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Estimating climate sensitivity from 3 million years ago

Posted on 9 May 2010 by John Cook

A key question in climate science is climate sensitivity. If the amount of CO2 in the atmosphere is doubled, the change in global temperature without feedbacks would be around 1°C. However, a number of feedbacks do occur - water vapour, snow albedo, sea-ice albedo and clouds. These respond relatively quickly, over a time-frame of years to decades, and are called "fast feedbacks". Modelling all the individual feedbacks can be problematic. However, we can empirically sidestep all this by using paleoclimate data to calculate the net response from fast feedbacks. A number of studies looking at various periods of Earth's past converge on a climate sensitivity of around 3°C (Knutti & Hegerl 2008). This means any initial warming is further amplified by positive feedback.

A new paper Earth system sensitivity inferred from Pliocene modelling and data (Lunt 2010) looks further into climate sensitivity as determined from the past. It examines the mid-Pliocene warm period,about 3.3 to 3 million years ago. This period is useful because CO2 levels and temperature were higher than pre-industrial conditions, giving us an insight into how climate responds when it's already warm. At this time, the main external forcing driving climate was tectonic changes in mountain ranges which led to changes in atmospheric CO2 (driven by both tectonic-related emissions and weathering).

What they find is the temperature response to changes in CO2 is 30 to 50% greater than the response based on fast feedbacks. This is due to other feedbacks operating over greater timescales (there is still uncertainty over the timescales involved, from hundreds to thousands of years). These slow feedbacks include changes in dust and other aerosols, vegetation, ice sheets and ocean circulation. This is confirmed by the geological record which records changes to ice sheets and vegetation over this period.

This means climate may be more sensitive to carbon dioxide than previously thought. On top of the fast feedbacks that cause the climate sensitivity of 3°C, additional slow feedbacks will add another 1 to 1.5°C warming. This result is confirmed by another recent paper also studying the Pliocene (Pagani 2010). This higher sensitivity should be taken into account when targets are set for limiting greenhouse gas emissions.

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Comments 101 to 115 out of 115:

  1. RSVP, you've got to make a stronger effort here.

    If by adding C02 to the atmosphere we cause it to be a more effective insulator, if we then remove C02 from the atmosphere we can expect it be a less effective insulator.

    In terms of net effect, this is simply insulation we're talking about.

    Do you live where it's necessary to have blankets on your bed? What happens when your blanket slips off? It's that simple. Please try a little harder.
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  2. johnd says:

    "If CO2 has been calculated as having a long residency time, what is the residency time of water vapour."

    CO2 has a short residence time (4-5 years) but a long adjustment time (50-200 years). CO2 needs modeling rather differently to other GHGs due to the large annual exchange flux.

    "Even though there is a high turnover of individual molecules, water vapour as a gas has residency time beyond measurement, a permanent presence that will exist whilst warmth from any source rises from the earth's surface. "

    Water vapour doesn't have a residence time beyond measurement, IIRC it is a couple of weeks. As I understand it warm air holds more water vapour than cold air, so if CO2 radiative forcing falls, and air temperature with it, then water vapour will precipitate out and there will be less radiative forcing from water vapour as well. It is a feedback in both directions.

    CO2 is a permanent presence in the atmosphere in exactly the same sense that water vapour is.
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  3. e says:

    "Long term processes such as rock weathering eventually start to remove CO2 from the atmosphere."

    I think the 200 year adjustment time is based on transfer of CO2 from the surface waters to the deep ocean, rather than chemical weathering, which acts on even longer time scales.
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  4. The need for postings to remain on topic is well understood for, each thread would quickly become chaotic, and people generally like to keep things neat and tidy not only in their minds, but in their forums as well. It has to be that way.
    The irony is that the matter being discussed, climate, is just the opposite, chaotic, and each individual factor somehow linked to interact with every other single factor.
    The subject is simply too vast and too complex for any single person to get their mind around, hence the need to break it down into handy bite sizes easily digestible, not only on forums such as this, but for the scientists as well.

    Ordered scenarios can be prodded and poked into a shape that than can be measured, and then modelled to yield results that reflect such an ordered system, but the focus is on the calculated outcome and the range of uncertainty essentially becomes off topic there also.
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  5. #47 Riccardo at 17:37 PM on 11 May, 2010
    there's no formal definition of slow or fast feedbacks, they must be considered relative to the time scale analyzed

    That's not an answer. It always bothers me that all kinds of feedback loops are discussed all the time without assigning proper time constants to them.

    For example atmospheric water feedback (including both vapor and clouds) has to be pretty fast. Residence time of water in the troposphere is about 9 days. Even in the lower stratosphere it is a month at most.

