Introduction to Climate Change

No, this post is not an introduction to climate change. It’s a question from Alex Hoffnung, who recently got his Ph.D. from U.C. Riverside after working with me on categorified Hecke algebras. Now he’s headed for a postdoc at the University of Ottawa. He’s a cool dude:

And he has a question that I’m sure many mathematicians and other scientists share, so I’ll make it a guest post here:

Have you come across anything like “Intro to Climate Change”? The big problem is that I have following the issues surrounding climate change are getting a handle on what the issues are. How hard is it to objectively state some of the more important foundational issues without running into controversy?

42 Responses to Introduction to Climate Change

  1. John Baez says:

    Alex wrote:

    Have you come across anything like “Intro to Climate Change”?

    I’m sure many people will point you to many sources online, but I’ll be glad to buy you this book…

    • John Houghton, Global Warming: The Complete Briefing, Fourth Edition, Cambridge U. Press, 2009.

    … if you tell me what address to have it shipped to. I guess your UCR math department address will still work?

    How hard is it to objectively state some of the more important foundational issues without running into controversy?

    I don’t think there’s anything you can say about climate change without someone calling you a liar or a fool. So, for example, this book has widely divergent reviews:

    • “…the best single-volume guide to the science of climate change.” Times Literary Supplement

    • “…a widely praised book on global warming and its consequences.” The Economist

    • “…this book is not science. It is a “briefing” in the sense that lawyers use the word “briefs.” It is a sustained argument for one point of view. It never takes the other side seriously. It is an advocacy piece.” Richard Gibson on Amazon

    • “… horsemanure.” Anonymous customer on Amazon

    Ultimately you need to make up your own mind. But this is a readable book, and it’ll help you get started on that process.

  2. Steve Bloom says:

    There are lots. The question is at what level? The IPCC AR4 Synthesis SPM is one, the Copenhagen Diagnosis is another, the EPA justification for regulating CO2 is yet another, the AR4 technical summaries are another, there are graduate textbooks on the subject (inc. a new one by Ray Pierrehumbert), and there are a raft of simple ones from short to book-length.

    For myself, I like the planetary big picture, which was laid out for the first time a few years ago in Hansen et al.‘s Target CO2 paper. As a follow-up to that Richard Alley’s Bjerknes Lecture at the last AGU fall meeting is good. These are basically focused on the fate of the ice sheets, which it has been realized only very recently are endangered by present CO2 levels.

    Then there’s ocean acidification, for which the EPA’s recent document starting their regulatory process is good.

  3. Steve Bloom says:

    Aha, I didn’t even know Houghton’s book had a new edition. I’ll be curious to hear what you think of it.

  4. Hybrid Moiety says:

    Our club has produced a flier on global warming and green energy. It will be used in our tabling events locally.

  5. John Sidles says:

    For history-minded folks there’s the AIP website The Discovery of Global Warming.

    If you know the history, then it’s pretty easy to fill in the basic physics. But the converse is not true … that’s why it’s a good idea to learn the history first.

    • Joe Kaplinsky says:

      I would strongly second Spencer Weart’s Discovery of Global Warming, linked above.

      If you want to study the science of climate in a bit more depth the problem is that a lot of work aimed at policy makers, e.g. the Copenhagen diagnosis and IPCC report summaries (I haven’t read Houghton but skimming it, it looks similar), tends to just present conclusions. The research literature, of course, takes for granted the important established results.

      The history helps show how and why problems arose and then how they were (or still remain to be) resolved. At least for me, it helps give the big picture.

      Another brief overview of the historical development is chapter 1 of the working group 1 fourth IPCC assessment.

      The present uncertainties and research priorities are a different (although related) question. Here is a paper which surveys experts on the most important uncertainties for future air temperatures. Number one is clouds, which I think few would dispute. Then come ice, water vapour, ocean processes, the carbon cycle and so on.

  6. Nullius in Verba says:

    I’d say that there was a significant problem with trying to find a single document summary, which is that you end up with only a single person’s opinion; a single viewpoint.

    (Or at most, that of a small number of people.)

