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Infographic: 97 out of 100 climate experts think humans are causing global warming

Posted on 11 May 2011 by John Cook

I was talking about climate to my dad last week (since the book launch, he will now talk to me about the subject) and I mentioned that 97% of climate scientists are convinced that humans are causing global warming. He registered great surprise at that statistic. "I thought it was more 50/50", he said. It made me realise just how good a job both the mainstream media and the fossil fuel funded disinformation campaign have done in confusing the public about the scientific consensus on global warming. At the same time, I was working on a consensus graphic (cribbed from the Guide to Skepticism) for a video presentation. So as a tool for anyone wishing to communicate the scientific consensus, I've added the following infographic to the Climate Graphics resource:

The 97% figure comes from two independent studies, each employing different methodologies. One study surveyed all climate scientists who have publicly signed declarations supporting or rejecting the consensus (Anderegg 2010). Another study directly asked earth scientists the following question "Do you think human activity is a significant contributing factor in changing mean global temperatures?" They found 97% of actively publishing climate scientists answered yes (Doran 2009). As "climate scientists actively publishing peer-reviewed research on climate change" doesn't really roll off the tongue, I abbreviated that down to "climate experts".

One feature of Doran's survey results is that while 97% of climate expert said "yes, humans are causing global warming", only 1% said "no, we're not". The other 2% were unsure:


Response to the survey question "Do you think human activity is a significant contributing factor in changing mean global temperatures?" (Doran 2009)

I've indicated the "I'm not sure" portion in the "97 out of 100 climate experts" infographic with grey colouring.

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Comments 151 to 179 out of 179:

  1. Harry Seaward,

    Perhaps getting back on topic, about the surveys, any thoughts on my comment 102?
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  2. 147, Harry Seaward,
    Thanks for the link, and, again, you could have left out the words within the parantheses.
    I apologize for letting my ire slip out, especially since you are not entirely at fault. In the past month a large number of "deniers" have inundated the comments with a fair amount of sarcasm, ignorance, and a lecturing tone, supported either by a gish-gallop of illogical facts, or by nothing whatsoever -- often, their tone and sarcastic, backhanded implications are all they have going for them. After a while, one loses patience, sometimes at the wrong moment.
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  3. 147, Harry Seaward,

    FYI, this page is where you want to start on Spencer Weart's site.
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  4. Harry Seaward @145, as is fairly clear from reading it, the IPCC reports make 'projections' (in its terminology) not predictions. The difference is largely semantic in that a projection is a prediction conditional on certain circumstances obtaining. The IPCC AR4 mean global temperature projections are:

    " The multi-model mean SAT warming and associated uncertainty ranges for 2090 to 2099 relative to 1980 to 1999 are B1: +1.8°C (1.1°C to 2.9°C), B2: +2.4°C (1.4°C to 3.8°C), A1B: +2.8°C (1.7°C to 4.4°C), A1T: 2.4°C (1.4°C to 3.8°C), A2: +3.4°C (2.0°C to 5.4°C) and A1FI: +4.0°C (2.4°C to 6.4°C).


    Reducing that to a single number without confidence intervals, and without reference to particular scenarios over simplifies to the point of misrepresentation.

    @146 Sphaerica referred to the fast feedback climate sensitivity which is approximately 3 degrees C per doubling of CO2. That is a prediction of the theory, not a projection. It can be and has been applied to past climate changes as a test of the theory, assuming you have a sufficiently strong paleo signal to make the test.

    @147 If you are interested in the history of the green house gas theory, Svante Arrhenius'1896 paper "On the influence of Carbonic Acid in the Air on the Temperature of the Ground" is the first attempt to calculate the effect of CO2 as a green house gas. Of particular interest are his predictions that:

    1) Greenhouse warming would be stronger towards the poles than the equator; and

    1a) The polar warming would be accentuated by the melting of arctic ice.

    2) The warming would be stronger at night than during the day, resulting in a narrowing of the diurnal temperature range.

    3) The warming would be stronger in winter than in summer.

    4) The warming would be stronger on land than at sea.

    5) The warming would be stronger in the Northern Hemisphere than in the Southern.

    (See page 265 for the exact details of these predictions.)

