Zeroing in on the true value of climate sensitivity

Climate sensitivity, defined as the increase in the Earth’s surface air temperature caused by a doubling of atmospheric CO2, is a curious metric. It isn’t used as direct input for anything – it’s derived from the output of climate models or from observational analyses – but no other variable in all of climate science is so controversial. This is because it tells us, in one single number, how serious a problem CO2-induced warming might be. If climate sensitivity is down around 1.5C, the low end of the IPCC’s range, the impacts probably won’t be serious, maybe not even noticeable. But if it’s up around 4.5C, the high end of the range, watch out.

So what is the true value of climate sensitivity?

The first estimate – 5.5C – was made by Svante Arrhenius in 1896. Ten years later in 1906 he lowered it to 2.1C. Then came Hulbert in 1931 with 4C, Callendar in 1938 with 2C, Plass in 1956 with 3.8C, Möller in 1963 with 9.6C, Manabe & Wetherald in 1971 with 2.4C and Sellers in 1973 with 0.1C. Figure 1 plots these and a couple more contemporaneous estimates by year. Clearly scientists were having difficulty putting a value on climate sensitivity in the early days:

Figure 1: Early climate sensitivity estimates

But things are different now, right?

Figure 2:  120 observational estimates of equilibrium climate sensitivity from 1896 to 2014. Only median or average values are plotted; the scatter would be much greater if high/low estimates were included. Data sources are listed at the end of the post.

A casual observer looking at Figure 2 might be excused for concluding that scientists are no closer to reaching agreement on the true value of climate sensitivity now than they ever were. As unlikely as it seems, however, there are trends, although the data have to be segregated into time groups to reveal them. I used six with a roughly equal number of estimates in each:

Before 1980: 19 measurements centered on 1960.8
1980 through 1993: 22 measurements centered on 1986.7
1995 through 2005: 20 measurements centered on 1999.9
2006 through 2011: 19 measurements centered on 2008.9
2012: 25 measurements centered on 2012
2013 & 2014: 15 measurements centered on 2013.3

Figure 3 plots the means, medians and standard deviations of these six groups. Means and medians show a downward trend since 1985, indicating that climate sensitivity estimates have been generally decreasing over the last 30 years, and the erratic downward trend in standard deviation since 1960 indicates that the level of estimation uncertainty has also been generally decreasing:

Figure 3:

Deleting the three high-value outliers seen in Figure 2 – Möller 1963, Pagani 2010 and Hansen 2012 – has only a small impact on the mean and median values but cleans up the standard deviation plot, which now shows a coherent and accelerating downtrend:

Figure 4:

Table 1 summarizes the results numerically. The means and medians of the estimates have fallen from 3 – 3.5C since 1981-93 to around 1.9C now – a substantial drop – and the decrease in the spread of the 90% confidence limits (equivalent to IPCC “very likely”) from 4.45C to 1.8C represents a significant improvement in the level of agreement:

Climate models are also used to obtain estimates of climate sensitivity, and while there aren’t enough estimates in Figure 5 to demonstrate convergence they also show a downward trend heading towards a 2013-14 value of around 2C, comparable to the 2013-14 observational values shown in Table 1:

Figure 5: Climate sensitivity estimates from climate models, excluding IPCC data

Figure 5, however, excludes the IPCC’s climate-model-based estimates, which place the value of climate sensitivity at around 3C with a “likely” range of from 1.5C to 4.5C. The IPCC’s estimates have in fact shown remarkable consistency over the years. Except for the increase in the lower limit from 1.5C to 2.0C in the AR4 they haven’t changed since the Charney report got the global warming ball rolling 35 years ago:

Figure 6: Charney & IPCC climate sensitivity estimates

The IPCC’s data nevertheless exhibit one interesting feature. The model-derived climate sensitivity estimates don’t match the model-derived temperatures. The mean of the 26 individual AR5 model-derived climate sensitivity estimates is 3.3C, but when the same models predict temperatures they operate at an effective climate sensitivity of only 2.2C. Figure 7 shows the near-exact fit at 2.2C for RCP8.5, the AR5’s worst-case scenario, which projects 1,250ppm CO2 by 2100 and 3.7C of warming to go with it. The fits for the RCP6.0 and RCP4.5 scenarios are similar:

Figure 7:  Model-predicted surface air temperatures vs. temperatures calculated using climate sensitivity = 2.2C, IPCC AR5 RCP8.5 scenario.

Why do the IPCC’s climate models operate at 2.2C and not 3.3C? Because according to global warming theory temperature equilibrium isn’t reached for hundreds of years after CO2 concentrations stabilize, meaning that model temperatures will keep climbing and eventually flatten out at a level compatible with a +/-3.3C equilibrium climate sensitivity if the models run long enough. But are computer model projections of what might happen hundreds of years into the future meaningful? Are they even relevant? Figure 7 shows an excellent fit to a climate sensitivity of 2.2C over a period of 200 years, which exceeds human predictive capability by a large margin and certainly my attention span. How far out do we have to go?