    So I’d be inclined to do it as a list of search terms and relevant terminology, that can be used to explore the space more rapidly. For “climate change” I would suggest Milankovitch cycles, Eemian climate peak, carboniferous climate, Bond events, Dansgaard-Oeschger events, interstadials, Holocene optimum, meltwater pulse 1a, medieval warm period, little ice age, Mayan civilisation collapse, Sahara climate history, Atlantic Multidecadal Oscillation, Pacific Decadal Oscillation, and so on. I’m sure others here could add some further useful terms.

    The aim is to be a help to the lost – but without guiding people down a specific path, and allowing a lot of exploration, a wider range of viewpoints, and an active engagement of their critical faculties.

    • John Baez says:

      Nullius wrote:

      I’d say that there was a significant problem with trying to find a single document summary, which is that you end up with only a single person’s opinion; a single viewpoint.

      True. And your list of buzzwords is quite nice! On the other hand, someone needs to be quite energetic and committed to run around and learn about Milankovitch cycles, the Eemian climate peak, Bond events, Dansgaard-Oeschger events, interstadials, and so on, without any rough preliminary framework to put all that knowledge. So it’s good start with a well-written book, even if after more study you wind up disagreeing with some of what’s in that book. And I think Houghton’s Global Warming: The Complete Briefing is a reasonable starting-point, though there are probably others, and maybe better ones.

      By the way: when Alex said he wanted to learn about “the issues surrounding climate change”, he probably meant it pretty broadly. In addition to climate physics and paleoclimatology, he might also like to know about different energy technologies, their possible impact on carbon dioxide emissions, the whole issue of modelling climate change and the amount of uncertainty in these models, and so on. And there’s a bit of all this in Houghton’s book.

      However, Houghton’s book definitely does take a specific viewpoint… so then Alex should try something else! Maybe something by a contrarian, for example.

      • David Corfield says:

        I wonder if there exists an introductory account to the ‘are we really sure we know?’-position. I guess that might not appeal to an editor commissioning work in popular science.

        The gentlest brief ‘don’t forget the sun’-account I know is Nir Shaviv’s There is nothing new under the Sun.

        • Florifulgurator says:

          David, isn’t that a little masochistic approach to learning something new?

          One could as well produce a list of scientists believing in young earth creationism (e.g. I know of an elderly game theorist who communicated a mathematical proof of it to Manfred Eigen). Would that be a good approach to learn about paleontology?

          Shaviv basicly denies the greenhouse effect (extremely doubtful) and sees cosmic rays as the cause of warming. Check’s A Galactic glitch and then justify why Shaviv is still worth considering.

          I’m confident any other scientist on your list can be disregarded as easily. But I promise John to not continue this game…

        • Steve Bloom says:

          Where do Shaviv’s ideas stand relative to the literature, David?

        • Nullius in Verba says:


          Young Earth creationism is an analogy with a lot of baggage. I always look forward to seeing it, because to a scientist it shouts “rhetoric” more effectively than anything I could say in response.

          Scientific history is full of examples in both directions: the old Earth theory was itself once the controversial outsider that overturned the young Earth consensus. (As with Lord Kelvin’s comments on the age of the Earth.) There was the Establishment opposition to black holes. There was the belief that the disease pellagra was caused by germs. Or that, when placed in a freezer, cold water always freezes faster than hot. And of course countless challengers turned out to be wrong too. Any scientist will be able to add examples to that list indefinitely.

          Science succeeds by seeking out the best arguments *against* a hypothesis, and checking that they all fail. I find that scientific understanding can be acquired most quickly by taking the same approach. Thus, the only reason for *not* looking at Nir Shaviv is if you think you know of a better argument against the hypothesis elsewhere.

          This example emphasises my point above about single points of view. Shaviv’s article lists about a dozen arguments and points, several of them acknowledged to be quite speculative or uncertain. The RealClimate page responds to one of them with their own view, which disagrees. If you only looked at the RealClimate page, you might not be aware that it only partially addressed a couple of the points made. If you only looked at Shaviv’s page, you might be unaware that there were any arguments against. (Although that seems unlikely!) By reading both, you would learn that meteorite analysis apparently showed a history of variations in cosmic radiation, but that these might not be the result of passing through the galactic arms as Shaviv indicated.

          Plus of course it’s just as possible for RealClimate to be wrong, too. It isn’t safe to take the existence of a rebuttal on RealClimate as the final word on the subject, either.