    To that we can add the prediction that:

    6) With increased CO2 content, the troposphere and surface will warm, but the stratosphere will cool.

    Arrhenius, of course, did not make that particular prediction because he did not know of the internal structure of the stratosphere, particularly the presence of ozone.

    These six predictions are unique to green house theory in that no other method of warming the Earth predicts all six of them. All six predictions are plainly falsifiable, but all six predictions have, of course, been verified. Five of those predictions where made 115 years ago, but still we get deniers accusing AGW theory of being without falsifiable predictions.
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  5. Sphaerica @ 150
    Your second paragraph demonstrates you have absolutely no understanding of the use of proper scientific terms. Even the IPCC calls it a projection.

    Directly from the IPCC AR4 Synthesis SYR Report:
    "3.2 Projections of future changes in climate <>For the next two decades a warming of about 0.2°C per decade is projected for a range of SRES emissions scenarios. Even if the concentrations of all GHGs and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected. Afterwards, temperature projections increasingly depend on specific emissions scenarios (Figure 3.2). {WGI 10.3, 10.7; WGIII 3.2}

    Since the IPCC’s first report in 1990, assessed projections have suggested global averaged temperature increases between about 0.15 and 0.3°C per decade from 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections. {WGI 1.2, 3.2}"

    Now, explain to me how my grasp of the IPCC glossary is weak.
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  6. Harry Seaward @155, instead of lecturing you might actually pay attention. Sphaerica clearly referred to the climate sensitivity for a doubling of CO2. You are referring to the projection of global mean surface temperature for the year 2100 (or more accurately, the average over the years 2090 to 2099). The projection follows more or less straightforwardly from a Business As Usual scenario and the climate sensitivity, but the climate sensitivity is not the projection.
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  7. dhogaza@148
    I mentioned that last line because of the words "substantial uncertainty". Based on the factors mentioned, modeling can give quite different results. The 3C rise should not be thought of or tossed around as an absolute.

    You are, as usual, absolutely correct in your statements.
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  8. Sphaerica at 152,
    Thanks for the apology and I probably owe you one as well.
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  9. Harry Seaward @140

    My point at @139 was that, since RW1 mistakenly calls the 3C rise projection (as I now know to call it :-) ) a theory, he then thinks he can attack climate science as being unscientific erproducing unfalsifiable "theories". In actual fact it is RW1 being unscientific since he chooses to obfuscate a projection and a theory.

    You ask what happens if after a century we get, say, a 6C rise instead of 3C ? Something was wrong with projection ! It behoves us (and our policymakers) to understand what theory and assumptions went into that projection so that we can judge it and do better next time.
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  10. Sorry, slightly mangled 159:

    he then thinks he can attack climate science as being unscientific for producing unfalsifiable "theories".
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  11. 155, Harry Seaward,

    I don't see, from your post, how A follows from B. Perhaps I got lost in what you were saying.

    The IPCC makes a projection (based on emissions scenarios) of a rate of warming for the next two decades.

    Individual scientists, or models, make predictions based on very specific sets of starting parameters, including a specific course of emissions which may or may not ever come to pass.

    The science defines "climate sensitivity" as how much the global average temperature will rise, after all feedbacks come into play, based on an initial (non-feedback) forcing which raises the temperature by some initial value. This is necessary because feedbacks are temperature based.

    Climate sensitivity is often stated in terms of a doubling of CO2, as a convenient metric, but it really applies regardless of the forcing, and for that reason I do wish that climate science adopted a different convention, specifically one based on forced temperature change (e.g. "3˚C total change per 1˚C forced change".

    But, specifically:

    The statement about 3˚C per doubling is a specification of climate sensitivity.

    A prediction about climate is a specific range of events/temperatures that would/should/could/might occur based on a specific set of initial conditions, and a proposed forcing (or set of forcings).

