An important paper on climate sensitivity

The nub of the AGW hypothesis is not simply that human actions, especially in burning fossil fuels, increase the concentration of carbon dioxide in the atmosphere and cause global warming, but also that the effect of those actions is multiplied by something called ‘climate sensitivity’. It is widely agreed, on all sides of the climate debate, that doubling carbon dioxide in the atmosphere is likely to lead, all other things being equal, to an increase in global mean surface temperature of about 1.1 degrees Celsius.

By itself such an increase is not all that alarming. Other things are rarely equal, and it takes quite a time to double the proportion of CO2 in the air — it was estimated to be about 280 ppm in 1780, and is now about 400 ppm.  So an increase of about a degree in global temperature is no reason to press the panic button. The daily range in temperatures for many parts of the world is twenty or more degrees. The panic button comes from the secondary ‘sensitivity’ hypothesis: an increase in temperature will, it is argued, lead to an increase in water vapour and cloud, which will in turn retain more heat in the air, which will lead to more clouds and therefore more heat… The notion of a runaway climate disaster, during which we all fry, and our countryside becomes desert or is flooded, flows from this proposition.

It has to be said that while it is plausible, climate sensitivity as a concept has proved very difficult to pin down observationally, and it is by no means obvious that its sign must be positive — that is, that warming must increase. It is based on several assumptions, none of which has yet been shown to be true. But it is there, in two forms, in the IPCC reports. The first form is ‘equilibrium climate sensitivity’ (ECS), which you can think of as the change in global surface mean temperature caused by a doubling of atmospheric CO2 concentration.

‘Equilibrium’ here is another assumption, and is related to the notion that somewhere, or at some time, our planet had a ‘stable’ climate. It is argued, by the orthodoxy, that by adding CO2 to the atmosphere we human beings have ‘forced’ the climate in an unusual way — jolted it out of equilibrium, as it were. It will take some time (maybe a very long time) for the climate to adjust, the argument goes on, but when it does, its average temperature will go up to a new level. More on that in a moment.

The second form is referred to as ‘transient climate response’ (TCR), and that is said to be the average temperature response over a twenty-year period, with CO2 increasing at 1% per annum. TCR is smaller than ECS. Among the assumptions that are involved here, apart from ECS and TCR themselves, is the notion that CO2 stays in the atmosphere for a very long time, that there really is something called global mean surface temperature that we measure accurately, that sensitivity actually means something, and that clouds and water vapour really do magnify the 1.1 degree C increase that would come from simply doubling the proportion of CO2 in the air.

No matter, the IPCC has no doubts, and indeed its whole case for an urgent and substantial lowering of greenhouse gas emissions  falls unless climate sensitivity is real, and works, and is pretty high. The IPCC’s AR4 in 2007 provided a range of 2.0 to 4.5 degrees C, gave a best estimate of 3 degrees C, said it was very unlikely to be less than 1.5 degrees C, and warned that high values couldn’t be excluded. Last year’s AR5 said the range was 1.5 degrees C to 4.5, but this time it did not offer a best estimate. The reasons for the range is that observationally you can’t actually measure ECS precisely, and most of the IPCC case rests on model simulations. The best anyone can do is to estimate and argue, and dozens have.

Increasingly, however, recent estimates have become lower and lower. A new paper by Nick Lewis and Judith Curry (yes,  that Dr Curry) published in Climate Dynamics took the line of accepting the IPCC’s own estimates of uncertainty, forcing and heat uptake, and then using matched periods and observational data, rather than model simulations. The desired periods would have the same amount of volcanic activity, and ocean heat that was much the same in all datasets. The results can be seen in the table below.

slide11

The authors’ preferred period comparison, because it is the longest, and thus allows the largest change in forcing and also the lowest uncertainty, is shown in the first row of the table. Their best estimate for ECS is 1.64 degrees C, and for TCR 1.33 degrees C. You will see that the best estimate is much closer to the lower edge of the range than to the higher, which suggests (to me at least) that the range has a weighted lower bound — that is, the reality is much more likely to be low than high. The same applies to all the other period comparisons.

Why is this paper important? Because the authors have said, ‘OK. We’ll use the IPCCs own definitions and estimates, and then use observations, not model runs, to see what we get’. There is a fascinating debate in the Comments section of the Climate etc website on the rationale for doing this. So many (and I’m one) have become so tired of the IPCC’s reliance on untested and unverified models that we tend to suspect anything that depends on them.