I contend that 200 years is quite far enough, in which case the IPCC’s best current estimate of climate sensitivity is 2.2C, not 3C. And at 2.2C the IPCC falls into line with everyone else. We have established a consensus.

And having done so the question becomes, what value will climate sensitivity estimates ultimately zero in on? The current value is around 2C, with estimates decreasing with time and with every prospect that they will continue to do so at least while the warming “pause” persists. My guess is that they will eventually settle somewhere below 1.5C – maybe even as low as 1C, which is generally agreed to be the climate sensitivity to a doubling of CO2 in the absence of feedbacks. At these levels the perceived risks of man-made global warming decrease very significantly, but that will have to be the subject of another post.

Data sources:


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52 Responses to Zeroing in on the true value of climate sensitivity

  1. Yvan Dutil says:

    Be careful, there is vary definition of climate sensitivity.According to your graph, 2.2 C is the transient version of hit, which is smaller than the 3 C that also include the slow feedback. This variation in definition creates plenty of confusion.

  2. Dave Rutledge says:

    Hi Roger,

    Very interesting post. Following up on Yvan Dutil’s comment, is there a way to separate the transient sensitivities from the equilibrium sensitivities in your graphs?


    • Roger Andrews says:

      Hi Dave:

      I was hoping no one would bring this subject up, but the answer is no, there is no way of separating equilibrium from transient climate sensitivities in my graphs, with the simple reason being that the data sources I used didn’t say which one the individual estimates referred to and it would have taken me for ever to find out.

      I think the basic question, though, is whether as a practical matter it makes any difference. If you look at the graph at the top of

      and compare it with my Figure 7 you will see that even as far out as 2100 we’re still on what the IPCC calls the “transient” part of the curve and there’s no sign of a flattening. Under these circumstances I would contend that the equilibrium sensitivity becomes academic. I also question whether there even is such as thing as equilibrium climate sensitivity but I’d have to write another post to say why.

      • Hi Roger,

        It is interesting is they assume that the forcing goes flat when it reaches the target level. The most important component of the forcing would be carbon dioxide from burning coal. Historically, when coal regions have declined, production has dropped quickly. 10% a year is typical. When I run the CO2 models that drop is quick enough that the CO2 drops too, and the forcing would drop rather than stay level.

        “Under these circumstances I would contend that the equilibrium sensitivity becomes academic.”

        I agree. The continuing temperature rise is just a consequence of their assumptions about forcing. The truth is that we can’t really get at what the slope of the temperature would be 200 years from now.


        • Hi Dave:

          The continuing temperature rise is in fact a consequence of the assumption that CO2 warms the sea as well as the air, and that while the atmosphere responds rapidly to CO2 stabilization the excess CO2-induced heat in the ocean takes many years to work its way out. There is, however, no observational proof of this, or at least none that I’m aware of, nor even that CO2 warms the oceans in the first place. It all comes from climate model output.

  3. Joe Public says:

    Thanks for a very educational posting, Roger.

  4. A C Osborn says:

    There is “Physics” Sensitivity and “Climate” Sensitivity.
    The Physics just looks at the “Gas” that is CO2 and it’s affect on temperature and retention of heat due to increasing it.
    Climate Sensitivity has to take in to account all possible feedbacks and the IPCC version only looks at positive feedbacks instead of looking at all the possible negative feedbacks.
    They also cannot possibly identify from the recent past Climate what part of the temperature changes are due to “CO2” and what part is purely natural.
    They say that the can and have, in which case they should have predicted the current “fall” in Surface Temperatures (they are only being kept level by “adjustments”) about 20 years ago, instead they currently have 40 or 50 different excuses for what COULD be causing it.
    Settled Science it is NOT.

    I have a question for all the people who understand the Logic of CO2 increasing the heat retention.
    Q. Where does the “Doubling” start in the Logorithmic Scale?

    Because if it starts at 1ppm then they have a real problem as many have pointed out in the past, only the first 20ppm or so will have any effect.

    PS, Satellites are not measuring the Surface Temps, they are measuring the heat escaping to space. Which is why we can get such discrepancies between what we experience on the Surface compared to the Satellite measurements.

    • Euan Mearns says:

      Where does the “Doubling” start in the Logorithmic Scale?

      AC – its a good question. I’m not well qualified to answer but will try anyway. I’m assuming current global average is 15 ˚C (Houghton) and 400 ppm CO2. For a CS of 4˚C we get:

      400 ppm 15˚C
      200 ppm 11˚C
      100 ppm 7˚C
      50 ppm 3˚C

      And for a CS of 2˚C we get

      400 ppm 15˚C
      200 ppm 13˚C
      100 ppm 11˚C
      50 ppm 9˚C

      Estimates for average surface temperature during glacial maxima are of the order 5 to 6 ˚C colder than now with about 180 ppm CO2 (Vostok). So you can see why climate science wants to have a high CS over 4˚C to get cold enough at just below 200 ppm.

      But this is where they go off the rails. It is well established that orbital criteria control ice ages. Once ice sheets become established they increase albedo cooling and a point Clive made recently, they dry the atmosphere reducing H2O greenhouse warming. These three variables are likely the main drivers, CO2 follows along and will have secondary effect. If you assume that ice ages are controlled by CO2 you will incorrectly pick an anomalous high value for CS.