    A projection is a broad approximation of a range of events/temperatures that would/should/could/might occur based not only on the information required for a prediction, but also factors outside of climate science (i.e. changes in global emissions, which are in turn dependent on economic, political and social factors that are far outside of the prevue of either the IPCC or climate scientists in general).
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  12. 157, Harry Seaward,
    Based on the factors mentioned, modeling can give quite different results. The 3C rise should not be thought of or tossed around as an absolute.
    But modeling is not the only source of information or confidence in climate sensitivity. Again, visit this page on climate sensitivity. A large number and variety of paleoclimate studies support the proposed climate sensitivity. A large number and variety of real world observations support the proposed climate sensitivity. A large number and variety of physical mechanisms support the proposed climate sensitivity. And lastly, a large number and variety of models support the proposed climate sensitivity.

    The number of completely different lines of evidence that support the proposed climate sensitivity is becoming overwhelming. The 3˚C rise is not being "tossed around," and the implication that it is being pulled out of a modeling hat is simply incorrect. It's the most important area of climate science right now (in my opinion), and there is a growing body of evidence to support it, and a complete dearth of evidence that would refute it.
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  13. 161 typo alert: I meant "purview", not "prevue" (dang auto-type spell checker!).
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  14. Tom @113 and Harry @118,

    Sorry been away from my computer for a while. Believe it or not, I had originally worded it that way (i.e., find a society that does not agree that human emissions are a major contributor of the warming), but in a moment of generosity I took it out so as to try and afford Harry some more wiggle room.

    A mistake, b/c now Harry is under the misconception that he has scored some kind of goal. Anything but. So Harry, here is the new challenge using Tom's more accurate wording (very similar to the wording provided in the link that I provided by the way):

    "Please list for us all the professional scientific societies of the same standing as the American Meteorological Society (for example) who state that human emissions of GHGs are not the major contributing factor to global warming."

    The list in my post @100 contains numerous agencies who support that understanding.
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  15. Utahn @151,

    Excellent point.
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  16. This is what the US National Academy of Sciences stated in a 2010 report:

    "A strong, credible body of scientific evidence shows that climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems….

    Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of subsequently being found to be wrong is vanishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities."
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  17. This is OT but in response to the aviation thing... The electric vehicle market should help aviation out quite a bit. Currently there are a couple of two seater electrics out that can fly for about 1.5 hrs on a charge. Battery technology is supposed to improve in efficiency by 3-5X this decade. That means the same plane will be able to fly up to 7 hours on a charge. That's longer than most small aircraft even with extended range fuel tanks.

    Electrics also have some huge advantages over ICE engines. Simplicity. You wouldn't believe it if I told you how much it costs to keep up a modern small aircraft engine. There's also the elimination of pressure altitude issues with normally aspirated engines. Service ceiling goes up significantly without adding complexity. The list goes on and on.

    My sense is small personal aircraft will go all electric in the coming decades, while larger corporate aircraft and commercial aircraft will move to algae based bio fuels as that becomes economical.
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  18. "The list goes on and on."

    Designing the powerplant to maintain power while flying upside down is easier, too ...
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  19. The sheer level of maintenance required for a high performance aircraft engine is astonishing. Even on a new aircraft you can spend $10k a year just to keep it airworthy. Add in any AD's (airworthiness directives) and that number goes up. Then the babysitting you have to do en route and in decent adds a ton of complexity to the already complex flight environment.

    If aircraft engines were electric motors... all (or most) of that goes away.

    I know of one incident where the owner of an older twin let his aircraft sit on the ramp too long without operating the aircraft - not long, maybe 6 months - and a small amount of corrosion built up in the cylinders... the aircraft was essentially totaled. An aircraft worth close to half a million dollars... gone. But had those been electric motors...

    Crap. Sorry. This is WAAAAY off topic.
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  20. Well, this is interesting.

    "I’ve read somewhere that about 5% of physicists have serious doubt that we really landed on the Moon. In both cases, science has moved on because the evidence is overwhelming. There was not a magic moment when it occurred, it is different with each person, but without doubt, it has."

    From the American Geophysical Union site.

    "Skeptics" really ought to actually read the first few lines of John's post about his dad to understand why the 97-98% figure is important.
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  21. Rob, I was making a poor joke...you know, me 'Albatross' suggesting 'gliding' :) I guess even winch launches require burning FFs. Thank goodness I'm an Albatross....