But Lewis and Curry have accepted the IPCC base as their starting point, and that makes their conclusions really powerful. For if climate sensitivity is as low as 1.64, what is all the anxiety about? This paper is not necessarily a game-changer, but it demands a response from the orthodoxy. You can see an attempt over at Skeptical Science, but I’m not sure I understood what it was about.

[Update: There is a better critique on Lewis+Curry at RealClimate: http://www.realclimate.org/index.php?p=17582]

Join the discussion 29 Comments

  • dlb says:

    The last sentence in that SkS article makes clear what they are about.

    “Without swift emissions cuts, we can expect a serious level of warming – whatever climate sensitivity ends up being”

    In other words throw science out the window when it doesn’t suit your agenda.

  • David says:

    Don
    I really liked this post.

    I saw a summary of their work in the Oz. The one thing I was a bit curious about was the way Lewis and Curry calculated their estimate. According to Oz they compared the years 1995-2010 with 1862-1875 (I doing this from memory).

    I’m not across the statistics but that does have the wiff of “cherry picking” to me. Their analysis drops all the observations in between. Why would you do that? Obviously if you pick some different years you would get a different result.

    But accepted for publication, for fair enough. 🙂

    • Don Aitkin says:

      David,

      If you look at the table, the first four rows provide different matched periods. It’s cherry-picking in the right sense: they look for periods which, as far as possible, include only apples, not apples and oranges.

  • LenRunciter says:

    Indeed its an important paper…any study adding to compendium of work regarding climate sensitivity is important.

    As someone who is not a climate scientist, however, I’m not sure why those who are skeptics are so fascinated by this study. After all, what conclusions can be drawn from it…climate is indeed sensitive to CO2 and therefore subject to AGW, rather than being the frauds that the IPCC has been accused of this new work tells all that the IPCC in fact ranged their potential outcomes and this one falls on the low end of those predictions, and lastly, even this study shows the potential for even more dangerous warming as it doesn’t rule it out (more than 3 degrees)…it just says they believe that the likely outcome will be lower than suggested by the IPCC. They just have a different opinion, and even Judith Curry says her work is NOT the final say on climate sensitivity as they themselves made assumptions (something the IPCC has been criticized for).

    Does it say there is a hoax underway? Not at all. Does it invalidate the work of the IPCC? No. In fact, it shows that IPCC has looked into all the data. Is it devoid of criticisms as to assumptions made? No. I would suggest one just look at climate scientist Kevin Trenberth’s detailed take on things left out of the study. Does it say warming will stop? Actually it says the opposite…that it will continue…we just have some more time than suggested by the IPCC. Myles Allen, a climatologist, suggests maybe it will buy us a decade or two IF they are right.

    Drew Shindell, a climatologist at Duke, did a study on particulates in the atmosphere including aerosols, and found that those who have been predicting lower climate sensitivity have ignored their effects.

    “…and I conclude that the lowest end of the range of transient climate response to CO2 in present models and assessments7 (<1.3 °C) is very unlikely."

    http://www.nature.com/nclimate/journal/v4/n4/full/nclimate2136.html

    I'm not so sure this study is really what skeptics think it is.

    • Don Aitkin says:

      I’m not sure quite how to respond, since I say some of the things you say, and I do not think that the IPCC has produced a fraud or a hoax. And I said that you can’t do anything about climate sensitivity unless you make assumptions. L&C have used observations, principally, apart from their energy budget, while the IPCC authors used models as the basis for their argument. That is the principal difference.

      And aerosols: surely it is the case that we do not know enough about aerosols, and that they, and clouds, are the two largest areas of uncertainty.

      • Don Aitkin says:

        I can’t read the whole of the Shindell paper, not being a subscriber to Nature. But he is another who relies on models: ‘results from recent climate modelling inter comparison projects…’

      • LenRunciter says:

        Thanks for responding. Not that I was looking for that, I was just making an observation about what the Curry paper actually says, at least to this observer. But I’m glad we can agree on some things.
        Like I said, even Curry stated the imprecise nature of studying climate sensitivity, as did the IPCC.
        I might add that you mention below the issue of climate models. Curry and Lewis are actually “modelling” as well, in the best scientific sense. It may be different that the models the IPCC uses, but its still modelling in order to make predictions.

