How Likely?

I had a good question the other day:


Now, my question is, if you know the answer: How high is the probability that arctic change is caused by AGW versus that is just natural variation of some kind (confidence level) ?

Good question. I’m not aware of any “official” estimate of the probability, but here’s mine.

First I’ll note that if AGW (anthroppogenic global warming) is basically sound science, then it’s overwhelmingly likely to be the cause of arctic change. Although it’s possible that AGW is sound science but something else has caused recent arctic change, I consider that to be so unlikely that the chance is negligible.

So we’re really trying to decide between two hypotheses. Hypothesis “A” is that AGW is basically sound science, so human activity is altering our climate, pushing it toward significant warming. Hypothesis “B” is that AGW is not basically sound science, that human activity has negligible impact on climate, and any notable changes we’ve observed recently are brought about by natural variation.

The observation “X” will be: arctic changes over the last several decades, namely warming much faster than the globe as a whole, and sea ice showing rapid and unprecedented decline. How likely were such changes (what’s the probability of “X”) if hypothesis “A” is true (AGW is sound), compared to when hypothesis “B” is true (AGW is not)?

Concensus AGW science predicts “X” as a consequence of “A.” But that doesn’t mean it has to happen — it’s still possible that AGW is sound but there something quirky about the Arctic that they missed, or that the changes will happen more slowly than expected, so it won’t warm faster than the globe and/or sea ice won’t decline. Nonetheless, if AGW is sound then the recent Arctic changes were pretty damn likely (seein’ as how they were predicted).

But no known mechanism of natural variation can explain “X”, a drastic change unlike anything in at least a century and probably thousands of years.

So if AGW is sound, then the changes we’ve observed are very likely, if it isn’t then they’re extremely unlikely.

How likely? My estimate: if AGW is sound science there’s at least a 90% chance of such changes as we’ve seen in the Arctic. So the first probability we seek, the probability of seeing “X” if “A” is true, is 90%

P(X|A) = 0.9.

How unlikely? With no AGW, a change such as hasn’t been seen for thousands of years, for which there’s no known cause, in fact the known sources of natural variation can be ruled out so there must be some entirely natural but unknown cause, is pretty damn unlikely. I don’t believe it’s sane to maintain that there’s more than 5% chance of the recent Arctic changes naturally. In fact I think that’s an overly generous estimate, but let’s say the probability of seeing “X” if “B” is true, is 5%

P(X|B) = 0.05.

With these conditional probabilities we can make a Bayesian estimate of the probability that AGW is correct.

A Bayesian estimate requires a “prior probability,” i.e., a probability that AGW is correct prior to knowing about the Arctic changes. We’ll call it p. Then the probability that AGW is not correct (prior to knowledge of Arctic changes) will be 1-p.

The Bayesian estimate of the probabilty that theory A is true, given the new information (Arctic changes, i.e., X happened), is given by Bayes theorem:

Prob(A|X) = {Prob(X|A) Prior(A) \over Prob(X|A) Prior(A) + Prob(X|B) Prior(B) }.

If we substitute our estimated values, we get

Prob(A|X) = { 0.9 p \over 0.9 p + 0.05 (1-p)}.

Now we need to estimate the “prior probability,” i.e. the probability that AGW is sound science, without taking into account Arctic changes. In my opinion, the laws of physics all by themselves make AGW more likely than not — by a fair abount. Let’s be conservative and go with a 60/40 split, so we’ll say the prior probability is p=0.6 and 1-p = 0.4. Then the Bayesian posterior probability becomes
P(A|X) \approx 0.964.

Like I said: arctic change is very strong evidence of AGW. So strong, in fact, that it increases a naive 60% probability estimate to 96.4%.

And frankly, 60% as a prior probability isn’t right. There’s more to AGW than the laws of physics, there’s a mountain of observed evidence so support it. I’d say a righteous estimate of the prior probability is at least 95%. At least.

And if we use that for the prior probability, the posterior probability becomes 99.7%.

That’s a pretty crude calculation. It’s also not a bad estimate of my belief regarding the question: is AGW sound science or not? Yes 99.7% — being conservative. And yes it’s the cause of recent Arctic change.

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79 responses to “How Likely?

  1. I think you need to send this to Judith Curry. She seems to be confused about how uncertainty works.

  2. and regardless of your (or anybody elses) prior probability that AGW is correct, the Bayes Factor (ratio of posterior odds to prior odds) is 18.
    So the observation of recent arctic change should cause 18 fold increase in your odds of AGW being true – which Jeffrey’s would rate as “strong” evidence – regardless of your prior belief in AGW (assuming you agree with P(X|B) = 0.05).

    Just out of curiosity, if it were possible that AGW was sound but the recent arctic changes were not actually (or wholly) caused by AGW – how would you answer the question – i.e. how would you calculate the probability that arctic changes were caused by AGW?

  3. You should also send it to Joe D’Aleo (sp?). Though he just seems to be confused, period.

  4. Mr. T, a much simpler version, if A is correct, then overall temps will rise. If temps rise, this means extra heat, and as everyone knows, heat rises. Since the Arctic is on top of the globe, then if A is correct, heat should rise upward to the Arctic, and the Arctic temps will rise quicker then elsewhere!

    Pretty simple really……

    :)

  5. Tamino,
    beautiful analysis as usual. i would love to get your thoughts on whether or not this criticism of cosma shalizi on bayesian analysis applies in the case and if not why not.
    weblog here: http://cscs.umich.edu/~crshalizi/weblog/664.html
    and paper here: http://arxiv.org/abs/1006.3868

    • David B. Benson

      That is a somewhat interessting paper about a philosopihical point regarding scientific methods. Thank you for the link.