    Anyhow.....back to business.
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  22. Ah yes. Albatross.... One of my favs! ;-)
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  23. Thanks for the laugh Rob :) OK, not that Albatross...!
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  24. This graphic reminds me of a way I've been trying to put the "controversy" into context for a while. I usually point at the imbalanced media coverage and say that to really represent the split, they'd have to gather about 34 climate scientists into one room for a discussion, and out of those 34 only one would be unconvinced of anthropogenic climate change. Wouldn't make for much of a debate, would it? Unfortunately it wouldn't make for a "fair and balanced" segment, just an accurate and informative one.

    I've also used the same numbers for a "what if you were diagnosed with cancer..." scenario.
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  25. Harry Seaward #158

    If you haven't already followed Sphaerica #153 to Spencer Wearts history run don't walk to read it. All the major theory and observational history is there. And most of the imprtant names - Arhenius, Hulbert, Callender, Plass, Revell Suess, Bolin. In may ways all the key discoveries and understandings of AGW were in place by the mid 60's. What followed was refinement of the understanding and people starting to get their heads around the idea that this was real and serious.

    Also this article by Ray Pierrehumbert in Physics Today covers a lot of the radiative physics side really really well. http://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf

    Particularly read the section related to figure 3 - the exquisite agreement between theory and observation wrt to the Earths outgoing IR spectrum. The two spikes highlighted are modelled so well because the stratosphere warms as you go higher and this effect is captured by the theory, but only at the frequencies with the highest CO2 absorption. A thing of beauty!
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    Response:

    [DB] Hot-linked URL.

  26. Think of how much money individual scientists could get from vested interests for stating that climate change isn't happening, or, if it is, that it can't be caused by humans.

    Now compare that to the amount of money they could hypothetically get from the government after slavishly writing multiple grants a year and being raked over the coals by reviewers.

    If opinion followed money, the 97:1 consensus for AGW should be in the opposite direction. So I find it amazing how few scientists have been actually been bought off in the end to hold a position they feel is scientifically untenable. It's reassuring actually.
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  27. Stephen Baines

    I think that being bought out requires a very different mindset than the general scientist. I say that as someone whose close relative (sibling) was a mouthpiece for the tobacco industry for denying "second hand smoking" dangers.

    Scientists know that if they state something ridiculous, they will be caught out. Whereas advocates don't care, as long as the discussion/disagreement continues, and no action is taken. The more irrelevant issues, the better, for them!
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  28. Tom Curtis @154

    These six predictions are unique to green house theory in that no other method of warming the Earth predicts all six of them.
    Except 6) these predictions are universal for all warming scenarios regardless of the causes and not limited to the GHG only.

    Simple physics:
    1) Greenhouse warming would be stronger towards the poles than the equator; and
    A logical behaviour because of the different content of water vapor in cold and warm air. A lower water content means a higher rise of temperatures with the same amount of energy.

    1a) The polar warming would be accentuated by the melting of arctic ice.
    This is of course obviously. Besides, a look to ice cores shows that the actual interglacial seems to be abnormal, because the ice shield of e.g. Greenland is much larger as in previous IGs.

    2) The warming would be stronger at night than during the day, resulting in a narrowing of the diurnal temperature range.
    This behaviour also occurs with more clouds. Warmer air will have more water and therefore the cloud cover will rise.

    3) The warming would be stronger in winter than in summer.
    Same reason as with 1).

    4) The warming would be stronger on land than at sea.
    In short time intervals this is true and also a normal behaviour because of the higher heat capacity of water.
    In longer timescales the temperatures of land and sea will adjust. Because of the inertia the sea surface temperatures will be higher if the trend turns to negative.

    5) The warming would be stronger in the Northern Hemisphere than in the Southern.
    This behaviour is also obvious because of the larger landmasses in the North.

    6) With increased CO2 content, the troposphere and surface will warm, but the stratosphere will cool.
    To your prediction 6) it is to say, that the observations (see the last of fig. 7: 'LST') counter it. As you may notice, the main drivers of stratospheric temperature rise are volcanic activities. Since the last major event (Pinatubo), what means since 1994 there is no trend in the LST. The lower stratospheric temperatures remain on a constant level.
    For this, one has to explain whether this prediction is correct at all.