      • Hi Euan:

        It’s now thought that CO2 in ice cores lags temperature by some hundred of years, meaning that we have to express climate sensitivity as the change in CO2 relative to an increase in temperature, not the change in temperature relative to an increase in CO2. (It works out to about 10ppm for each 1C increase).

        Trying to figure out what climate sensitivity really means and how we should go about measuring it is like trying to shovel a cloud into a bucket. But if you don’t mind mathematics one of the better accounts is the one Clive Best gave a year or so ago:

        Using a different approach Clive also gets an ECS value of 2C.

        • Euan Mearns says:

          It’s now thought that CO2 in ice cores lags temperature by some hundred of years,

          Roger, I thought sceptics have argued this for some decades while the CC community has resisted. I am revisiting ice core data in light of what I have learned in recent years. It is vital to understand that in ice cores the CO2 signal is carried in gas bubbles and temperature signal carried in snow. And the difference between ice age and gas age is determined by snow fall rates that vary enormously between Vostok and Siple station. Trapped gas compositions vary as a function of time it takes to make ice out of snow. 1000 years at Vostok, a couple of decades at Siple.

          Gut feel is that CO2 and temperature vary more or less as one. But CO2 still varies more or less as a function of temperature – in ice cores.

          • Euan: I don’t have any specifics on how the lead-lag argument finished up, but we don’t hear much any more on how the correlation between ice core temps and CO2 proves that we’re doomed.

            One of the problems in accepting the ice core CO2/temp relationship as proof that CO2 controls temperature is the R squared value of ~0.8 you get when you correlate the two, which implies that 80% of ice age temperature changes were caused by ~100ppm fluctuations in CO2 while the combined effects of Milankovitch cycles, changes in albedo caused by ice sheet growth and decay, changes in ocean and atmospheric circulation patterns and everything else had only a 20% impact. That’s kind of hard to do.

  5. Euan Mearns says:

    If climate sensitivity is down around 1.5C, the low end of the IPCC’s range, the impacts probably won’t be serious, maybe not even noticeable. But if it’s up around 4.5C, the high end of the range, watch out.

    Roger, this is the key. The failure of the climate science community to differentiate the energy policy response required for these two scenarios is a disgrace. If the policy responses were differentiated then the need to zero in on “truth and reality” would become very apparent.

    • louploup2 says:

      The word “probably” is more to the point. There are so many uncertainties I don’t see how any firm conclusions can be drawn. We are conducting a long-term, non-controlled (we only have one planet) experiment; expecting the scientific community to spell out a range of specific needed policy responses is rather a stretch.

      Also, under BAU we continue emitting GHGs without knowing the precise lag time to a new thermodynamic equilibrium or the sensitivity at various levels of GHGs. Centuries? We are less than two centuries into a rise in CO2 from c. 280 to c. 400 ppm, and yet the impacts are already significant. IPCC AR5, WG II: “Chapter 18. Detection and Attribution of Observed Impacts”. I think those impacts are already “serious.”

      • louploup2:

        Yes, your comment did make it through (I deleted the two follow-ups).

        I don’t think there’s any point my responding to your observations on the allegedly already-demonstrated impacts of rising CO2 because we’re unlikely ever to reach agreement on this subject and it’s off topic anyway. The purpose of this post was to show that science is in fact zeroing in on “(climate) sensitivity at various levels of GHGs”, and that the final consensus value is likely to end up at a rather low level. If you have evidence to the contrary feel free to present it.

        Euan may have something to add later.

        • louploup2 says:

          “If you have evidence to the contrary…” Nope; I’m not a scientist (lawyer by profession) and my policy analysis focus is adaptation. That brings me to the reason I bring up impacts; I read about them daily. I work on forested communities throughout the U.S. and I know very few experts or lay people who work in the field who don’t agree that fire and insect damage has been significantly increasing.

          Also, it’s totally anecdotal and subjective, but in my 40+ years living in the same place (Western Washington State/Pugetopolis), I am perceiving changes in weather patterns especially in timing of seasons and extreme events, such as Arctic air mass incursions and heavy precipitation.

          Thus, I disagree when you say “allegedly” about impacts already occurring but I agree it’s somewhat off point from your original post so that’s that. I say somewhat because the biophysical impacts of increases in mean or extreme temperatures overlaps with the calculated sensitivity. We are dealing with a very complex simple.

          In my adaptation work, documenting current changes and trends is a crucial initial step in the impact assessment. I hope you will consider addressing such issues yourself at some point.

          Thanks for dealing with multiple posting; I’ll let WP deal with my posting. But it did give the “duplicate” message on first hit of prior “Post Comment.”

          • Euan Mearns says:

            Forest fires: often due to poor forest management in that forests in some places are supposed to burn regularly. Man preventing this leads to accumulation of vast amounts of forest litter. And when it eventually burns, perhaps by a careless dropping of a cigarette, the fires are much worse than normal, especially if there is real estate in the forest that is not really supposed to be there.