    It means it should respond to any changes in SST (Sea Surface Temperature) on this timescale. That is, if the water cycle is supposed to amplify CO2 forcing threefold as claimed, we should be able to detect the effect even in short records (several years), we do not need many decades of data for this particular purpose.

    This is exactly what Roy Spencer is doing recently. His upcoming paper in the Journal of Geophysical Research will be an interesting read.

    Spencer, R. W., and W. D. Braswell (2010),
    On the Diagnosis of Radiative Feedback in the Presence of Unknown Radiative Forcing,
    J. Geophys. Res., doi:10.1029/2009JD013371, in press. (accepted 12 April 2010)

    He does not address water feedback directly, but claims to have found a strong short term (~1 month) negative feedback based on 7-9 years of NASA CERES radiation budget data. For detecting fast feedback loops that much data should be more than sufficient.

    On the other hand, it is hard to imagine that there could be a strong short term feedback in the climate system other than atmospheric water. At least no one has found one so far.

    Anyway, if short term (up to a month or so) feedback is negative, all feedbacks operating on longer time scales (years, decades, centuries, millennia) can only take this controlled signal as input.

    It means that any long term positive feedback loop that would bring temperature anomaly up to 4-5°C for CO2 doubling as claimed by Pagani should supply a gain close to 10 (Dr. Spencer has found a 0.5°C short term equilibrium value for CO2 doubling, as opposed to the 3°C IPCC "consensus" figure).

    With an f value of ~0.9 the climate system would be dangerously close to a runaway feedback (f > 1). In this case any number of slight structural changes over the ages could push it over the limit. As it has never happened in billions of years, there can be no such a strong positive feedback whatsoever on any timescale.

    Therefore we should look for some explanation of past excursions of climate other than carbon dioxide "forcing" amplified by multiple positive feedbacks of different origins operating on all timescales.
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    Moderator Response: The right place to post this comment should be Climate sensitivity is low. Please always find the appropiate topic for your comments.
  6. doug_bostrom at 03:13 AM, whilst many people try to use a blanket as a simple analogy, that is making it too simple.
    For a start you need to clarify whether the house is on fire or not, because if it is, and your blanket slips off........
    The greenhouse effect is not really a mechanism providing our comfort as a blanket usually does, but one providing us with protection, as a blanket also can, so a more useful analogy than a blanket would be a firefighters protective gear.
    So what happens if the firefighters jacket slips off?
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  7. Not the case we're speaking of, johnd. Flipping the analogy is pointless.
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  8. Whilst the residency time of the various gases is often discussed, is it relevant?
    It is the residency time of the heat energy carried by the various gases and exchanged and transferred between the various gas molecules that is more important yet doesn't seem to be considered as something separate.
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  9. Johnd, regarding residency time there are scads of information on that topic liberally sprinkled hither and thither all over the Web, as well likely in your public library. Why do you want us to go through the exercise of repeating what has already been expressed hundreds if not thousands of times, available just a few keystrokes away?

    Make an effort.

    If you can't think of where to begin, consider starting here:

    Weart's Discovery of Global Warming


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  10. johnd at 07:14 AM on 13 May, 2010

    "The need for postings to remain on topic is well understood.......The irony is....The subject is simply too....handy bite sizes..."

    Not really johnd. This thread is about an estimate of long term climate sensitivity based on analysis of CO2 temperature relationships 3 million years ago. Two people (you and RSVP) have chosen to sidetrack the thread into a tedious pretend "argument" about a very well understood subject (waste heat generation and its relation to Earth thermal energy balance). A pretence that we don't know what we do know about a subject doesn't equate to an indication that a phenomenon is inherently "chaotic". It just means that it is easy for posters on blogs to hijack discussions. It happens all the time, especially in poorly moderated blogs.

    The reality is that the natural world doesn't conform to one (or two) persons real or pretend ignorance of a subject. Likewise flooding threads with false arguments, pursuing inappropriate "analogies", and acting as if scientific knowledge hasn't progressed beyond the junior school level doesn't equate to "skepticism". Skepticism only really has meaning in relation to an informed and honest appraisal of a subject.

    In reality, while weather is "chaotic", the thermodynamics of radiative balance in the Earth system isn't. That's not to say that we understand everything - the range of likelihood in climate sensitivity (very unlikely to be below 2 oC per doubling of [CO2]; unlikely, but with greater uncertainty that climate sensitivity is greater than around 4.5 oC) is an indication of that. However the latter has got nothing to do with "chaos". It's largely to do with uncertainty in quantitating atmospheric aerosol forcings, cloud feedbacks and the various response times of the climate system.
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  11. #105 Moderator Response: The right place to post this comment should be Climate sensitivity is low.