      However, the point being driven home by Tamino here is too straightforward to require the model testing advocated in the paper.

  6. I might have framed it differently:

    Chance that AGHGs lead to planetary warming: 99.9%
    Chance that planetary warming (and AGHGs) lead to Arctic warming: 95% (with the 5% being something weird like the counterintuitive Shindell black carbon result that the right kind of heating in the Arctic troposphere in the right amounts could lead to changes in atmospheric convection patterns such that increased BC forcing actually led to cooling in at least one GISS simulation).

    But that still leaves a question of how much of the Arctic warming is due to AGHGs (and in this case, I separate out AGHGs from aerosols etc. to make a clearer picture). My off-the-top-of-my-head guess would be about 70% of the Arctic warming is from AGHGs and about 30% from BC, with cooling aerosols actually having very little effect, and I don’t have any good reason to think that net natural variability (AMO, PDO, solar, whatnot) would lead to either cooling or warming, but that isn’t to say that such information does not exist somewhere (obviously, globally, the best bet is that natural forcings are net cooling, as are net aerosols, as is net land-use albedo change, which means that the best bet for AGHG contribution to global warming is greater than 100%…).

    Also, I guess my approach doesn’t really get at the question of whether Arctic warming is additional evidence for AGHG warming… but then, my AGHG warming prior is already 99%, so…

    -M

  7. I think you need to send this to Judith Curry. She seems to be confused about how uncertainty works.

    She’d simply point out that she’s not convinced by what is something of a back-of-the-envelope calculation, and that her *intuition* trumps it … :(

    • You mean the classic ‘Any uncertainty whatsoever =Zero knowledge’ denialist argument.

      Certainly JC seems completely convinced of some sort of internal variability being the cause.. without doing anything like actually find evidence for it.

  8. Shouldn’t P(X\B) be 0.025 (with alpha of 0.05, warming will be randomly colder half the time)

  9. Hi Jim,
    I don’t have a good answer to your question, but contemporary changes in the Arctic aren’t just down to anthropogenic causes, it’s those + natural variability. The record low in 2007 wasn’t just because it was warm, it was warm + there happened a large dosage of local conditions that produced a very dramatic low.

    What the anthropogenic component alters is the trend. If it were just Natural Variability it would be reasonable to expect lows to be mixed with highs and in-betweens and the Arctic Ice would recover in short order.

    Unfortunately, because of the persistent warming from the anthropogenic component and because Arctic Ice is recovery is a fragile thing with single year ice being less robust than multi-year ice, a recovery in ice extent in following years is much less likely and the resulting changes in albedo and sea-level rise will likely persist and further compound the issue.

    The first (and possibly an awful) analogy that comes into my head is HIV. It’s common, everyday illnesses that do the damage – the advanced stages of HIV just make it increasingly unlikely that there’ll be a good recovery.

  10. Let me see if I can formulate some questions – possibly not and possibly they are nonsensical anyway:

    Presumably the same anaylsis could be done for any predicted effect of anthropogenic global warming. Could these individual probabilities be combined to say anything meaningful? Would the prior probability that AGW is correct need to exclude only the particular predicted outcome, or if their probabilities were being combined would the prior probability then have to exclude all of these prediction / observation pairs in each probability calculation?

    What are the calculations and implications for any predicted effect that has not subsequently been observed, like…. er, I’m sure there are some (troposhperic hot spot?)?

  11. I’m not quite sure how you can claim that ‘…no known mechanism of natural variation can explain “X”, a drastic change unlike anything in at least a century and probably thousands of years.’ when most of the long Arctic temperature records show a similar warming only 60-70 years ago. So guesstimating P(X|B) to be 0.05 is the weakest point of your argument. Suppose we say it’s 50/50 (i.e. P(X|B) is 0.5), then, leaving all other numbers equal, you get P(A|X) ~= 0.73, not quite as impressive.

    [Response: As Walt Meier himself posted (on WUWT no less), conditions 60-70 years ago were not like they are today. There was no similar decline in sea ice. The weakest point in your "logic" is that you ignore the facts you don't like.]

  12. Sea Ice conditions 60-70 years ago may not have been similar to the conditions of the last 5-10 years, but temperature conditions were indeed very similar, and that was my point.

    [Response: So you "point" was that you would ignore or alter condition "X" when computing conditional probabilities based on "X".

    How Wattsian of you.]

  13. Please don’t be rude, I did not alter condition X. I don’t need to prove the existence of a completely similar situation 60-70 years ago to make my point, which is that there are huge natural temperature variations in the Arctic, and thus that I think you’re underestimating P(X|B) (I won’t even bring up the ice free Arctic in the early Holocene or the mild Greenland climate with viking Farms during the MWP, since one can always argue that insolation in the Arctic has changed since then, so it “should” be colder now anyway).

    [Response: You persist in finding fault in my P(X|B), based on past conditions which are NOT X. Please don't be an idiot.]

  14. I’m not sure why you need to be so rude, and I should probably leave and never come back … [edit]

    [Response: I'm not sure why you need to be so stupid. Please leave and never come back.]

  15. Espen,
    “when most of the long Arctic temperature records show a similar warming only 60-70 years ago.”

    I challenge this assertion wholeheartedly.
    How do you substantiate this claim? Because if you look at the following graph I put together of ALL land stations above 66°N, I don’t think the evidence supports your conclusion?
    See this image for yearly

    And this image for every season!