    Regarding the topic one may say that the consensus is based on predictions that are generally valid for all cases of warming. Of course, this makes it easy to confirm.

    But, Arrhenius' theory has a small but important mistake. He did not account for clouds in the right way.
    Correctly he regarded the cloud albedo for the incoming radiation but he did not for the outgoing. (We are able to confirm the reflection by comparing the radiations at a clear sky contition to those at cloudy conditions. If there were no reflection, what means no albedo for the outgoing radiation, one would see the atmospheric window almost unchanged.
    This, of course, has influence on the emission coefficient of the whole system Earth and with it on the equilibrium temperature.

    So we finally have to ask whether the consensus is on the right basis.
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  29. KR @ 177

    I agree whole-heartedly. I guess I was pointing out that the consensus on climate change is proof positive that what you say is true. Given the potential for personal material gain for naysayers, we would see a much more even distribution of opinion if scientists didn't care as much about the truth and "being caught out."
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  30. This is a repost of a response to Bud in another thread.

    Bud said "I have following this subject since 1980 when many of these same people were alarmed about global cooling."

    I remember 1980 Bud. Your timeline is wrong. I was only an undergraduate at the time, but the NAS Charney Report on global warming came out the year before in 1979. We discussed it in class. At that point there was already a consensus on the action of GHG on climate among climate modelers - and that consensus, based on the known physics at the time, suggested warming not cooling.

    There were still a lot of scientists who remained unconvinced at that point and into the 90s. They have been gradually convinced over the ensuing three decades by the accumulating evidence until virtually all of them now agree. Those people have not arrived at their opinions by appealing to authority, but by evaluating evidence.

    Look...What if you took your car to 100 mechanics, and 97 of them said you needed to replace your radiator to prevent an engine failure while providing good reasons for their position. One disagreed without giving you a good reason, claiming simply that the others don't know what they are talking about and are not completely sure. Who would you listen to?
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  31. Tom Curtis @23

    I share Harry Seaward's concern over what boils down to a climate relevant sample size of 79. I discuss AGW with deniers all the time, and they will simply dismiss me if I tell them that the sample size from which the 97% figure was derived was a mere 79 people. I'm getting up in years, and have no children. Maybe I should be content with letting inmates now running the asylum roll the dice--after all, it is only them and their descendants who will suffer the consequences.

    In at least one other respect, the survey confirms my experiences: meteorologists are quite resistant to the idea of AGW. I can't explain why this is so.
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  32. "... the survey confirms my experiences: meteorologists are quite resistant to the idea of AGW. I can't explain why this is so. "

    Two reasons I suspect. Firstly, meteorology training doesn't have the same emphasis on basic physics as climate researchers do. And this exacerbates the main reason ...

    Secondly, meteorologists know very well that weather can't be predicted more than a few days out. Because it's just not possible to enter, let alone compute, all the possibly relevant information hour by hour, day by day. Most meteorologists work in a framework limited, almost entirely, by initial conditions.

    They have to abandon and invert that thinking to deal with climate, which is all about boundaries and limits - regardless of initial conditions. And they need to pick up on some physics and more statistics into the bargain. Most could do it. The real problem arises with those who don't try and don't want to try.
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  33. Eric the Red - Your arguments about 90 vs. 97 percent of actively publishing climate experts are really just dancing around the margins.

    The great majority of people who work in this field find that the evidence supports anthropogenic causes for recent warming, with solid evidence for the interactions of CO2 and consequences thereof. There are plenty of areas where there is active dissent over aspects of this (ocean heat content profiles, cloud feedback, pinning down the climate sensitivity, etc.), but the core of the theory is solidly supported.

    Pointing loudly to the edges of the theory where details are being argued does not invalidate the larger theory.
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  34. KR,

    I am not arguing that the theory is invalid, but the details.

    Most scientists will argue that the feedbacks in AGW theory are the most critical, as the direct warming of a doubling of CO2 is only ~1C. That is where the bulk of the differences occur, and why such a wide range of scenarios exist.

    The problems with these surveys is that the arrive at a number; 80%, 90%, 98% believe that CO2 is responsible for the recent warming, but then tie that response to a particular statement about climate sensitivity, future warming, etc.