          If you want to know which bits are controversial and which not, you have to read more widely than a single source, and you have to actively engage your critical faculties, even (some would say especially!) on subjects thought to be well established.

          I would certainly agree that it is helpful to have read some basic meteorology or atmospheric physics texts first to be familiar with the concepts and terminology. But that is not a quick study.

          As Tom Wigley once said with regard to climate science: “No scientist who wishes to maintain respect in the community should ever endorse any statement unless they have examined the issue fully themselves.”

          Wise advice, I’d say.

        • John Baez says:

          Nice reply, Nullius!

          I hope you and David Corfield talk a lot here on this blog, because you seem a bit less eager to “shoot down” his questions than some here, and perhaps a bit more interested in actually engaging with them — and that would be interesting, because David, as far as I can tell, is actually interested in talking about ideas, rather than pushing an agenda.

        • Florifulgurator says:

          I guess I know pretty well what is science and what is rhetorics. I know referring to creationism sounds a bit rhetoric… First I tried giving an example from mathematics: Learning number theory from studying pre-Wiles Fermatism. (But then, maths is perhaps a bit different business. Still, the working mathematician has to take the word of others when it comes to large theorems. E.g. many use Atiyah-Singer index stuff without having checked every detail of proof(s) before – those will one day sink in by osmosis. Likewise, I trust the Poincare conjecture is true, the four color theorem is true, etc. – even while I never ever will check the proofs.)

          OK, so back to Shaviv. (Note that I haven’t bothered giving this more than half an hour of thought, for I think it’s a waste of time. My arguments should be checked independently.) Shaviv is actually an excellent example of how to proceed for the amateur climatologist:

          1) The greenhouse effect is undisputable. But Shaviv states here,

          In summary, there is no direct evidence showing that CO2 caused the 20th century warming, or as a matter of fact, any warming.

          2) Climate models can reasonably reconstruct historic temperature (cf. e.g. the figure in Kerry Emanuel’s essay) – even while clouds are a major factor of uncertainty (cf. op. cit.). So, it is highly improbable that cosmic rays (impacting cloud nucleation) are a significant climate forcing. Anyhow, the cited RealClimate article states,

          it seems that Shaviv’s “periodicity” estimate for crossing of spiral arms by the sun does not hold up under scrutiny when using current astronomical results as the work by Kranz et al. This comes in addition to the previously shown fact that the correlation of cosmic ray flux with paleoclimatic data proposed by Shaviv and Veizer (2003) only arises “by making several arbitrary adjustments to the cosmic ray data” (Rahmstorf et al. 2004).

          3) The Sun’s influence on climate is well-known by mainstream climatology, so it is not to be expected Shaviv has anything significant and substantiable to add (otherwise it would have been accepted by mainstream wisdom).

          Conclusion: Studying Shaviv in detail would be quite a distraction from learning the real stuff. You can safely disregard him – except you actually don’t want to learn about climatology.

          (P.S.: And yes, I know this is heuristics. But pragmatico-necessarily so. I will now take my conclusion seriously and say no more.)

        • David Corfield says:

          I’d like to know the range of proposed mechanisms for solar climatic influence. Is the decreased solar activity-more galactic cosmic rays-more low altitude clouds mechanism the best supported? The CLOUD experiment must presumably have been well regarded for CERN to have accepted it. Jasper Kirkby gives a lecture on the experiment.

        • Steve Bloom says:

          I’m a little less impressed by NiV’s reply, John, probably because I’ve seen variations on that theme so many times. It doesn’t seem to me to have much content.

          In any case I agree that examination of some actual ideas would be a good thing. Maybe a thread on Shaviv, if David agrees with NiV that it’s defensible material, or if not something else he can point to?

        • Steve Bloom says:

          Thank you, Florifulgator. There’s a lot more that could be said, but what you said is sufficient. I hadn’t spotted the “any warming” passage on my quick read-through; that’s pretty funny.

          Re the influences, David, it sounds as if you’ve already looked into this. If you’d like to discuss them, probably you should lay them out for others to comment on.

          Re the CLOUD experiment, it’s a reasonable effort to get a handle on a plausible minor effect. Proposals that it’s a significant effect fail trivially on paleoclimate grounds. I’m sure you’ve seen that material as well. Why are you so interested in such a thoroughly rejected set of ideas?