    To me it seems that the last 10 years are roughly 0.65°C warmer than any 10 year period during the previous warm period. This graph does not include the ridiculously warm year we’ve had this year (2010) either.

  16. I don’t particularly like this analysis. Not because I think it is wrong, or because it has no value, but because any discussion based on Bayesian statistics inevitably devolves into pointless bickering about the choice of numbers.

    If the result is sound, a non-Bayesian analysis should pop out a near identical number. Probability is my weakest subject, so I will leave the actual analysis to people able to – you know – do it right.

    • “a near identical number” for what? If you want to calculate the probability that a hypothesis is true, e.g. Pr(A|X), then you have to use Bayes theorem – there is no alternative (its just conditional probability). You might be able to use repeated model simulations to estimate P(X|A) and P(X|B) – the later arguably could be calculated form very long term climate & ice data but the former cannot – AGW has never happened before). But however you arrive at those probabilities, if you want P(A|X) then you have to use Bayes theorem. The frequentist approach would be to estimate P(X|B) only , and draw conclusions based only on the probability of the observations given the “null” model (regardless of the plausibility or otherwise of that null or the altnerative). This leaves just as much scope for bickering about the choice of P(X|B), and tells you very little about the probability of A or B.

    • Didactylos,
      This is not necessarily true. Use of minimally informative priors and empirical Bayesian methods really involve less subjectivity than many so-called frequentist approaches. I’m working on one such analysis at present in my own field.

      The reason to use Bayesian approaches is that they are sufficiently flexible to accommodate any type of data. One can quibble with the exact value Tamino chose within limits (limits Espen has clearly breached). The question is whether the difference is significant.

      [Response: I agree that with minimally informative priors, Bayesian analysis is far less subjective than often thought. But it's still *thought* to be subjective, so its reputation alone leads to disputes. Not logical -- just human.]

  17. Robert, I think some of that data may be tainted (e.g. by the development of air traffic – this is certainly the case for e.g. Barrow, Alaska), but I’m certainly open for the possibility that this period is slightly warmer than the previous warm period, and I certainly think the difference may be partly due to AGW. What your charts show, is that large multi-decade warming and cooling periods are the normal in the Arctic, as far as we know from the limited instrumental record.

    However, on skepticalscience.com I read: “Decline in sea ice is the major driver of Arctic amplification. This is evidence by the pattern of atmospheric warming over the Arctic. Maximum warming occurs over the surface during winter while less surface warming is found in summer when heat is being used to melt sea ice. This pattern is consistent with sea ice amplification.”

    This doesn’t quite match what I see in the seasons graph you supplied, because according to that graph, the recent winters are quite similar to the previous warm period winters, while the summers are warmer now…

    [Response: First you criticize my estimate of P(X|B) based on conditions which are NOT X. Then, when the entire basis for your "in the past" argument is shown to be a sham, you resort to the "data may be tainted" meme.

    Which makes us wonder: if the arctic data is tainted, then what the hell was the basis for your claim about conditions 50-60 years ago?

    Not only will you ignore the evidence you don't like, you'll deny the very *data* you don't like. You are no skeptic, you're in denial.

    Bye.]

  18. Thank you, Tamino, excellent post and nice ideas (and I think this should be in textbooks describing conditional probability too, very nice and very descriptive example how these formulas shall be used in practical example). And yes, even if I’d be denier and I’d put even more “denialist” input parameter (with at least somehow honest params), there would be clear that AGW theory and currently behaviour of Arctic Ice are strongly correlated (so I’d be a AGW denier, I’d have to work much stronger and really find much of the opposing evidence – I don’t say it is impossible, but I’d rather climb Mt. Everest).

  19. Tamino, I’m not a denier, in fact I have a very open mind (otherwise I would, as a green activist in my youth, never have started asking myself critical questions about AGW at all)… Also,, I’m convinced that CO2 warms the atmosphere – but not that climate sensitivity is high. I’m not convinced that warming has already damaged the earth, in fact I think that a little more warming will only be beneficial. It’s much more rational to worry about a possible return to little ice age conditions, where the Arctic may have been quite close to the onset of a new glaciation.

    I think it’s sad that you don’t want a dialogue with people who don’t completely share your views. And I’m sorry to say that this failure to engage in dialogue makes you half blind: You think I’m in denial just because I see similarity where you see differences.

    [Response: what a crock. You're a denialist because:

    1: you criticized my estimate of P(X|B) based on conditions which were NOT X.

    2: When the basis for your "50-60 years ago" argument was shown to be a sham, you resorted to the "data is tainted" meme.

    It seems that you're willing to use data when you think it supports your belief, but when it contradicts you, you'll deny the very data itself. Even after you yourself make a false claim about Arctic temperatures! What goddamn data did you use for that claim?

    It's pathetic that you can't do any better than that. What's worse is that you actually pat yourself on the back for it. And when your shenanigans is shown for what it is, you call yourself "open minded" and me "half blind." You are totally blind.

    I love to engage in dialogue with people who don't completely share my views. I'm sick and tired of refuting the same old astoundingly stupid arguments from the blind. That's you.]

  20. Espen

    The aforementioned data comes from combining 117 land stations between 66N and 90N using the proper methods as outlined here by tamino and developed into a command prompt program by Joseph at Residual Analysis.