    In reality, these scientists believe that CO2 is a greenhouse gas, and that it has caused an increase in global temperatures. However, there is a very wide range about what individual scientists believe will result from the increasxe in CO2.

    I do not believe these are the edges of the theory, but the crux of the argument. DO you feel differently?
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  35. Eric the Red - There's certainly room for discussion on climate sensitivity, as the data doesn't impose terribly tight boundaries on it.

    However, those boundaries range from no less than 1.5C per doubling of CO2 (very solid boundary) to 5+C per doubling (less solid, it could be higher). So the limits run from bad to extremely bad in terms of consequences to us, with a consensus view of 3C. Even the lowest reasonable numbers for sensitivity are going to cause us a lot of trouble.

    Your decrying of consensus gives the appearance of wanting to claim that climate change won't be a problem. If that's not your opinion, my apologies. But that's certainly how you are coming across in these discussions.
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  36. KR,
    I know that I have the reputation of being the "no problem guy" just because I do not hold many of the positions as being very solid. Just because I stick to the notion that all these scenarios are probably (not just possible), is no reason to think that I do not believe the science.

    I will not accept or dismiss premises based on one or two papers, especially when those papers are written by people whose sole purpose is to promote their hypotheses. When the data becomes convincing, then I will join in. I have a particularly disdain for models (some based on personal biases dealing with modelers in the past) that cannot be verified with data.

    Remember, there is a big difference between evidence in support of a theory, and evidence that confirms a theory. Currently, we have a a lot of evidence in support of AGW, however, many scientists are extrapolating well beyond the limits of the data, and scientists will tell you that is introduces a large amount of uncertainty.
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  37. Eric the Red - In regards to this particular issue, climate sensitivity, it's not "one or two papers".

    The Climate sensitivity is low thread contains a dozen representative papers alone. See also the IPCC report at this link, additional information here, with specifically observational work described here.

    A Google Scholar search on "climate sensitivity estimate", limited to just the last 10 years, no patents, and {Biology, life sciences, environmental sciences / Chemistry and materials science / Physics, astronomy, planetary science} only (as a reasonable set of restrictions), gives ~68,400 matches. Out of that list I believe there are perhaps a couple of dozen papers that claim sensitivities below 1.5C/doubling of CO2, most of which have already been refuted. You appear not to be looking at the considerable body of evidence.

    Even beyond that, the limits of uncertainty on sensitivity, while broad, have serious implications at both extremes.
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  38. Eric the Red - Here's the search I mentioned in my previous post. I believe this an adequate response to your statement of "premises based on one or two papers".
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  39. Eric,

    Re the estimate of climate sensitivity, you say "One or two papers"

    Oh good grief. Come on that is a very, very weak argument, you have to do better than that if you wish to make a compelling case. BPL provides a summary of 61 papers (between 1896 and 2006) that calculate estimates of climate sensitivity. It is by no means comprehensive or up to date, but is shows your statement to be demonstrably wrong.

    FWIW, the mean is of the 61 papers is ~+2.9 C (plus/minus one sigma 1.4 to 3.9 C)and the median is ~+2.6 C, compare that with the best estimate of +3 C reported in the IPCC AR4. And that is for doubling, we will very likely treble CO2 if we continue with BAU.
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  40. Yes,
    I know BPL had a list which included a half dozen papers below 1.0, and 3 above 5.0. Neither of these convince me that it is below 1 or above 5. Does it convince anyone else. The range is quite large, mostly due to the large uncertainties associated with the various feedback loops. Much more work is needed in this area in order to narrow the range.
    Albatross, you display a one sigma uncertainty. Have you done the math for the 95% range.
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  41. Eric the Red The IPCC states that "...‘equilibrium climate sensitivity’, is likely to lie in the range 2°C to 4.5°C, with a most likely value of about 3°C. Equilibrium climate sensitivity is very likely larger than 1.5°C.". (Emphasis added)

    As per IPCC definitions:

    "Likely" > 60% probability
    "Very likely" > 90% probability

    So yes, that math has been done.
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  42. KR and Eric,

    I did not see the point of undertaking a rigorous stats analysis of the papers involved.