  • DaveW says:

    LenRunciter – I’m not sure why you think ‘skeptics’ are fascinated with the the Lewis & Curry paper. If you read the comments at Climate Etc, WUWT, etc you can find lots of scepticism on show. Don refers to ‘a fascinating debate’ in the comments. Not to a fascinating paper.

    Papers by lukewarmers don’t generally seem to fascinate people who reject climate orthodoxy. Lewis & Curry accept the assumptions and data of the IPCC and test them against observation and find that the models run hot. That is hardly surprising to anyone who has paid any attention over the last few decades. I suppose it is a bit surprising that they were able to get it published. Perhaps the climate mafia’s stranglehold is weakening. That would be a good thing.

    • LenRunciter says:

      Now why would you think its surprising they got it published? Do you really believe there is something like a climate mafia? The IPCC models that were used ran BOTH hot and lukewarm…not just hot. Its why they ranged it. Curry and Lewis found it to run lukewarm, and also ranged it to include the possibility of hot.

      The skeptic blogosphere ran with this story, starting with WUWT. That is a site that claims a hoax is underway, the IPCC can’t be trusted, that there isn’t anything to be worried about, and in fact that the warming has at a minimum paused and maybe even stopped. The Curry paper says no such thing, so its hardly supportive of that mindset. In fact, as I stated, it really says the opposite…it says climate is sensitive to CO2, certainly manmade CO2, and the planet is warming because of it. It isn’t stopping, and there is a chance that it could hit over 3 degrees warming. In their eyes not likely, but in the IPCC eyes more likely.

      That is what I find strange. Its hardly supportive at all of the things some of these sites have stated. If this is now what a site like WUWT is accepting, then so be it.

      • DaveW says:

        LenRunciter – read the Climategate emails and you will find all the evidence you need for a Climate Mafia. You could also do some googling to see what has happened to a number of scientists who made the mistake of not being orthodox enough. I’d also note that Professor Curry has stated that she requests that Michael Mann not be a reviewer when she submits papers.
        Curry posted the paper before WUWT, and as I’m sure you know, Professor Curry is skeptical of some claims of the CAGW crowd and has been vilified by many of them. You seem to be reading a lot more into the Lewis and Curry paper than is there or that the authors claim and, except for not trusting the IPCC, you are also misrepresenting what Anthony Watts seems to believe. Maybe you are confusing some commentators with the site owner?

      • Peter Kemmis says:

        Hi Len
        It’s pleasing to have you join the discussion. My conclusion after first becoming quite interested some four years ago in what we may well call the ‘climate war’ (although I think it has been more of a Clayton’s debate), is that for quite some time it has been very difficult for the sceptical side to be heard. This has applied in the media from print to TV and radio, for example with the BBC and Australia’s ABC notably trumpeting every report supporting the pro-AGW position, but rarely reporting on a contrary view or notable climate observations.

        I have attended a number of public fora at Australia’s national university, and the conviction of speakers in favour of the pro-AGW view is so strong that presentations are often quite anti-intellectual, where the purpose is to persuade by emotion, not by argument. In a recent one I attended, one speaker expressed the view that sceptics (whom he called ‘denialists’, which is quite insulting), should not be given equal time and space in the media! Ha! That’s a joke – the sceptical side has been receiving a fraction of the attention given to climate matters. This was a forum where the whole tenor was of persuasion by emotion, not by evidence.

        So the sceptic side of the debate has had to be carried on very much in the blogosphere. I have read quie a number of comments about the difficulties of getting sceptical papers published. Given the prevailing ready acceptance of the IPCC position, I am not at all surprised that sceptical papers are not well received. The herd mentality operates among editors and reviewers as well as anywhere.

        I think that what has changed over the last twelve months, is that more people in the professions are starting to question the orthodox view. But it still surprises me how little the tertiary trained that i meet with, actually know about the climate. One of my brothers-in-law has been a science teacher, and quite convinced that the IPCC’s projections are sound. But I discovered last month when I finally got him to discuss the issue, that he knew very little of what is currently the case with the climate. He had no idea that global temperatures have been more or less even over the last 17+ years. He had assumed they were rising as predicted. He said “well, the ice-caps are melting”, with no reference to any observations, and no awareness of the extent of sea ice in the Antarctic, or the recovery in the Arctic. This is the climate of opinion that I find around me all the time among the very people you would expect to be reasonably informed. You’ll find the same story about sea level rise – as you will know, it’s not accelerating, It’s rate has been 200-300mm per century, as we complete a warming phase from the last Little Ice Age.