        • Nullius in Verba says:


          I quite agree that as a practical matter, a scientist must take a lot on trust. But such trust, while fully justified on a practical level, is not itself scientific. If challenged, it needs to be checked. And anything should be open to a plausible challenge.

          The four colour theorem is a case in point. Kempe proved it in 1879, and I’m sure that for a lot of mathematicians that was good enough. But in 1890 Heawood challenged it, and mathematicians on hearing of it would have checked, and then reset it back to ‘conjecture’. Note, Heawood didn’t prove the four-colour theorem wrong, he only challenged whether we had the evidence to prove it.

          On Shaviv, I’m not sure if this is the appropriate place to be arguing this. If John wants us to stop, I’m happy to do so.

          On point 1, I agree that the greenhouse effect is indisputable, but I also agree (subject to definitions and caveats) with Shaviv’s statement. They are talking about two different things.

          I am in no doubt that there is a greenhouse effect, and that increasing CO2 will tend to increase temperature. But we cannot deduce from that that an observed increase in temperature is therefore due to CO2. There are *lots* of things that can change temperature, including I am sure many that are as yet unknown. The whole process is non-linear with many feedbacks and time delays, so you can’t even say it has made a particular additive contribution.

          I have seen Shaviv elsewhere say that the solar correlation only explains about half of the observed modern change (IIRC), and I’d be confident that he’d agree that CO2 probably caused a part of the remainder.

          On 2, climate models can reconstruct the global average, but they’re not so accurate on the details. The variances are wrong, the response to volcanic eruptions too large, and they come up with a different answer for the upper tropical troposphere trends – although the enormous uncertainties in the observations mean the significance of the difference is borderline.

          The problem with models that have adjustable parameters is that they are capable of ‘curve-fitting’. Take any set of linearly independent functions (with generally similar smoothness characteristics to your target) as a basis, and you can approximate an arbitrary function with a weighted sum of them. Climate models are more complicated than that, of course, but the same principle applies. John von Neumann said “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”

          The global mean temperature anomaly is a one-dimensional summary for the entire planetary climate system. Fitting that is no challenge. Several sceptics have done so with linear combinations of PDO, AMO, and ENSO. I agree that climate models do a better job over a wider range of variables, but it’s not a perfect job, and I consider there to be a significant risk that the fit is partially due to a more subtle curve-fitting.

          I take your point that they did make a reference to the cosmic ray data. I’m not going to chase the reference right now, but I’ll bear it in mind.

          On point 3, I have seen mainstream climatologists discuss variations in the *thermal* contribution of the sun to climate, but that does not address the possibility of non-thermal influences.

          The argument that if there was anything to it the mainstream wisdom would already have accepted it, I’ll put down as one of your ‘heuristics’. :-) I ‘ll say no more about it.

          Commenting on the cosmic ray hypothesis generally, my view is that it is only another competing hypothesis, with even less confirmation so far than the AGW hypothesis. It’s based mainly on correlations (which do not imply causation) and some plausible but as yet unconfirmed possible mechanisms, and little in the way of experimental back up. But I think that’s mainly because it’s very new, and not a lot of people are working on it.

          But I do think that as a hypothesis it is very much still in play, and that as people challenge it, it will continue to improve.

          Challenging it on a technical level is healthy, and I’ve no objection to people not agreeing with it. I was only disagreeing with the idea that it wasn’t worth a newcomer to the subject *even looking at it*, or taking it seriously. More, that a deliberate effort should be made to steer them away from it, and towards only looking at ‘approved’ science. That doesn’t sound right to me.

          Anyway, I think I’ve made my point. I’ll leave any further discussion to another thread.

        • Tim van Beek says:

          Thanks for the reference to Houghton’s book, I took a look at it.

          I wonder if there exists an introductory account to the ‘are we really sure we know?’-position.

          That was one of my thoughts, too, after reading the introductory chapter. Starting with a list of extraordinary weather events may be a good way to get the attention of the audience, but to me it just sounds like propaganda, and I don’t like that. I would like to have a book with more technical details, too.

        • Florifulgurator says:

          David asks:

          I’d like to know the range of proposed mechanisms for solar climatic influence.

          M. Lockwood, C. Fröhlich: Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature, Proc. R. Soc. A 8 October 2007 vol. 463 no. 2086 2447-2460:

          Abstract: There is considerable evidence for solar influence on the Earth’s pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.