    The thing about using so many stations is that you don’t run into the problem of a single site influencing the overall series much. Also note that the “airport warming” signal that you say exists, has no impact on large scale reconstructions as determined at the following URL

    http://clearclimatecode.org/?s=airport

    The season graphs are 10 year averages. Your diagnosis is incorrect. Winter shows a much stronger linear warming trend than summer which is much more flat. I can compute the actual yearly change if you like but you should be able to eyeball it yourself. What you see from the graphs is that warming affects the winters and falls greatest. In terms of the multidecadal variability you speak of, there is undoubtedly significant correlations with the AMO and solar radiation. However note that the current warm period has included the deepest solar minimum in the last century and that overall solar values have plummeted.

    And if by similar you mean that all seasons show greater than 0.5 °C warmth compared to then? and an overall warming of 1.5 °C above the baseline (1951-1980) (arctic amplification).

  21. The Arctic warming of the 1930s ought to be a warning signal, not a comforting confirmation of “natural variability”, because it suggests that the Arctic system is sensitive to much lower levels of forcing than it’s currently experiencing.

    It might also be worth noting that current ice loss is almost certainly being driven primarily by ocean heat content increases, and that is determined mainly in the tropics (because that’s where most of the earth’s surface is). Models that don’t ship enough heat north underpredict ice loss and Arctic amplification. See Mahlstein and Knutti, Ocean heat transport as a cause for model uncertainty in projected Arctic warming.

    In other words, T’s 90% estimate for X is probably low…

  22. Espen, Let me try to explain something to you. Most of us here are scientists–and science does not allow us to make claims without evidence.

    You say you are “…not convinced sensitivity is high…” OK, what evidence do you base your doubts on? There are over a dozen independent lines of evidence that preclude a value less than 2 degrees per doubling and favor 3 degrees per doubling. For 2 doublings, which we’ll likely reach early next century, we’re talking a temperature rise of 6 degrees–well into the deanger region. And it could be up to 10 degrees if we include all feedbacks.

    You say you think a “… little more warming will only be beneficial.” Again, based on what evidence. The evidence I’ve seen shows agricultural and fishing yields drop with increasing temperature. Drought increases, and the precipitation that does occur tends to be in more intense bursts, which are more likely to cause floods and less likely to recharge aquifers.

    So my question to you is: If you aren’t presenting any evidence on which you are basing your opinions and you are ignoring the evidence presented by experts, how is it again that you aren’t a denialist?

  23. David B. Benson

    Espen — Here is a zero dimensional, zero resevoir climate model which is nontheless sufficiently good enough to produce an ECS of about 3+ K for 2xCO2; same as in IPCC:

    http://www.realclimate.org/index.php/archives/2010/10/unforced-variations-3-2/comment-page-5/#comment-189329

    and here is the drought prediction from NCAR for 2060 CE:

    http://climateprogress.org/2010/10/20/ncar-daidrought-under-global-warming-a-review/

  24. Espen: I think that a little more warming will only be beneficial.

    BPL: What part of “more and more of Earth’s land surface will be in severe drought until human agriculture collapses altogether” do you not understand?

  25. However note that the current warm period has included the deepest solar minimum in the last century and that overall solar values have plummeted.

    It has also included two strong la Nina (cool) phases (2008, 2010), the current one possibly the strongest since 1955, while the most recent el Ninos (warm phase) have been relatively moderate.

  26. The last strayed a little off-topic. I understand that ENSO has a negligible effect on Arctic sea ice. (Been looking for info on such a connection, any help appreciated)

  27. It is always true that global warming events have other factors involved. The question is could the ice have got so low without global warming?Clearly not, but Tamino would say almost certainly not.

    Anthropogenic climate change is responsible, beyond reasonable doubt. It is just barely possible that a previously unknown mechanism will be found that explains what has happened. That would be akin to finding out alpha is not constant throughout the universe.

    • Actually, given that there are String Theories that predict a variable alpha, such a discovery would be less surprising. For the current warming epoch not to have an anthropogenic cause, everything we know about climate would have to be wrong–and there is NO coherent alternative theory.

  28. Espen’s meanderings above demonstrate how the term “skeptic” is frequently misused.

    It is NOT skeptical to question one dataset yet accept another with open arms, without question. It is using confirmation-bias to drive a non-scientific, political agenda. Yet time and again this is what we see. “Skeptic” has to be the most abused word in the dictionary!

    Cheers – John

  29. BPL human agriculture is not going to collapse due to warming even with 10 billion people. I was a ‘warmist’ for years but now I see how the ‘science’ is severely flawed and that is from the training, science classes, field work, etc… not from guesses and speculation.

    [Response: How nice of you to dismiss a mountain range of evidence gathered over a more than a century and studied by the world's most knowledgeable experts -- as well as the laws of physic themselves -- as "guesses and speculation." Enjoy your time in fantasyland.]

      • Marco |

        Poe…

        One is never sure, by definition, but I believe what we have here is a lost locust from the WUWT swarm. The chittering noise is unmistakable.

      • Adam R. wrote:

        … I believe what we have here is a lost locust from the WUWT swarm. The chittering noise is unmistakable.

        I believe we have a winner!

        Stastical thermodynamics proves why thermodynamics cannot be violated at the micro-level. The correspondence principle is what this is called. Thus global warming cannot be depicted by the hockey stick accurately or have a run away effect ever.

        October 11, 2010 at 5:55 pm

        http://bit.ly/bytoCn

    • And here I stand, stuck in the middle between two nutters.