            You may want to believe that this is due to an increase in CO2 from 260 to 400 ppm and impose energy poverty and a risk of financial collapse on an economy that is built around fossil fuel. But a careful person would analyse the real causes in more detail.

            Where I live in NE Scotland it did seem that the climate had changed late 1990s when we had a run of mild wet winters. But now its gone back to what it was like when I was a boy in the 1960s. All part of a natural rhythm, in this case linked to the North Atlantic Oscillation.

            Those who believe that CO2 forcing of climate may lead to extreme cold events have been totally brain washed by Green propaganda. You should wake up and smell the coffee.

          • louploup2 says:

            Forest fires
            The number and intensity of fires in Western North America has been increasing over the past few decades. The causes (and consequences) have been explored in dozens if not hundreds of peer reviewed papers, such as Westerling’s 2006 study (“Warming and Early Spring Increase Western U.S. Forest Wildfire Activity”) doi:10.1126/science.1128834 ; and Williams et al. 2012 (“Temperature as a potent driver of regional forest drought stress and tree mortality”) doi:10.1038/nclimate1693 The causes of these fires and stresses are not solely or even primarily ”due to poor forest management” (although as a forest policy advocate for 40 years, I agree there’s plenty of that as well).
            You may want to believe that this is due to an increase in CO2 from 260 to 400 ppm
            I don’t have to “believe” anything; the facts are apparent in the woods. Whether or not the increase in fires and beetle damage is due to increased CO2e is irrelevent from that perspective. It is clearly due to increased warmth, drier soils, etc. However, I do happen to believe the rapid change in radiative forcing due to anthropogenic GHGs is largely responsible for the conditions increasing fire threats to forests in my part of the world. Fires are not the only such “apparently” AGW caused forest problem: “Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects” doi:10.1525/bio.2010.60.8.6 Bottom line—we have a big problem out here: “Widespread Increase of Tree Mortality Rates in the Western United States,” doi:10.1126/science.1166505 .
            And BTW, the changes in our forests are related to changes occuring in regional hydrology; if you wish I can refer you to similar studies and graphs showing shifts in runoff timing, snow melt dates, total precip, etc. all over Western N.A. Non AGW natural variation only? Maybe, but fewer and fewer people around here believe it, and close to none among the numerous university and agency scientists working on the subject.
            impose energy poverty and a risk of financial collapse on an economy that is built around fossil fuel
            This is an amazing line from someone who has studied peak oil, EROEI, and energy issues generally. It sounds like those who deny the science of global warming altogether because they don’t like the policy implications: “Solution Aversion: On the Relation Between Ideology and Motivated Disbelief,” doi:10.1037/a0037963
            Where I live in NE Scotland it did seem that the climate had changed late 1990s when we had a run of mild wet winters. But now its gone back to what it was like when I was a boy in the 1960s. All part of a natural rhythm, in this case linked to the North Atlantic Oscillation.
            I’m not familiar with climate conditions and trends in Northern Europe, but it wasn’t difficult to find studies that disagree with your unqualified underlined conclusion. E.g. Hurrell and Desser, “North Atlantic climate variability: The role of the North Atlantic Oscillation,” doi:10.1016/j.jmarsys.2008.11.026 .
            Those who believe that CO2 forcing of climate may lead to extreme cold events have been totally brain washed by Green propaganda.
            Huh? I’m relying on the work of Jennifer Francis and others; the warming of the Arctic is likely causing the jet stream to weaken leading to intense and long-lasting incursions of Arctice air masses deep into North America. Francis & Vavrus 2012 (“Evidence linking Arctic amplification to extreme weather in mid-latitudes,” doi:10.1029/2012GL051000); Cohen et al. 2014 (“Recent Arctic amplification and extreme mid-latitude weather,” doi:10.1038/ngeo2234) It has always happened; now it’s happening more and lasting longer. Brrr…
            You should wake up and smell the coffee.
            You should be polite to your guests who are not being rude, and simply trying to add to the conversation and ask questions

          • Euan Mearns says:

            Loup, I’m sorry if I came over as impolite. I’ll follow up on Roger’c initial comment which was that no matter how much we talk, neither of us will change our minds. A few links:

            UK climate change



            Change to the jet stream

            Solar forcing of winter climate variability in the Northern Hemisphere
            Sarah Ineson1*, Adam A. Scaife1, Jeff R. Knight1, James C. Manners1, Nick J. Dunstone1, Lesley J. Gray2 and Joanna D. Haigh3

            1Met Office Hadley Centre, FitzRoy Road, Exeter, Devon EX1 3PB, UK, 2National Centre for Atmospheric Sciences, Department of Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford OX1 3PU, UK, 3Blackett Laboratory, Imperial College London, London SW7 2AZ, UK. *e-mail:

            Climate change in SW USA


          • louploup2 says:

            Thanks for responding.

            It troubles me that you presume an inability by people engaged in dialogue to “change minds.” If that is so, why bother engaging? I am perfectly willing to agree that climate sensitivity might well be lower than many scientists have argued.