          See Figure 1. there.

          Which leads me back to the thread’s main topic:

          There’s a very entertaining source for learning about the science as well as the denial: The video collection Climate Denial Crock of the Week.

          Part of said Fig. 1 appears in Solar Schmolar.

        • Joe Kaplinsky says:

          Certainly the sun has a big impact on earth’s climate which can be seen in a variety of places and these are worthwhile debates. It just seems to me a real stretch to claim they are the main event. even basic features like the day/night warming pattern tend to go against it.

          I won’t repeat points that have already been made, but to add a few:

          Specifically on Shaviv’s work on cosmic rays which he cites in part III, so far as I know he hasn’t replied to the criticism in this work.

          On cosmic rays more generally, they may well have some effect but it looks to me to be small. Reading Svensmark’s book The Chilling Stars what was striking was how he claimed that because his cosmic ray mechanism was important and no one else can explain climate, conventional climate science must be wrong but without explaining how. It also relies quite heavily on the poor understanding of clouds (no one knows, so it must be cosmic rays…). But while clouds remain mysterious certainly much more has been learnt known about their nucleation in the last few years (dust, acid, bacteria, lead, etc.) That in itself puts pressure on the cosmic ray argument.

          It seems to me that while climate science is very much a work in progress, unless the solar and cosmic ray people can show where it has gone wrong I think they will be better off trying to fit in somewhere in that picture rather than replace it.

          Skeptics first pointed to changes in Total Solar Irradiance as a cause of warming, which was not implausible. Satellite observations date from 1978, and you need a couple of 11 year cycles to begin to draw conclusions. It now looks like changes in TSI cannot account for warming. (People have tried to derive climate sensitivity from what changes are observed and conclude it lies in the range 2.3-4.1K per CO2 doubling.)

          But most of the variation in TSI is in the UV. Fred Singer, for example, has claimed that this could be important. But again the observations seem to go against it. (e.g. here – I think there is more on this I’m not familiar with.)

      • John Baez says:

        Nullius wrote:

        On Shaviv, I’m not sure if this is the appropriate place to be arguing this. If John wants us to stop, I’m happy to do so.

        Thanks! Since I’m too busy moving into my new apartment and writing the last issue of the old This Week’s Finds to properly kick off a thread on Shaviv’s ideas, or more generally issues of solar variability and the like, I don’t mind if you guys discuss them in this little subthread.

        As always, please keep the discussion fact-rich, rhetoric-poor, and scrupulously polite.

      • David Corfield says:

        Naturally, there were responses to the Lockwood and Fröhlich paper. For example, Svensmark and Friis-Christensen, wrote Reply to Lockwood and Fröhlich – The persistent role of the Sun in climate forcing, and Shaviv’s reply is here. No doubt there were replies to the replies.

        I see Lockwood argues for regional change due to solar activity. We Europeans may be in for some chilly winters, he thinks. The full article is here.

        Is there anywhere predicted to have warmer winters through a Maunder-like minimum?

        • Steve Bloom says:

          But were there any replies to begin with (in the usual sense of the term)? Neither of those seem to have appeared in the literature. Maybe you could explain why.

  7. Florifulgurator says:

    N.i.V., that’s the reason why I generally trust only the math books written by the Bourbaki group: Most others are opinion pieces by single authors…

    An unimportant search term you forgot is greenhouse effect. To get a grasp on the history of this term I suggest a glance on the 1896 paper by S. Arrhenius, On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground. To get the full picture it then suffices to meditate on a temperature chart covering at least 50 years (not 10, for you would miss the forest). Then follow the news (not Fox News, of course) – This year has quite a lot of climate action happening. It is very important to sort out the climate crackpots, which needs some experience and wits (e.g. “There was no warming since 1998” scores at least 50 points, as obvious after above meditation). If interested in technical details start clicking at

    • Nullius in Verba says:


      The greenhouse effect is a different topic, and I think I’d offer different search terms for that.

      Unfortunately, while Arrhenius was historically important, his understanding of the physics was limited, and the theory has been significantly modified since to account for the effects of convection and feedbacks. (The controversy is mainly about the feedbacks.) I would recommend something more recent: at least post-Manabe and Wetherald. Ramanathan is pretty good.

