 eyeball confused the denier with the skeptic. Fair enough. 30% it is.

  35. Lab Lemming says:

    Hey Phil,
    There is a Annan and Hargraeves paper which uses a Bayean approach to estimate climate sensitivity: see their blog here for explanation and (free preprint) link.

  36. Great post and website. I've just noted that we seem to share an eerily similar impression of the IPCC estimates.

    But it's probably more productive to use a term like "contrarian" rather than "denier" unless you want to provoke distracting responses like "It's so insulting that you compare us to Holocaust deniers!"