    I see. In reply to an article claiming climate sensitivity to be high, anyone who thinks otherwise should post comments elsewhere. Makes sense.
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  12. Berényi Péter at 07:22 AM on 13 May, 2010

    Although the moderator doesn't like Peter Berenyi's post here, I would have thought it was appropriate. After all the thread is about Earth climate sensitivity and that's what Peter's post is about. So in the assumption that posts about climate sensitivity are O.K. (as opposed to digressions into "waste heat"!), I'm going to respond to Peter.

    A problem with your post (Peter) is that the conclusion ("Therefore we should look for some explanation...") is built on a set of premises some of which are unlikely to be justified:

    (i) It's worth stating from the outset that Spencer and Braswell don't conclude anything about the Earth climate sensitivity from their paper in press that you refer to, and in fact state explicitly that their analysis doesn't necessarily have any bearing on Earth climate sensitivity as commonly regarded (the change in Earth surface temperature at equilibrium in response to radiative forcing equivalent to a doubling of atmospheric [CO2]; see bottom of post [***]). Nowhere in their paper do they suggest that the climate sensitivity is 0.5 oC (they don't conclude a climate sensitivity at all). I think you may have fallen for blogosphere over-interpretation, whereby someone pretends on his blog that a paper means something that it doesn't actually mean!

    So your major conclusion ("Therefore we should look for some explanation...") is invalid (non-sequitur).

    (ii) It’s also worth having a look at what Spencer and Braswell have actually done. One can usually get a good idea by looking at the abstract; it's reproduced at the bottom of this post [*****].

    A previous analysis (Forster and Gregory, 2006) has made a much more substantial and quantitative analysis of this subject with a good discussion of the problems, and is rather more understandable than Spencer and Braswell. The aim is to make the most direct measure of feedback by direct analysis of the combined long wave IR emitted from the earth surface (and shortwave IR reflected by the atmosphere) in response to changes (fluctuations) in the measured sea surface temperature. If the temperature rises, the change in emitted IR (measured at the top of the atmosphere by satellites; TOA) should be equal to the “blackbody” radiation determined using the Stefan-Boltzmann relationship in a system with no very fast feedbacks. This is around 3.3 W.m-2.K-1. In other words just as the Earth should warm by around 1.1 oC per doubling of [CO2] with a climate system with zero feedbacks, so an Earth temperature rise of around 1 oC should result in an enhanced IR emission of 3.7 W.m-2 as the climate system “tries” to recover radiative equilibrium. If there is a very fast positive feedback the change in emitted IR will be less than this (because the positive feedback acts to "trap" some of the LWIR escaping to space). If there is a very fast negative feedback, then the increase in TOA IR will be less than this. Forster and Gregory found that the change in TOA IR is 2.3 +/- 1.4 W.m-2.K-1, (i.e. positive feedback since the change in TOA emission is less than the blackbody value), consistent with a (fast) climate sensitivity response of 1.0 – 4.1 oC per doubling of [CO2]. i.e. unfortunately poorly constrained.

    (iii) Spencer and Braswell use a variation of this in which they compare monthly averages of sea surface temperatures with monthly averaged satellite TOA measures and make regressions of the data to pull out a TOA radiative response to temperature changes. At this point their analysis becomes somewhat obscure (to me anyhow), and they compare the patterns of their regressions with model data and identify “striations” and “spiral patterns”. Their apparent change in TOA emission is 6 W.m-2.K-1. However (see [***] below), they conclude that this doesn’t necessarily relate to a climate sensitivity.

    (iv) There is an inherent problem with the claim of a negative fast feedback (Spencer claims this on his blog even if he doesn’t in the paper) and this relates to the fact that the fast feedback is bound to involve water vapour. There is no question that the atmosphere responds to warming with an increase in absolute humidity (see papers on this here ). So there really has to be a positive feedback to the warming from enhanced [CO2] with respect to water vapour. Cloud changes could provide a fast feedback (and presumably that is where Spencer and Lindzen – although the latter’s analysis was incorrect – would source their putative negative feedback). However Forster and Gregory conclude (with poor certainty) that the cloud response was neutral over the time of their measurements. The only other direct analysis of cloud feedback concludes that the cloud feedback is positive (over the NE Pacific anyhow!)

    (v) Finally, the temporal evolution of warming since the middle-late 19th century rather precludes negative feedbacks (clouds or otherwise) on the timescales relevant to climate sensitivity. Since we’ve had around 0.9 oC of warming during this period in response to enhanced [CO2] (290 ppm – 388 ppm) which should give around 0.4 oC of warming at equilibrium with a climate with zero feedbacks (let alone negative feedbacks) , significant negative feedbacks seem very unlikely (after all where have they been??), particularly as the solar contribution to this temperature rise is likely no more than 0.1 oC, and that some of the enhanced greenhouse warming has been offset by anthropogenic aerosols . Of course analyzing this properly requires a proper analysis! (try e.g. here, or here
    -------------------------------------------------
    [***]e.g. Spencer and Braswell conclude:

    "Although these feedback parameter estimates are all similar in magnitude, even if they do represent feedback operating on intraseasonal to interannual time scales it is not obvious how they relate to long-term climate sensitivity."

    and
    "Since feedback is traditionally referenced to surface temperature, extra caution must therefore be taken in the physical interpretation of any regression relationships that TOA radiative fluxes have to surface temperature variations."