      How did you determine that 50 years was sufficient? How is that number calculated? Because it would be just as easy to say that you shouldn’t look at the current month, you have to look at a full year so as not to miss the forest, and there isn’t anything in the statement itself that makes it clear why one is correct while the other is not. (If either of them is correct.) Why not 500?

      A post hoc choice of threshold selected specifically to be long enough for a consistent upward trend to be seen is obviously scientifically unsatisfactory. You need to identify an interval based on general considerations and only then look at the data. Time series analysis already has a range of standard tests and approaches for doing this, depending on the autocorrelation structure of the data.

      Whether there was “warming” since any particular date depends on which aspect of temperature you are looking at (e.g. are you looking at temperatures or temperature anomalies, how high above the ground, adjusted or unadjusted for ENSO/vulcanism/air conditioners/barbecues/etc.), your statistical model of the signal and noise, and your choice of significance level, and is a far from simple question.

      And I wouldn’t start with RealClimate – although obviously it’s relevant to any study of the subject at some point. A lot of what they say can be easily misinterpreted if you don’t know the background to the topic fairly well. It is very much a case of a single viewpoint. :-)

  8. Florifulgurator says:

    Oops, I forgot N.i.V. forgot another perhaps relevant search term: Keeling curve.

  9. Tom Leinster says:

    Oxford University Press’s series of Very Short Introductions is generally pretty good, and they have one along these lines: Global Warming: A Very Short Introduction. I haven’t seen it.

  10. John Baez says:

    I had to delete a bunch of comments here, because the authors were unable to resist:

    1) using the subject of this blog entry as a springboard for unrelated rants,

    2) arguing about partisan politics, and

    3) generally being rude and/or aggressive.

    I won’t allow comments like that. This WordPress blog has an option where if I approve a post, further posts by the same author will be accepted until they do something naughty and I “unapprove” the offending post. But since a bunch of you just took advantage of that option to post political diatribes while I was asleep, I think I’ll change settings so that I need to approve every post.

    Too bad: more work for me, and more delay for you. But there are already lots of blogs where you can get your daily adrenaline fix by stomping on your political opponents. The world doesn’t need another of those.

    To help everyone understand the rules, here are some examples of comments that won’t make it onto this blog, especially if you’re replying to the question what is a good introduction to climate change?

    “At some point since 1955, American conservatism became stupid…”

    “A lot of the non-scientists who’ve hopped in the AGW/Green bandwagon have come from the old Left of politics.”

    “This posting is the kind of thing you see at Tea Party events, vacuous statements made without recourse to facts or proof, just rhetoric and argumentation and arm waving.”

    Note that besides making people angry and eager to fight, none of these comments actually address the question at hand!

    You’re all my friends… I’m hoping you can help me do something productive here, not beat each other up.

  11. Nathan Urban says:


    I haven’t read Houghton’s book, but I’ve heard it’s good. An even shorter intro is David Archer’s Global Warming: Understanding the Forecast, and even shorter than that is Kerry Emanuel’s What We Know About Climate Change (summarized in his Boston Review essay).

    As for what the main issues are, here are some points from what I would consider the “mainstream” view, on what’s important and what conclusions we have more/less confidence in:

    1. The enhanced greenhouse effect of CO2 is uncontroversial.
    2. The modern temperature history of the Earth is uncontroversial.
    3. There is a fair bit of uncertainty about how Earth system feedbacks modify the CO2 greenhouse effect — the general view is that the “climate sensitivity” is somewhere between 2 and 4.5 degrees C of warming per doubling of CO2. This is the largest overall climate uncertainty.
    4. Much of the uncertainty in climate sensitivity, as well as uncertainty in decadal scale natural variability, comes from limitations in our understanding of clouds.
    5. There is some uncertainty about how much natural atmosphere-ocean variation has contributed to decadal climate trends, but CO2 is agreed to be a large part of the long-term (50+ year) temperature trend.
    6. Projections about global temperature (conditional on a given emissions scenario) are fairly uncontroversial, within the range of uncertainty about climate sensitivity mentioned above.
    7. Projections about global precipitation are more controversial, but there are some general conclusions about how it will change in wide regions such as tropics vs. mid-latitudes.
    8. Regional climate projections (say, country-scale and below) are much more uncertain, especially with respect to precipitation. It is unknown how reliable these projections may be.
    9. Even more so for regional ecosystem impacts.
    10. There is deep uncertainty about ice sheet stability, which greatly influences sea level rise projections. However, total ice sheet disintegration is probably not possible within the span of a couple centuries. It is possible that a threshold for disintegration could be passed sooner than that. (i.e., it won’t happen right away, but it could be “triggered”).
    11. There is also a fair amount of uncertainty about the stability of permafrost carbon, but this too is something more likely for future centuries.
    12. I’m skipping over the non-scientific climate change uncertainties like our future emissions trajectory, economics, politics, etc.