      BPL is probably right about the eventual collapse of human agriculture. But he thinks it will be a massive global catastrophe, and will happen not only in our lifetime, but very soon. To me, that sounds like the babblings of a fundamentalist predicting the Second Coming. It’s not supported by the data or any sane analysis I have ever seen.

      Then on the other hand, we have Jacob “it’s not happening it’s not happening” Mack. He can’t tell the difference between a possibility and a certainty. He has no concept of science and a poor grasp of data, but like BPL, thinks his version of reality is better than anything.

      Why do we bother? I have no problem with insane. It’s wilfully stupid I can’t stand.

      [Response: I'm uncertain about the future possibility of massive drought, but I suspect BPL might be correct.]

      • Didactylos,
        Remember, we do not face solely the problem of climate change. The climate is changing as human population reaches its peak of 10 billion (probably mid century) AND as we further damage the globe’s carrying capacity (by depleting aquifers, causing fisheries to collapse, depleting topsoil) trying to meet food and energy needs for that number AND as we deplete the most easily exploitable energy sources–fossil fuels. In some of these areas (e.g. resource depletion and aquifer destruction), we are permanently damaging the planet as it has never been damaged before. I do not care to speculate as to timescale, but it does look to me as if human civilization is at risk.

      • Oh yes, human civilisation is definitely at risk. But BPL completely fails to take into account things such as water management. There is a huge difference between an unmanaged water supply under water stress, leading to major drought, and a managed water supply where water is moved long distances, and rationed in order to keep food supplies safe. I’m not saying that scenario doesn’t involve some downsides. But there is no way we are going down without a fight, and treating the worst case as the only possible case is just idiotic.

        It’s developing countries that will feel the pain first. Without water management, and without huge sums of money to throw at the problem, there is no way out. But there won’t be any abrupt collapse. These countries already suffer from these problems, and more besides. We will see more conflict, more famine, more refugees. But even at the top end of IPCC projections, developed countries in temperate regions will be able to mitigate for the next 50 years at least.

        I’m not a denier. We need to act now to prevent vast problems a century from now. But I’m not an idiot, either. Global warming isn’t going to kill us all next week. And if BPL is right with his unlikely predictions, then the IPCC is very, very wrong. Is it possible BPL knows more than all those scientists? Is it likely? BPL would be the first to mock any denier pulling the same stunt, so I say he should be held to the same standard.

      • Response: I’m uncertain about the future possibility of massive drought, but I suspect BPL might be correct

        Yes, BPL *could* be right. He is overconfident of the certainty (IMO), but there is an argument to be made there.

        While, on the other side, we have this:

        I was a ‘warmist’ for years but now I see how the ‘science’ is severely flawed and that is from the training, science classes, field work, etc… not from guesses and speculation.
        </blockquote

        Argument from personal incredulity, not at all the same thing BPL is doing. BPL does work hard to try to put his opinions in a quantifiable context.

        And, of course, I really am looking forward to how Jacob Mack will turn basic physics into "science" (i.e. not science). Does he deny "gravity" (quotes attached to make clear that gravity doesn't exist outside "science", which obviously doesn't exist due to being "quoted").

      • Didactylos,
        I don’t know how old you are, but my ideas about sudden collapse changed when the Soviet Union imploded in 1991. Perhaps it made more of an impression on me as I was in a remote African village and did not get news regularly. When I try to imagine such a collapse under the additional stress of food insecurity and severe environmental degradation, it is not a pretty picture. What is more, the rest of the world was able to cushion some of the blow to the FSU (albeit not as much as they should have IMHO). When the degradation is global, there will be conflict rather than aid. Collapse within a generation? I don’t know. However, what may be worse is the impact of an impending collapse that 3 or 4 generations of humans will know is coming and that they cannot avoid.

      • “According to Russian government statistics, the economic decline was far more severe than the Great Depression was in the United States in terms of Gross Domestic Product. It is about half as severe as the catastrophic drop borne out of the consequence of World War I, the fall of Tsarism, and the Russian Civil War.” (Wikipedia)

        All this really shows is that humanity is quite good at surviving “collapses”. Humanity, note. That isn’t going to include every last human.

        I’m no Nostradamus, but a simultaneous global collapse this century seems unlikely, and distinctly alarmist. Regional problems? Absolutely. Unrest? I would bet on it. Hundreds of millions of deaths from climate change this century? A possibility. It will certainly be many millions.

        You need to remember that there are already millions of people dying from preventable disease in third world countries. Without the news media, I’m not sure we would even notice if those numbers tripled.

      • I remember quite clearly from when I was child that everything around me seemed so permanent–in my mind, the houses, people, even pets were just as they had always been, and would always be. And many adults have had the same feelings about many things–steadily rising real estate values, immutable gender roles, (going back a bit) the inevitability of “progress,” or (going back quite a bit further) the existence of legal slavery in the United States.

        But sometimes what feels like “realism” is actually a failure of the imagination. And I don’t mind admitting that, like Ray, I did not imagine the fall of the USSR. Or, for that matter, the (relatively) bloodless end of the apartheid regime in South Africa. (And I really, really tried to imagine the latter!)

        I fervently hope Didactylos’ realism really is. . .