            To me the sensitivity number is not nearly as important as what is happening. Thus it troubles me even more that you appear to conflate impacts with policy solutions, inherent in your statement implying that my position will or is intended to “impose energy poverty and a risk of financial collapse on an economy that is built around fossil fuel.”

            My perception is confirmed by your references intended (I assume) to refute the conclusions in the various studies about Western U.S. I posted (notwithstanding that we cannot “change minds”). I looked at the publicly available references in your post (and am awaiting arrival of the two behind paywalls via a friend with access). I see nothing that contradicts the content of what I posted.

            IMO, your work on energy issues has been (and continues to be) excellent. The net energy cliff graph and explanation is a brilliant addition to the overall dialogue. Please be more careful and thorough in your research and analysis when you explore areas that are not directly within your expertise.

          • Roger Andrews says:


            Please be more careful and thorough in your research and analysis when you explore areas that are not directly within your expertise.

            Euan does tend to concentrate more on energy while I am maybe a little more familiar with the climate aspects. Your statement, however, does raise the question of how qualified you are to exercise independent judgment on climate issues, and to find out here is a little test which anyone familiar with the global temperature record would find fairly trivial.

            The graph below shows the GISS (1200km) global surface temperature record between 1965 and 2004. The 1997-1998 El Niño is clearly visible and the trend line shows close to 0.6C of warming between 1965 and 1998 followed by the “pause” after 1998. I have, however, deliberately altered something. Can you identify what?

          • louploup2 says:

            I believe I am competent to evaluate the literature of climate change impacts and identify relevant policy issues. I am an expert policy analyst. I am also an experienced attorney (both barrister and solicitor in British terminology) and have worked extensively with numerous experts on various natural resource issues (fisheries, water quality, geomorphology, silviculture and forest ecology). I have been studying sustainability for a number of years and have read extensively in the relevant fields (energy, relationship of energy to economy, thermodynamics, systems analysis, history, history of science, history of capitalism, etc.). That’s why I am aware of Euan Mearn’s posts at TOD.

            Your “test” is like asking me to get into a debate with Tamino about climate statistics (but it appears he has not posted since July?). I do not have the hubris of Anthony Watts. I do not hold myself out as an expert able to look at that graph and identify an error or change. I probably could, if I spent the time, search out the original data and compare them. I did not post anything remotely close to arguing about the accuracy or relevance of surface temperature data.

            Please look again at what I posted: forestry and climate impacts on North America are my focus, and that’s what I posted on. I got a response from Euan Mearns that clearly did not show knowledge of the field and moreover indicated little interest in doing some basic research before posting on the subject (first paragraph of November 11, 2014 at 10:29 am). In response I quickly posted a few of the studies because it’s the area in which I work. In Euan Mearn’s response some references were given, but I see little to contradict the material and conclusions I posted (and that’s exactly what I said in my follow up). Thus my last post.

            If Euan Mearns (or you) don’t have time to engage on the specific points I raised, don’t. I have been trying to engage because I respect what you are doing with this site, but am disconcerted by authors’ drifting into areas that are clearly not of great interest or focus leading to statements that undermine the overall credibility. Instead of asking me to take a test that is wholly irrelevant to the issues I raised, why don’t you address my substantive points?

          • louploup2 says:

            I’ll take a wild guess though: It appears that the y axis has been moved up. It does not appear that the 0 is at the average for the period shown.

          • Euan Mearns says:

            Roger’s test. I dunno, but make the following observations:

            1. The temperature rise on the chart 1965 to 1998 is 0.5˚C and not the 0.6˚C claimed 😉

            2. As Loup observes, the 0 datum is in the wrong place and I’d speculate that this may be down to you using a different reference period for calculating anomalies

            3. I observe the graph stops in 2003 and wonder why? Perhaps you have applied a filter to the data?

          • OK, time’s up

            I lied about the dates. The graph doesn’t show the period of surface warming between 1965 and 2004. It shows the period of surface warming between 1907 and 1942.

            Why did I lie about the dates? To show that we have had two periods of warming during the 20th century that are effectively identical but which according to the IPCC had quite different causations – the 1965-2004 warming was caused mostly by man-made greenhouse gases (it must have been or AGW theory is falsified) while the 1907-1942 warming was mostly natural (it must have been because GHG radiative forcings can’t explain it.)

            And what was my point? (apart from testing analytical skills, but I won’t comment further on that.) Simply to highlight the fact that the IPCC is being somewhat less than objective in attributing most of the recent warming to man-made GHGs. (The ongoing warming “pause” conclusively demonstrates that natural forcings are a lot more important than the IPCC is willing to admit.) The implications relative to the numerous claims that increased temperatures caused by man-made GHGs are having such-and-such an environmental impact in such-and-such a place should be obvious.

          • Euan Mearns says:

            🙂 Good test Roger. I looked at the Gistemp chart on Woodfortrees and eyeballed it with yours. The similarity is quite striking. And I’m familiar with this Lindzen “trick”.