    • Steve Bloom says:

      A couple of points, Nathan:

      The best guide to sensitivity is the glacial cycles. Estimates from those are in the vicinity of, you guessed it, 3C. The nice thing about this approach is that it necessarily accounts for whatever it is the clouds are doing. Note that we’re discussing the Charney sensitivity, essentially the short-term result of doubling CO2 (from 280 ppm pre-industrial) but without carbon and ice-albedo feedbacks. The true equilibrium sensitivity is known from studies of the mid-Pliocene to be about 3C for our current level of CO2 (390 ppm). Transient sensitivity can be even higher if the transition is fast enough (paper).

      The scientists studying the permafrost have recently become unconvinced that major near-term melt is unlikely. I think “quite worried” is a good description of their present state (see here e.g.).

      Similarly, major short-term losses of the GIS and WAIS can no longer be excluded.

      • Nathan Urban says:

        I’m not convinced that the glacial-interglacial cycle is necessarily the best guide to climate sensitivity, although it is a strong constraint. Well, even if it is the best guide, there are still problems with the range of sensitivities so derived.

        First, there are still instances in the glacial-interglacial cycle where the climate doesn’t completely match up with the Milankovitch forcing. This suggests we may not understand the natural modes of variability completely. Therefore it can be hard to disentangle how much of the response is feedback to forcing, and how much may be complex Earth system variability which may not be easily interpretable as a “feedback” to the Milankovitch forcing.

        There are some who think that there can be periodic variability in continental ice sheets even without Milankovitch forcing, and the orbital cycles merely synchronize the phase of the cycle rather than cause the entirety of the response.

        Second, climate sensitivity can be state- and forcing-dependent. (For example, consider the strength of the ice albedo feedback in a world with ice, compared to one without.) You allude to this in your mention of Charney sensitivity. The Pleistocene record contains only brief examples of what happens to the climate when forcing is applied in warm periods (interglacials), and none of them reach temperatures as warm as it is possible for us to reach in the future anthropogenically.

        Finally, there are some discrepancies in which sensitivities are favored by the glacial-interglacial data, depending on the data. Last year I saw a poster by Tamsin Edwards from the UK PalaeoQUMP project which indicated that temperature proxy data and precipitation proxy data favored different sensitivities when compared to an Earth system model’s predictions. Of the two, I’d trust temperature more (both in the proxy data and in the modeling), but it’s still a cause for some concern.

        I think the glacial-interglacial cycle gives a pretty good first order approximation of climate sensitivity, but the error bars may be overconfident.

        As for ice sheets:

        Significant sea level rise from ice sheets on the short term (meaning, within this century) are possible. Pfeffer gives an upper bound of 2 meters/century with 0.8 being are more plausible bound. We don’t know yet how close to the bound we may approach.

        But I was talking about losing the entire ice sheet. That can’t happen this century, and likely not within the next few. See, e.g., Pollard and DeConto with respect to WAIS. Pollard gets a WAIS disintegration time of 1000-2000 years, IIRC.


        With respect to permafrost, my understanding is that we’re not likely to see a huge carbon response this century. This is based on talking to folks like Kevin Schaefer and Oleg Anisimov. I think Schaefer gets something like 30-50 ppm over the next century (or two)? Anisimov is more worried about permafrost thaw impacts on civil engineering (unstable soil).

        However, Schaefer’s work suggests that some permafrost regions could pass a melting threshold within this century (i.e., the whole soil column will eventually thaw unless temperatures are lowered). We should certainly be worried about crossing thresholds even if we don’t see the full response immediately, because it means that we’ve committed future generations to a lot of problems. (Ditto for ice sheet disintegration.)