      • Didactylos,
        The thing is that we are already doing irreparable damage to the planet trying (and failing) to provide for 6 billion and with fossil fuels (on which agricultural productivity depends) abundant and cheap. The real stresses on the system will come in the latter half of the century. That is when human population will crest. That is when the resource crunch and resulting conflict will be most intense and the environmental degradation most severe. The critical question is how much damage we will do to the planet’s carrying capacity in this period. If it drops from 2 billion (where I think it now lies) to about 100 million, it will be very difficult to maintain civilization. I’ve tried to rule out such a collapse, but I have not been able to bound the damage. Saying “That’s alarmist,” misses the point. The point is to bound the risk. If you cannot, you have to assume the worst.

      • Ray, my complaint is that BPL won’t even admit the possibility that he is wrong (on this and any other subject you care to name). And he will stoop to any denier trick he feels like in support of his pre-determined position.

        You know that the future is uncertain, and you aren’t monomaniacal about your prognostication – so while I may disagree with you on a few point, you don’t irritate the heck out of me. In fact, I respect your views a lot.

        2 billion seems low. What assumptions are you making?

        Kevin: The USSR was falling apart at the seams long before it ultimately collapsed. Maybe you weren’t watching for it, but I imagine anyone with an inside view had a good idea what was going on.

        My own vision of the future isn’t based on fantasy, but actual projections. In the very near term, there are positive effects of climate change that will offset some of the problems in some areas. Yes, it’s storing up trouble for later, but it makes it less likely that anything catastrophic will happen in the short term. Negative effects won’t even begin to impact western civilisation until we reach 3-4 degrees. We will be well on the way to destroying the biosphere, but food will still grow, where there is water anyway. That much temperature rise may have locked us in to absolutely enormous sea level rises, but we won’t see even a fraction of it by 2100.

        So, probability of global collapse before 2050: infinitesimal. Before 2100: Low. Before 2200: buy your ticket offworld now….

        This is why old people can have such a callous disregard for the truth. They won’t see it. Their children will get by. But what about their children’s children?

      • Didactylos, While I sympathize with your frustration, we certainly have not demonstrated a flaw in BPL’s analysis. Until we do, I don’t think it is fair to label him denialist. Dissident perhaps. And if he is correct–and he certainly thinks he is–then alarm is not inapproproate. My own guess is that BPL is on the pessimistic side. However, his estimate coincides with the period where maximum stresses on the environment begin.

        My estimate of 2 billion carrying capacity is based on a SWAG from the graph of population vs time. That would put us at about 1940-50 levels–prior to the green revolution, which I believe is based on unsustainable inputs. I think we have already done damage to that carrying capacity.

      • Ray, maybe you know more about the details of BPL’s analysis than I do, but he never answered any of the questions I had concerning it – which is what leads me to believe that his analysis makes no allowance for reduced emissions as a result of crashing economies, and no allowance for water management. According to the little I know, BPL assumed an emission scenario that conflicts with the results he claims.

        Without allowing for such details, it is hard to see how a drought analysis can produce anything that speaks to “global collapse” in any form at all.

      • Didactylos, I’m not convinced that “water management” is a plausible answer to drought of the severity Dai et al. predict. It’s expected to be much, much worse than the 30s. And I don’t think that decreased emissions will have much effect on warming realized by 2050. I just put out a new article–a review of “Keeping Our Cool,” by Andrew Weaver; you may have seen the notice at RC–and as part of the process Dr. Weaver was kind enough to email me a Powerpoint he did. It included this figure:

        You’ll note that the differences in warming by 2050 resulting from quite different emissions trajectories are pretty small. I don’t have any details on BPL’s paper, but I suspect that the differences of a couple of tenths C shown by the figure will not affect the drought severity or extent much.

        So I still hope you are right, and that things are indeed “less worse.” But honestly, I don’t think that any of us has the methodology to make a really good assessment at this point. Hence, I don’t feel all that reassured–especially since the likelihood of effective mitigation action soon seems rather low.

        “Keeping Our Cool” article:

        http://hubpages.com/hub/Keeping-Our-Cool-a-review

      • I’m not daft enough to claim that water management will mitigate anything completely. I’m just saying it needs to be taken into account. Vörösmarty et al. (2010) show that water management has an absolutely game-changing effect on water stress.

        And the graph you provide simply confirms that we will almost certainly see less than 2 degrees above preindustrial by 2050. Such a rise will be damaging for third world countries, but for developed countries, the worst you can reasonably call it is “inconvenient”.

        You keep talking about Dai et al.’s “predictions”, but Dai et al 2001 says nothing about PDSI for 2050 or water stress for 2050. It focusses on precipitation changes by 2090. Precipitation increases overall, but in a very region-centric manner. I am absolutely sure that for some large regions, this will equal severe drought by 2100. But other regions getting more water may be able to cope, particularly if they employ effective water management.

        They comment on the US specifically:

        Indeed, over the twenty-first century as a whole, the trends in winter precipitation over the southwestern United States, as well as over the whole contiguous United States, are not statistically significant. The increasing trends of summer and annual precipitation averaged over the contiguous United States are, however, statistically significant at the 5% level

      • The penny drops. You mean Dai, 2010, Not Dai et al.

        Certainly cause for concern. But my earlier remarks remain valid. The difference between c and d in fig. 11, for example, is small enough anyway – then consider that those pink areas in Europe and the US are already a healthy green on maps that account for water management. Even if we have reached the absolute limit of benefits from water management (and I don’t believe we have) then the additional stress will produce effects only equivalent to moderate drought at worst.

      • Sorry for the bad cite, Didactylos. You are of course quite correct about which paper I had intended.