        So I look forward to that mythical 50% of media exposure to the sceptic side of the debate.

        • LenRunciter says:

          H, Peter. I just wanted to take a step back and analyze, at least from my eyes (again, not a climate scientist) what the Curry study actually says/implies. I just found it interesting that it hardly is supportive of some of the skeptical theories out there. Like I said in my first post, regardless, its an important addition to a field (climate sensitivity) that we probably will never understand 100%.
          Is hard for me to respond to anything other than what I hear and see in the States (I live in New York), and I can tell you here that nearly every debate in the media on climate is covered from both sides. If its in the political arena, both a skeptic and believer are always on together. In the media, its always covered from both perspectives. In fact, the complaint among believers is that the media portrays it as if it is a 50/50 issue, when as you know, they believe that over 90% of climate scientists believe in AGW.
          Regardless, its always a heated debate. Let’s hope cooler heads will prevail (pun intended).

          • Peter Kemmis says:

            Hi Len

            That’s very interesting that you have a 50/50 coverage there in New York. Here in Australia my impression is that it runs at 95/05 – the latter the sceptical side. It may have something to do with our long history of breeding sheep, and perhaps our convict history, where we learned that to stay out of trouble, it was best to keep one’s head down. But we still get complaints that the media shouldn’t be giving so much air time to the “denialists”.

            Over the last few months, some interesting scientific facts have drifted across my glazed vision, pertinent to your thoughts about climate sensitivity. Another poster on this site, John Morland, sent me a little while ago this information (and I’m sure he won’t mind my quoting it here – and I’ll just reproduce the first link he mentions):

            “Have a look at the following links. The first one shows (at p 5) the high resolution of absorption wavelengths of atmospheric gases. You can see that both CO2 and CH4 relevant “greenhouse” absorption lines lie within water vapour (CH4 is at the edge). Note that CO2 15 micron absorption line is relatively wide and 100% whereas Methane’s (at 7.6 microns is narrower and substantially less than 100%.). Other absorption lines less than 5 microns are irrelevant as they are outside Earth’s IR emission into space ( in effect those lines tend to partly shade us from solar radiation – ie a mild cooling effect).
            http://irina.eas.gatech.edu/EAS8803_Fall2009/Lec6.pdf

            John’s point is that we know from lab tests how much (or how little) of the infrared spectrum is absorbed by these two greenhouse gases, and he has made the point to me that water vapour has a very much larger capacity to absorb infrared. This bears very much on the issue of climate sensitivity – and the question is, to what is it most sensitive?

            The second interesting fact relates to sunspots. As you will know, we’ve had a recent period of high solar activity in the latter part of the 20th century, and are now entering a period of lower solar activity with fewer sunspots. Sunspots emit ultraviolet light – that is why they appear black against the sun in the visible part of the spectrum. Ultraviolet light heats water (and water vapour) with about ten times the intensity of visible light, which might explain why I’ll get burnt here in sunny Oz even on cloudy days – perhaps I should drink less water and more beer! So with high sunspot activity, I’d expect to see more warming of water vapour, and of exposed ice and seawater. The key question is, if it’s not carbon dioxide and methane that are the significant ‘villains’, then what are major determinants for changing temperatures?

            The third fact is about the role of cosmic particles emanating from all kinds of galactic misbehaviour reminiscent of Guy Fawkes (he was the chap who tried to blow up the British Parliament in 1605, so you folk might approve of his attempt). Our Sun protects us somewhat from this persistent particle showering with its own magnetic shielding, but the effectiveness of that shield is cyclic. Svensmark and others have demonstrated that those cosmic particles can seed the beginnings of aerosols, and I find that very interesting, as I’m asking “what are the mechanisms that drive climate? and why do we have Ice Ages?”

            So if you have not yet done so, you might care to have a look at these points. I’ll be very interested in your thoughts.

          • LenRunciter says:

            Peter-
            The technical issues in some of those points are a tad above my pay grade, so I doubt I would have anything germane to add, other than to say yes indeed, natural variables are still and always will be part of the climate debate.
            One thing I could add…indeed…stick with more beer! Its always been my philosophy…particularly on Guy Fawkes Day!