    • Steve Bloom says:

      Oops, I just looked at your website, Nathan, and see that you’re just a little more qualified than I am (me = not at all, other than having followed the science closely for some years). I don’t think I over-stated anything in terms of what I’ve heard from some other scientists, but please excuse the lecturing tone.

      • Nathan Urban says:

        No need to worry about tone. I’m fairly familiar with the climate sensitivity literature, but I’m not as expert on Plio-Pleisotcene paleoclimate, and I’m only beginning to get into ice sheets and permafrost, so I could well be off there.

    • John Baez says:

      Hi, Nathan! Thanks for the rundown! I hope to be seeing more of you here! We haven’t talked much in public since the good old days of sci.physics.research, back when we were both interested in quantum gravity. I’m sort of nostalgic for those days — the days before I started working on n-categories and made myself incomprehensible to all but a few specialists. Maybe Azimuth can recapture some of the charm of that era. Having you around is certainly a start!

      • Nathan Urban says:

        Hi John. I have your blog in my RSS feed now. (I didn’t notice it before; I’ve been busy with my recent move.) I’m looking forward to seeing what you do with it. And sorry about drifting away over the years … but as you allude, I had trouble following the n-Category Cafe once I stopped thinking about quantum gravity and mathematical physics full time. Through some quirk of fate we now appear to be reconverging on a new set of problems to discuss. It should be fun.

    • Hi Nathan,

      Thanks for the breakdown. This list is full of ideas I have never even thought about before, but also not too overwhelming. I think it could really be quite helpful for me.

      I am also in the middle of a move, so I will take a closer look at some of the topics you mention when things settle down a bit.


      • bane says:

        Since you mentioned this is stuff you haven’t thought about before, I’ll just mention something that’s not strictly climate change but has the potential to have a large effect on ecosystems: ocean acidification. Here’s a recent paper on the topic:

        Since this appears to be another consequence of atmospheric carbon dioxide levels, it naturally connects with other reasons why (some people are of the opinion) something needs to be done about carbon dioxide levels. (Here’s the abstract:

        Increasing concentrations of carbon dioxide in sea water are driving a progressive acidification of the ocean. Although the associated changes in the carbonate chemistry of surface and deep waters may adversely affect marine calcifying organisms current experiments do not always produce consistent results for a given species. Ocean sediments record past biological responses to transient greenhouse warming and ocean acidification. During the Palaeocene–Eocene thermal maximum, for example, the biodiversity of benthic calcifying organisms decreased markedly6, 7, whereas extinctions of surface dwellers were very limited8, 9. Here we use the Earth system model GENIE-1 to simulate and compare directly past and present environmental changes in the marine realm. In our simulation of future ocean conditions, we find an undersaturation with respect to carbonate in the deep ocean that exceeds that experienced during the Palaeocene–Eocene thermal maximum and could endanger calcifying organisms. Furthermore, our simulations show higher rates of environmental change at the surface for the future than the Palaeocene–Eocene thermal maximum, which could potentially challenge the ability of plankton to adapt.


        • John Baez says:

          I’m definitely going to say a lot about ocean acidification and other threats to the ocean (like overfishing, dead zones and the decline of phytoplankton) in the new This Week’s Finds. Hence the penultimate line in week300.

          (By the way: a satellite has spotted a 377,000 km2 algal bloom in the Baltic Sea — that’s about the size of Germany, but apparently some such bloom has occurred every summer in recent history; I don’t know if it’s getting bigger.)

  12. Hank Roberts says:

    One good place to begin–pictures with citations to the studies, here:

    Take any question or search string in this area, and put it into first Google, then Google Scholar, then Google Image Search: you’ll get rather amazingly different results, often revealing spin, by making the comparison. Scholar isn’t nearly as good as search by a real librarian with academic tools, but it’s the best most citizens have. Gotta watch its results though.

    Don’t trust any site claiming to describe a published paper, even if they use what appear to be direct quotes, even if they claim to be a science site. Some are; many are spinning PR.

  13. westy31 says:

    Some time ago, I decided to write my own introduction.

    After “climate gate”, I wanted to check a couple of things for myself. The most simple model I could think of predicts a couple of degrees of global warming due to human CO2 emissions.

    I also tried to do some estimates of how you could replace fossil fuels.


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