        I’ll take another look at Dai’s figures, and see what that Vörösmarty et al. (2010) have to say. . . some reassurance would be welcome, particularly if well-founded. Though at least “cause for concern” remains, either way.

      • Here’s some recent evidence of non-linear tipping points leading to prolonged droughts in the Andes:

        Nonlinear climate change and Andean feedbacks: an imminent turning point?

        1. M. B. BUSH,
        2. J. A. HANSELMAN,†,
        3. W. D. GOSLING,‡

        Article first published online: 25 FEB 2010″

        Source here

        Here’s the part supportive of BPL:

        “Given a rate of warming in the Peruvian Andes of about 0.3-0.5 degrees Celsius per decade, the tipping point ahead would be reached between 2040 and 2050.”

        The Yooper

      • Yes, we should expect severe regional problems, and some may come sooner than we expect.

        But the Andes aren’t the first place I think of when someone mentions civilisation….

      • Didactylos says, “But the Andes aren’t the first place I think of when someone mentions civilisation….”

        Well, there has been a civilization there for thousands of years, and some of it even survived the conquest. And the foods introduced to Europe (e.g. tomatoes and potatos) substantially increased calories and nutrients per hectare of cultivation and so were in part responsible for avoiding the realization of Malthus vision (or at least posponing it) in Europe.

    • Jacob, I can only hope you are kidding. Or do you really think that the National Academies of every country in the world are conspiring to dupe the world’s population?

  30. I was a “Gravityist” for years but I’m okay nooooooowwwwwwww…… splat!

    Cheers – John

  31. I want examples of where the science is “severely flawed.” My best friend works for an oil company and he can get a simply gigantic Christmas bonus if he can demonstrate any aspect of global warming science is severely flawed; in fact, they will give him a Christmas bonus each Christmas for the rest of his life – the once-a-year mother of all Christmas bonuses.

    I told him about Jacob, and he really has his hopes up.

    One thing, it has to to right. The oil companies employ top-notch scientists, guys who should have uncovered this severely flawed science ages ago, and the usual denier BS will not stand up to their tests. They will demand that the science ACTUALLY be flawed.

  32. SEVERELY flawed.

  33. David B. Benson

    This is very alarming because if the drying is anything resembling Figure 11, a very large population will be severely affected in the coming decades over the whole United States, southern Europe, Southeast Asia, Brazil, Chile, Australia, and most of Africa.

    from

    http://onlinelibrary.wiley.com/doi/10.1002/wcc.81/full

  34. Feed 10 billion people, I don’t think so. Agriculture collapse tomorrow, probably not. Stress in the food production system, already happening.

    Have you looked at the impending phosphate shortages. Followed the cost of fertilizers and other agricultural inputs. Agricultural water quotas in Australia are being reduced, the water table in India is falling. Russia has banned wheat exports. Crop damage due to storms in the US.

    Yes droughts and starvation have always happened, but elsewhere in the system there have been surpluses. Now the food supply chain has little in reserve, any shortage anywhere in the world is likely push up prices.

    In the not so distant future I might have to give up my Shiraz, others might have to give up breakfast and lunch.

  35. “Collapse” is a loaded and imprecise term, unfortunately. I don’t know what qualifies, precisely. But the drought projections from Dai et al are really, really, scary–particularly for Mexico and adjacent areas of Central America, most of the Mediterranean basin and adjacent portions of the Middle East, and Chile. The American Southwest, western Australia and much of Amazonia are going to be pretty damn dry, too.

    And while these are modeled projections, and various caveats certainly apply, my understanding is that these results are fairly robust across model runs. Moreover, as BPL and Dai et al. both point out, the drought trends over the last 20 years are–well, IIRC, Dai characterizes them as a “sharp increase.”

    It’s not a recipe for happy days. Serious conflict, violence, upheaval and famine seem quite probable. Timescale? Well, BPL ran numbers in a fairly serious way; he may be right. The Dai et al projections I think are for 2050, which some of us will see.

    Let’s see if I can find the link again, it’s worth it for those who haven’t seen the paper yet.

    Yes, from RC:

    http://onlinelibrary.wiley.com/doi/10.1002/wcc.81/full

  36. Kevin,

    I ran my simulation 10,000 times to get a good sample. The fraction of Earth’s land surface in severe drought always reaches 70% by 2050-2055, with a mean of 2052 sd 0.6.

  37. Should probably be in “Extreme Heat” really, but since water scarcity is discussed above… I mused:

    Fremen-style stillsuits anyone?

    New use for whale hunting ships after Cetacea collapse: iceberg hunting?

    Strip mining of Greenland glaciers? And those of Antarctica? The thin end of a dangerous wedge of Antarctic “minerals” mining?

    Water exports from water-rich areas?

    Water wars?

    • P. Lewise wrote:

      Should probably be in “Extreme Heat” really, but since water scarcity is discussed above… I mused:

      Fremen-style stillsuits anyone?….

      I don’t know about Fremen stillsuits, but assuming we continue with business as usual and then burn the non-traditional fossil fuels (e.g., shale, tar sands, synthetic oil from liquified coal) the planet is going to look very different.

      There will be the dried out continental interiors due to oceans having higher thermal inertia than land. Absolute humidity may remain the same as moist ocean air blows inland but the higher inland temperatures will mean that the relative humidity drops.

      Less moisture reaching the soil and what moisture is in the soil evaporating more quickly will dry out the soil and result in the loss of vegetation that might otherwise hold on to some of the moisture. This will reduce moist air convection which will result in higher surface temperatures.