    [*****] abstract: “The impact of time-varying radiative forcing on the diagnosis of radiative feedback from satellite observations of the Earth is explored. Phase space plots of variations in global average temperature versus radiative flux reveal linear striations and spiral patterns in both satellite measurements and in output from coupled climate models. A simple forcing-feedback model is used to demonstrate that the linear striations represent radiative feedback upon non-radiatively forced temperature variations, while the spiral patterns are the result of time-varying radiative forcing generated internal to the climate system. Only in the idealized special case of instantaneous and then constant radiative forcing -- a situation that probably never occurs either naturally or anthropogenically – can feedback be observed in the presence of unknown radiative forcing. This is true whether the unknown radiative forcing is generated internal or external to the climate system. In the general case, a mixture of both unknown radiative and non-radiative forcings can be expected, and the challenge for feedback diagnosis is to extract the signal of feedback upon non-radiatively forced temperature change in the presence of the noise generated by unknown time-varying radiative forcing. These results underscore the need for more accurate methods of diagnosing feedback from satellite data, and for quantitatively relating those feedbacks to long-term climate sensitivity.”
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  13. Tiny error in my long post just above.

    Though no one will likely notice, under (ii) it should say:

    "If there is a very fast negative feedback, then the increase in TOA IR will be more than this."
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  14. doug_bostrom at 03:13 AM on 13 May, 2010
    "RSVP, you've got to make a stronger effort here."

    For the sake of a possible "new" reader, and so that you know I understand what you are referring to, allow me to first clarify the context of your remark.

    Correct me if I am wrong..., I am somehow not able to see why the Earth's temperature should stabilize to where it "should" be if all radiative forcings were to be "pre-Industrial Revolution", etc.

    I am actually basing my question around the radiative forcing model, which assumes that only "changes" in forcing can cause "changes" in temperature. Kind of like an aircraft's bank angle... it doesnt right until steering corrects, etc. Another analogy would be voltage or drift in a circuit. You can have unwanted voltages lingering due to poor grounding. No extra energy is needed.

    So if intuition leads one to think that the Earth's temperatures would return to normal if all humans just disappeared all at once, it probably has to do with the fact that the amount of IR radiated is a function of temperature, and that that radiative model is flawed as it doesnt take into account this nonlinearity.
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  15. Berényi Péter,
    "That's not an answer."
    Indeed! The question was ill posed, mine was a request for clarification. Now I see that you were not interested in asking.

    Lunt et al. paper looks at the equilibrium climate sensitivity, i.e. at the long (thousands of years) time scale; already in the abstract they point out that this long time scale response is often negleted. You contrasted it with a sensitivity derived over an extremely short (months) time scale. Your comparison has not much value.
    Let me remind you that upon looking at progressively shorter time scales you'll "discover" the strongest and fastest negative feedback we know, thermal emission. And let's not forget that feedbacks are additive.
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  16. RSVP - you stated that the radiative model doesn't take into account nonlinearity of IR vs. temperature - I would have to beg to differ, as ALL models do take this into account. Any model using the Stefan–Boltzmann equations, as all radiative models do AFAIK, incorporate the fact that the power radiated scales as the 4th power of the temperature.

    Incidentally, if all humans disappeared at once, it would be quite some time before the CO2 forcing (and all the feedbacks) backed off, allowing temperatures to lower. I believe a number of people have modeled that already. But removing CO2 emissions would definitely change the forcings on temperature!
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  17. Berenyi Peter: #46
    "What are the timescales involved? I understand slow feedbacks are supposed to operate on timescales from hundreds to thousands of years. But how fast are fast feedbacks? Days? Weeks? Years? Decades? "

    See, here's where, crazy as it seems, *reading the paper* might help. The term 'fast feedbacks' as used by Lunt et al. is defined in the first paragraph:

    "[Climate sensitivity] is usually defined as the increase in global mean temperature owing to a doubling of CO2 after the ‘fast’ short-term feedbacks, typically acting on timescales of years to decades, in the atmosphere and upper ocean have had time to equilibrate5"

    Ref5 is
    Hansen, J. et al. in Climate Processes and Climate Sensitivity (eds Hansen, J. E. & Takahashi, T.) 130–163 (American Geophysical Union, 1984).

    HTH.
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  18. FYI The Spencer & Braswell (2010) thing is continued here
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