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  7. When I was a boy in the 1960,s the weather was nothing like this wet all year round . We had good hard winters and good weather in the summer and again in the autumn .Take a look out the window and smell the breeze

  8. edhoskins says:

    An alternative way to look at Climate sensitivity using IPCC data. There is only about 13% of the effectiveness of CO2 remaining between 400 -1000 ppmv

    So the magic 2degC limit could never be attained even using the most extravagant IPCC data for the effectiveness of CO2 as a greenhouse gas.


  9. Those who would like to make independent estimates of how much CO2-induced warming there might be between now and 2100 can do so using this graph:

    The procedure is simple:

    1. Take your best guess as to what the atmospheric CO2 concentration will be in 2100.
    2. Take your best guess as to what the climate sensitivity is.
    3. Pick the point where your CO2 guess intersects your climate sensitivity guess.
    4. Trace left to where it intersects the Y-axis.

    And there’s your warming.

    Tenth-scale divisions are provided for precision work.

    • Euan Mearns says:

      Roger, this of course needs to be a post on its own – with formulae used etc – but it would be really interesting to see the curves back casted. I know you have probably already done this 😉 Since time scale is flexible, you could mark x-axis with dates (in the past).

      I will bet £10 that atmospheric CO2 is about 600 ppm in 2100, CS is about 1.5˚C and so we land at less than 1˚C warming that will be indistinguishable from natural variation.

      • Here’s HadCRUT4 plotted against temperatures estimated from CO2 forcings at four different climate sensitivities. (Hate using HadCRUT4, but everyone else does so I guess I have to just to be consistent). I estimated forcings using watts/sq m= 5.35 ln(C2/C1), which is an approximation but should be good enough for climate work:

        As you can see, the basic problem is that temperatures don’t fit CO2. But if you have to pick a number then somewhere between CS = 1 and 2 fits best.

        The 60-year periodicity in temperatures is very clear. Looks like another 30 or so years of no warming in the pipeline.

        • Euan Mearns says:

          Roger, why the dog leg in the curves in 1965? CO2 related?

          The climate science community asserts that warming 1910 to 1940 is natural. And their is increasing consensus that there is a “natural variability within the climate system”. I also think you should be allowed to start your curves on Hadcrut 4. Its difficult to see a climate sensitivity much higher than 1. We have the baseline 1.2 (Houghton) with a little negative convection and cloud feedback.

  10. Euan Mearns says:

    @ LoupLoup2

    This post, authored by Roger, is about a review of climate sensitivity. Your interest and knowledge of forest fires in the SW USA is duly noted and acknowledged but is way off topic.

    You may want to believe that this is due to an increase in CO2 from 260 to 400 ppm
    I don’t have to “believe” anything; the facts are apparent in the woods. Whether or not the increase in fires and beetle damage is due to increased CO2e is irrelevent from that perspective. It is clearly due to increased warmth, drier soils, etc. However, I do happen to believe the rapid change in radiative forcing due to anthropogenic GHGs is largely responsible for the conditions increasing fire threats to forests in my part of the world.

    So you want to believe that the increased rate of forest fires is down to a small increase in CO2. Whilst, as you point out, I am no expert in this field, I continue to apply my common sense which tells me that a change in the observed forest fire statistics may be done to one or a combination of the following:

    1. The way statistics are recorded
    2. Greater intensity of built environment in forest areas
    3. Changes in forest management practices
    4. Natural changes to multi-decade scale climate, especially rainfall
    5. Man made changes to climate, especially rainfall

    Now, if you can assure me that the possibility of natural causes are cleary understood and accounted for then your argument for man made / CO2 causes will cary more weight. Without that, you postings are indistinguishable form thousands of others that observe a change in local climate and attribute this to man made influences without adequately quantifying what any natural effect might be.

    You need to understand that there is one of me and one of Roger and on any given day 30 or more commenters. We do our best to respond where a response seems appropriate.

    • louploup2 says:

      Thanks for your response.

      I am not arguing that “the increased rate of forest fires is [caused by] a small increase in CO2.” The observation you quote from my post is a personal opinion. But I do wonder why you characterize an increase in CO2 from a pre-industrial c. 250 to present 400 and climbing as “small”. Along with the increase in numerous other GHGs.

      None of the numerous studies on forest fire that I’ve reviewed try to pin down the increased rate and impact of forest fires on a specific “small” or large increase in CO2e. The immediate cause is warmer temperatures, a quantified reality. It takes very little increase in average temp to lead to a reduced level of humidity in forest soils, and then in the trees, causing fires to be more catastrophic than they would be under a cooler moister regime. A similar linkage holds for the impacts of a slight increase in the minimum winter cold temps; this change leads to a higher survival rate of various species of bark beetles, who proceed to weaken and kill millions of trees. And there’s the change in hydrology; slight increases in temp can lead to dramatic changes in timing of runoff and other hydrologic changes (e.g., earlier snow melt and runoff ==> soil dries out faster, go back to fire risk…).

      Again: The issue to me is not the calculated impact (degrees of warming) of a specific quantity of GHG. The issue is that under already extant warming we are seeing impacts occurring.