    Annan and Hargreaves however have looked at the upper range of climate sensitivity. They determine that:

    "A+H [Annan and Hargreaves]combine three independently determined constraints using Bayes Theorem and come up with a new distribution that is the most likely given the different pieces of information. Specifically they take constraints from the 20th Century (1 to 10ºC), the constraints from responses to volcanic eruptions (1.5 to 6ºC) and the LGM data (-0.6 to 6.1ºC – a widened range to account for extra paleo-climatic uncertainties) to come to a formal Bayesian conclusion that is much tighter than each of the individual estimates. They find that the mean value is close to 3ºC, and with 95% limits at 1.7ºC and 4.9ºC, and a high probability that sensitivity is less than 4.5ºC. "

    Good enough?
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  43. Albatross - Quite clear, thank you.
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  44. The range of sensitivities (ie constrained by physics) is much lower than that found in empirical estimates of sensitivities as you would expect. The important point is that empirical estimates are consistent with model estimates. A sensitivity of less than 2.0 requires a so far undiscovered negative feedback.
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  45. scaddenpm
    the reason many lower ranges are below 2.0 is the uncertainty of the cloud feedback, which could be a large negative. It is not undiscovered, just unknown with enough certainty to enforced a higher limit. That is just one, there are others. The 95% ranges for the three previous analyses are 1) 1.5 - 5.0, 2) 1.7 - 4.9, and 3) ~0.1 - 5.1.

    There are several others with their own ranges. These are just three. This is why I cringe whenever someone makes a claim that the climate sensitivity is a certain value and then makes projections based on that. Hence, I disagree with anyone who makes such a claim.
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  46. Eric the Red - Given the best evidence we have to date, the climate sensitivity could be under 2ºC. It could be over 4.5ºC. Our evidence indicates that it's most likely about 3ºC.

    You seem to be picking strictly from the lower estimates, of low probability - you might as well pick from the high estimates of low probability. Neither is a sensible choice.
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  47. But evidence we have is that cloud feedback is near neutral. There is uncertainty but below 2 is not the way to bet.
    0 0
  48. I am not picking anything. These are the ranges chosen by the three other posters. Betting under 2 appears to have a similar likelihood as over 4.
    0 0
  49. Eric @198,

    No. I'm flattered, byut my simple analysis of BPL's data is really not something you should be using to bet. Also, look at Knutti and Hegerl, the PDF is not symmetrical, it is skewed towards the high end.

    And AGW is not a betting game.
    0 0
  50. Eric the Red, given the range of independent studies based on independent data that indicate a range of values centred close to 3, it is very foolish to assume the marginal values are at all likely. There are several reason for this.

    First, as a matter of pure science, if you combine the evidential weight of multiple independent lines of evidence, you get a much more tightly constrained value than any of the independent lines. This can be seen by comparing the ranges of the three independent lines of evidence discussed by Annan and Hargreaves (mentioned by Albatross @192). They, of course, just discussed three lines of evidence. Given that nearly all the lines of evidence generate ranges centred close to three, if a similar technique where applied to all the evidence, a range very tightly centred on three would be the result. It is certain that the range of such a study would be within the range determined by Annan and Hargreaves, and is likely to be narrower. (My guesstimate would be a 95% confidence range around 2.5 to 4)

    Second, as a matter of policy, given a range of uncertainty, you should base your decision on a probability weighted estimate of loss or gain for each outcome. As the expected adverse impacts of global warming rise with increasing climate sensitivity, to that extent the climate sensitivity you should assume for policy purposes should be weighted towards the upper limit rather than the lower limit.

    Personally I don't expect that to happen, or argue for it. I want politicians to just accept the IPCC mean value (3) and base their policies on that. I expect that because, given available evidence, it is the more likely to fall close to the actual value than any other choice. As such, I am advising people to ignore standard decision theory or mini-max strategies both of which (as noted above) would require using a significantly higher value for policy settings.

    Now, can you give one good reason why it would be sensible for politicians to bet on a low climate sensitivity when the probability that the value is wrong is significant, and assuming a low climate sensitivity in their response to global warming will maximize the adverse consequences of global warming?
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