      The expanded subtropics will likewise dry out much of the globe, including the US. And one of the points where models fell down was in terms of modelling the rate of expansion. The subtropics were expanding at three times the rate that the models were showing. But we may be doing a better job of modelling the expansion nowadays.

      Meanwhile more rain will fall at the higher latitudes, e.g., in Canada. But as the rate of evaporation and absolute humidity roughly doubles for every 10°C the water cycle will contain more energy, resulting in stronger storms. Flash floods become more likely.

      Some of this rain will occur even in the otherwise desiccated continental interiors. But as there will be less vegetation and roots to hold on to the top soil much of the rich soil that makes farming possible in the US today will be lost to the sea.

      In time there will be more bare rock, and with harder rainfall carbon dioxide will be slowly, naturally sequestered through the process of mineralization. A negative feedback that operates over tens of thousands of years.

      Meanwhile the coasts will provide some solace from the baking of the continental interiors but cities will keep having to be moved inland — and we will no longer be able to afford the high investment in infrastructure. And of course even once the temperatures plateau the oceans will continue to rise with the melting of the ice, the loss of glaciers and the icesheets that reside on land, and most importantly the expansion of the oceans.

      Harvests from the midlatitude farms will all but disappear. The high latitudes may take over to some extent, but so much of the soil has been locked in ice, will subside with the thermokarst lakes. Rocks will have to be cleared.

      The high latitude soil will provide a poor substitute for what had once existed in the midlatitudes. Tundra is acidic and brown forest soil becomes acidic with increased precipitation. And of course the infrastructure to support farming will be largely missing.

      But with temperature continuing to rise more quickly in the high latitudes than the rest of the globe it won’t make much sense to invest heavily in farming. Whatever crops the land might support will tend to be gone a few decades later.

      Much of our ocean harvests come from the coral reefs which act like tropical rainforests in preserving diversity in an otherwise largely desolate ocean. But coral reefs will largely disappear due to periods of high temperature and increased acidity. And the latter of these will eat away at the calcareous protists that lie at the very base of so much of the ocean food chain.

      Increased drought and the occasional flooding will put at risk much of the fresh water supply — already threatened by our depletion of the water tables and many of our freshwater lakes. Rising ocean levels will likewise result in the contamination of water tables by salt and even red algae. Stronger storms will likewise contribute to this.

      With shortages in freshwater you will see people increasingly relying on whatever water they can find — despite the risks. And the risks will include waterborne illnesses. The places that are hardest hit will become a breeding ground for disease. Likewise the resource shortages will make people desperate, vulnerable to extremist ideologies — and with war comes the spread of further disease.

      • Sounds like the Australianisation of everywhere. Thin, nutrient-poor soil, blowing away or washing away, just getting thinner and more impoverished.

        At least there’ll be expanded scope for rainwater tank manufacturers. Biiig rainwater tanks.

      • Timothy, I’ve saved a copy of this comment. I think you did a nice job of deducing consequences.

      • Just a few nit picks.

        Surely we should expect more rain in a warmer world? Certainly rainfall may have a different pattern to now, with possibly a case of heavier/less often, but all things put together there should be more.

        As far as soil goes, yes, much of out best farmland is a result of glaciation and the resultant erosion of rock, meaning that high quality soil exists in a band in temperate latitudes. Outside of that band you only really get good soil in river valleys or other ‘special circumstances. So the oft-cited argument that agricultural zones can just shift north is wrong; the soil isn’t going to conveniently shift a few hundred miles.

  38. “Water exports from water-rich areas?

    Water wars?”

    Well, there’s been a slight “war of words” about that on RC, with one poster advocating persistently for a vast new forest to be created in the American West, to draw down carbon; water would come from Canada. Many don’t think it would be quite so easy as all that, somehow.

    But there’s certainly the possibility of major irritation in the US-Canada relationship, as Southwestern US cities have real water problems while Canadian lakes store really, really massive amounts of fresh water perceived by some as “surplus.” Never mind that, like the Southwestern plains and deserts, they are ecosystems of their own, adapted quite exquisitely to what is there now (and still recovering from the clearcutting of surrounding forests on massive scales, for that matter.)

    • I have a hunch you may have already read it, but “Cadillac Desert” by Marc Reisner is a good overview on water and its impact on western (North American) civilizations. Even though it was published 25 years ago, the issues really haven’t changed much in that time. The book has some interesting stories about some of the plans the U.S. Army Corps of Engineers for Canadian water.

  39. My brother is a famous Canadian, so you’ve probably never heard of him. He’s in the Canadian Baseball Hall of Fame. Yeah, they have one.

    But anyway, he says the USA can have some Canadian water about the same time the USA apologizes for lying about the Canadian healthcare system and elects to use one just like it (he’s an employer and he likes their healthcare system.)

  40. Andrew Dodds,
    More drought in a wetter world is exactly what we can expect. Rainfall will tend to be concentrated in extreme events as we have just seen in Pakistan.

    In my little corner of the world those who rely upon rainwater tanks would think a 100,000 liter tank the bare minimum. In another state 10,000 liter tanks are common and 30,000 liters are considered excessive, yet their total rainfall is less. Less rain, but more consistent. Right now it is a fairly safe bet that the other states 10,000 liter tanks have more water in them.

    • Well, I live in the south west UK, which probably counts as the part of the world least likely to be negatively impacted by AGW – if we ever need to store our own water it probably means the end of the world for the rest of the planet..