      I think my points are relevant to the subject of Roger Andrews’ post to the extent that I call into question the necessity to spend so much effort trying to determine the climate sensitivity metric so precisely rather than working to reduce GHG emissions, and adapt to the changes already under way. As our exchange revealed, policy implications are always lurking close to the surface of these discussions, and I believe it does a disservice to the long term need to adapt to a much lower energy supply. Civilization’s metabolism can not continue at current levels for much longer.

      I do not understand why you spend so much effort to minimize the impact of GHG emissions. And I’m really surprised that you do not appear to want to explore the policy implications of peak oil, and instead imply that I want to “impose energy poverty and a risk of financial collapse on an economy that is built around fossil fuel.” In your Three Nails post you agree we appear to be near “the end of capitalism.” Seems like cognitive dissonance.

      • Euan Mearns says:

        But I do wonder why you characterize an increase in CO2 from a pre-industrial c. 250 to present 400 and climbing as “small”.

        Well if you think CO2 is a potent GHG and is endangering the Planet you might argue that 150/250 = 60% rise. But on the other hand, if you accept that the absorption bands of CO2 to re-emitted IR are already saturated at surface at below 250 ppm (which I believe most do) then you’d understand that increasing CO2 has no direct impact on radiative warming at surface. The enhanced greenhouse effect is dependent upon a process called radiative transfer up through the troposphere and its impact upon surface temperatures is currently unknown as far as I can tell.

        And so if you are sceptical you would observe that a 150 ppm increase is a 0.15% increase in the atmosphere. There is a big difference between 60% and 0.15% – both numbers are valid.

        My engagement in the climate science debate has as much to do with auditing the science as being sceptical about the impacts of CO2. You’ll find I am rather cautious on emissions, but distinguish between a trajectory towards 1000 ppm and the hysteria caused by weather events being attributed to CC.

        I won’t refute that your forests are burning because of CC. What I’d question is the cause – natural versus Manmade. I suspect rainfall patterns are the key to soil moisture.

        Policy implications of peak oil = we need to get building affordable nukes.

        • Raff says:

          The 0.15% is “valid” but of little relevance (actually 150ppm is 0.015%). 99% of the atmosphere is not “radiatively-active”, so although the increase in CO2 represents 0.015% of the overall atmosphere, it represents 60% of the part of the atmosphere that is involved in IR absorption. It is only the IR active part that keeps us warm, oxygen, nitrogen and argon don’t.

          Saturation is a red herring too. As Science of Doom says in “The “Greenhouse” Effect Explained in Simple Terms” (

          If we add more radiatively-active gases (like water vapor and CO2) then the atmosphere becomes more “opaque” to terrestrial radiation and the consequence is the emission to space from the atmosphere moves higher up (on average). Higher up is colder. See note 6.

          So this reduces the intensity of emission of radiation, which reduces the outgoing radiation, which therefore adds energy into the climate system. And so the climate system warms (see note 7).

          This isn’t controversial. SoD is a good source of unbiased physics for those interested in climate.

          • Graeme No.3 says:

            You seemed to have missed the bit about water vapour. Since it is often present (near the surface) at levels 100 times that of CO2 it must have some effect.

            Also SoD and (most of) the commentators agree that an increase in CO2 will cause the height of the tropopause to rise, which it seems to have done in the 1980’s, yet recent measurements indicate that it is dropping. The atmosphere is a complex system and I don’t think that the old approach “2 legs bad, 4 legs good” works very well.

          • Euan Mearns says:

            The missing “0” bugged me when I was walking my dogs this afternoon 🙁

            I’m sure I’ve read the SoD piece before but will read it again. I have been trying to write a piece on the CO2 greenhouse effect for beginners, but its very complex. i’ve been working off Clive Best’s posts;


            The concentration of water vapour varies enormously laterally and vertically in the atmosphere. As does the IR emission height. This dwarfs the CO2 effect, not to say that CO2 is not significant.

            I reached the conclusion myself that CO2 appears to set the height of the tropopause, and am therefore astonished to read Graeme’s comment!

            I have long assumed that increased convection was a powerful negative feedback that kept the climate stable, BUT increasing the height of the tropopause will reduce the troposphere temperature gradient thereby reducing, not increasing convection.

            So I’m still trying to work out what is going on. But be patient, I’m commenting on several threads, both here and on other blogs. (and mainly on energy matters)

            But the foregoing bears no resemblance to the “popular debate”.

          • Euan:

            Allow me to contribute my vast, or more correctly half-vast, fund of knowledge on this topic.

            I don’t think the devil is in the details here. Ninety-nine-point-nine-something percent of the heat in the atmosphere and the oceans is in the oceans, so the critical question is whether GHG-induced back radiation heats the oceans as well as the atmosphere, as AGW theory says it does (if it doesn’t then all the climate models are wrong). Here are some articles from SoD saying it does:


            And a couple from skeptic blogs saying it doesn’t.