      • DaveW says:

        Here’s a quick summary of the Climategate emails from WUWT:

        http://wattsupwiththat.com/2009/11/25/climategate-men-bahaving-badly-a-short-summary-for-laymen/

        You should read the WUWT About page too. I think Watts has a pretty much a middle of the road view on most topics. And I think I’ll retract my statement that Watts doesn’t trust the IPCC. My impression is that he doesn’t trust the executive summaries, but I may be just projecting my own distrust.

  • […] you read my post last week on the Lewis/Curry paper, you’ll know why she is […]

  • donaitkin says:

    A correspondent who found he could not get his comment on to the site has asked me to do so, and here it is:

    ‘I tried to post this in your comment and would appreciate some comment here

    More focus on the net energy contributed by Renewables is urgently needed so I am grateful for introduction to this research

    The approach is very Germanic – thorough and technical rather than economic

    I fear tho that the approach is too comprehensive to refreeze the melting ice feared by the Eco Energy Lobby

    So solar, biomass and wind, all of which need storage of some kind to be practical

    This seems to me the wrong emphasis. Clearly storage will improve with research but to rely on this is very risky and long term.

    A more simple approach is to estimate the net fuel contributed by Renewables to a Grid at the current or target level of acceptable risk of Black Out after allowing for fuel required for back up

    The capital cost of back up must be added the the cost of the Renewable

    I seem to remember some thought a decade ago that the net contribution of energy was near to zero

    The Grids must have these estimates but I cannot find any. GWFP and other interested Groups are shy of attempting a guestimate

    My hypothesis is that the net contribution of energy from Renewables is near zero and the cost fatuous.

    Can no one refute this hypothesis?

    If so Renewables are an empty cosmetic designed to meet Treaty obligations and harness a The Floating Green Vote.’

  • Don Aitkin says:

    A correspondent has asked me to help with his comment:

    ‘I tried to post this in your comment and would appreciate some comment here

    More focus on the net energy contributed by Renewables is urgently needed so I am grateful for introduction to this research

    The approach is very Germanic – thorough and technical rather than economic

    I fear tho that the approach is too comprehensive to refreeze the melting ice feared by the Eco Energy Lobby

    So solar, biomass and wind, all of which need storage of some kind to be practical

    This seems to me the wrong emphasis. Clearly storage will improve with research but to rely on this is very risky and long term.

    A more simple approach is to estimate the net fuel contributed by Renewables to a Grid at the current or target level of acceptable risk of Black Out after allowing for fuel required for back up

    The capital cost of back up must be added the the cost of the Renewable

    I seem to remember some thought a decade ago that the net contribution of energy was near to zero

    The Grids must have these estimates but I cannot find any. GWFP and other interested Groups are shy of attempting a guestimate

    My hypothesis is that the net contribution of energy from Renewables is near zero and the cost fatuous.

    Can no one refute this hypothesis?

    If so Renewables are an empty cosmetic designed to meet Treaty obligations and harness a The Floating Green Vote

    • LenRunciter says:

      Don-

      The one thing to always be encouraged about is human ingenuity. Whatever costs one may associate today with energy systems like solar, you can safely bet will be reduced as we continue to explore new technologies.

      This was just published on ExtremeTech…an Ohio State University initiative that just may very well address the issue of storage, in addition to reliability, when it comes to solar.

      http://www.extremetech.com/extreme/191438-this-air-breathing-solar-panel-stores-its-own-electricity-cutting-the-cost-of-solar-power-significantly

      “The new device, developed by Ohio State University, is essentially an air-breathing lithium battery that recharges via a built-in solar cell. This is significant, because one of the biggest problems with wide-scale solar power deployment is that you also need huge banks of batteries to store electricity — to even out spikes in generation when it’s cloudy or dark – and not only are those batteries expensive, but a lot of electricity is lost simply by traveling from the solar panels to external storage. An integrated solution is both cheaper and more efficient — about 25% cheaper and 20% more efficient, according to the researchers.”

  • […] in the three chapters dealing with observations. I made the same point recently about ‘climate sensitivity’: if it is small, then what are we worrying […]

  • […] wrote about climate sensitivity a while back, and for those who are new to this term, it refers to the assumption that while human […]

  • […] think that climate sensitivity is likely to be low, and since I’ve thought so too (see here), I’ve added that to the list. Then Lucia Liljegren (see my Blogroll — The Blackboard), who […]

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