            What do I think? I can’t comment on the theoretical aspects but the observational data I’ve looked at show compelling evidence that the sun heats the oceans and no compelling evidence that GHG-induced back radiation does. But let me not influence you unduly. Form your own opinion 😉

          • Euan Mearns says:

            All I will say for now from my position of sublime ignorance and partial intoxication is three things;

            1. I don’t think that heat can be significantly conducted from atmosphere to ocean (near zero mass to huge mass), especially if that excess heat is in the upper troposphere.

            2. The amount of cloud is likely to have a large impact on direct radiative heating of ocean.

            3. Average global wind speed may have some impact on ocean temperature – latent heat of evaporation linking into clouds and Sun.

            So are ice ages more or less windy than the warm ages?

            I spent the day plotting data from Vostok 😉

          • Euan Mearns says:

            Skim read the SoD article – its a bit simplistic. Some good comments, this one caught my eye


            It provides an interesting perspective

            Convection is the main cooling mechanism
            No GHG no convection therefore no cooling

            But on the other hand no GHG and the surface would be freezing.

            Here’s a chart that Clive posted attributed to Richard Linzen. I think this sums up how I am seeing things. But an increase in CO2 leads to a reduced T gradient through the Troposphere that may impact a lot of things including convection rate – its just that this type of process never figures in the IPCC debate – maybe I don’t follow enough of the detail.

            And here is the main 15 micron CO2 band emitting at 215˚K = -57˚C which is close to tropopause, as measured from space.

          • Raff says:

            Graeme, yes there can be a lot of water vapour near the surface. But convection is the most important mechanism for transporting heat in the lower troposphere. At higher altitudes, much water vapour condenses to clouds. So CO2 becomes more important.

            As far as troposphere height falling, can you provide references on that? AGW observer gives a long list of papers discussing the subject at Have you cross checked against those?

            Roger, why do you question “back radiation”? A fire is a bit warmer if you stand in front of it than if you don’t. The sun is a bit warmer for the earth orbiting it than if the earth were absent. And the earth is a bit warmer because the background temperature of space is a few degrees Kelvin than it would be if it were zero. Similarly, the ocean is warmer because it is covered in a warm atmosphere. These are not up for debate. I think you should read and understand the subject on SoD (or the text books he refers to) before you write about it.

            Euan, what conclusion should we draw from your drawing attention to “an interesting perspective” in a comment by “Kristian”? SoD says of him:

            “I haven’t got much patience for people who have no interest in science, no interest in learning, make ridiculous claims as a substitute for learning and start insulting people when they keep pointing out the basic flaws in your claims.

            Proven experimental science is not armchair science because you don’t like it, or don’t like the conclusions it leads to.”

            In what way do you find it interesting? Because you agree with it? Because you don’t know whether it is right or wrong but you prefer it to what SoD says?

          • A C Osborn says:

            SoD may be a good source for information, but not as good as NASA Scientists.
            Their latest findings have surprised them, the added CO2 in the Tropopause has not caused it to get “hotter, it has caused an increase in radiation to space instead.
            ie it is causing faster COOLING of the earth.

          • A C Osborn says:

            Raff, you obviously believe in the Back Radiation in Trenberth’s cartoon diagram of the earth’s heat budget.
            So you actually belive in 333 Watts of DWIR?
            And you believe the Sun only provides 161 Watts?
            Do you think that all Watts are created equally?

      • I think my points are relevant to the subject of Roger Andrews’ post to the extent that I call into question the necessity to spend so much effort trying to determine the climate sensitivity metric so precisely

        Climate sensitivity is a measure of the amount of warming increased CO2 will cause, condensed into one single number. If it’s only a half or a third of the ~3C the IPCC has historically claimed then projected warming decreases by a factor of two or three and the magnitude of the perceived threat to the planet posed by any given CO2 increase decreases in proportion. That’s why so much effort is spent trying to determine its value.

  11. Toby Thaler says:

    the magnitude of the perceived threat Yes, understood. I’m focused on the impacts actually occurring so I am less concerned with that metric. I’m also more concerned with how much lag time there is between cessation of emissions and the new equilibrium.

    • A C Osborn says:

      Would you like to provide us with a list of those “impacts actually occurring” as I can’t actually find any.

    • Euan Mearns says:

      Loup, I’ve been thinking about this. Where I live I can see virtually no climate change either natural or manmade in the 50 odd years I’ve lived here. I spent a lot of time researching this and my memory was confirmed by the facts as recored by climate data – I think I posted the links already. But we did need to have a CO2 forcing with CS about 1.4˚C to explain the data. So the UK has maybe warmed by 0.5C since pre-industrial as a result of CO2. But here the impacts are virtually zero.

      You live in a place experiencing climate change and you are positively disposed to attributing that to CO2 and other manmade causes, even although much of the change you are experiencing may be natural.

      I think this in part explains our different outlooks.

  12. Graeme No.3 says:

    no reply button below your last Nov. 25 @ 3.33p.m.

    Sorry, can’t find the reference. I think I must have seen it while on holiday (August) and using iPad for viewing. From memory it was a comment in discussion and referred to a recent N.O.A.A. paper or report. Definitely this year.

    It surprised me which is why it stuck in my mind.

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