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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How sensitive is our climate?

What the science says...

Select a level... Basic Intermediate Advanced

Net positive feedback is confirmed by many different lines of evidence.

Climate Myth...

Climate sensitivity is low
"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)

 

Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we double the amount of carbon dioxide in the atmosphere.  This includes feedbacks which can either amplify or dampen that warming.  This is very important because if it is low, as some climate 'skeptics' argue, then the planet will warm slowly and we will have more time to react and adapt.  If sensitivity is high, then we could be in for a very bad time indeed.

There are two ways of working out what climate sensitivity is. The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence, by looking at climate changes in the distant past:

adapted fig 3a

Various paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras.  Adapted from PALEOSENS (2012) Figure 3a by John Cook.

These calculations use data from sources like ice cores to work out how much additional heat the doubling of greenhouse gases will produce.  These estimates are very consistent, finding between 2 and 4.5°C global surface warming in response to doubled carbon dioxide.

It’s all a matter of degree

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against reducing greenhouse gas emissions because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

Basic rebuttal written by GPWayne

Last updated on 1 August 2013 by gpwayne. View Archives

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Related Arguments

Further reading

Tamino posts a useful article Uncertain Sensitivity that looks at how positive feedbacks are calculated, explaining why the probability distribution of climate sensitivity has such a long tail.

There have been a number of critiques of Schwartz' paper:

Comments

1  2  3  4  5  6  7  Next

Comments 1 to 50 out of 312:

  1. Hi John,

    Excellent blog.

    With regard to the climate sensitivity issue, I did an analysis based on Schwartz' assumptions for climate sensitivity (1.1 deg C) and time constant (5 yrs) described here.

    I wanted to see what kind of warming (due to CO2) that his assumptions would predict for the past 3 decades and how that compares to what actually happened. His predicted result does not appear to jibe with what actually happened.
  2. Do all of these people make the same basic mistakes as Annan? That is to assume CO2-warming with reference to volcanic cooling or solar warming?

    Annans snapshot of 20,000 years ago is an example of studiously IGNORING the empirical evidence rather than empirically showing anything. We know that glaciations come and go. We also know that the CO2 FOLLOWS AND DOES NOT LEAD the changes in temperature. So for him to grab that snapshot in time was just him filling out a troika of Unscience and non-evidence.

    Hopefully someone will let me know if any actual evidence is contained in any of the studies.
  3. So what method do you propose, GWB?
  4. GMB
    Apparently you have some good backup out there. The last 3 papers I read on prior warming maximums all said that CO2 lagged temps by quite a bit. Dr. R. Spencer of NASA is explaining to congress that the sensitivity is much less than what the IPCC says it is.
  5. Re #2 GMB

    You've got that wrong. Schwartz is the one that is using very short time series (largely 40-50 year periods in the 20th century) to estimate his "sensitivity". Your comments about CO2 and its relation to temperature change during ice age cycle transitions are not relevant in the manner that you might wish your capitalized phrase to insinuate.

    There are two essential methods of determining climate sensitivity in relation to real world measurements (a third would be to use a completely theoretical analysis). These are:

    (i) Determination of the relationship between equilibrium temperature and atmospheric CO2 from paleodata. This is in principle preferable since the analyis can be made with respect to "equilibrium" situations. i.e. since the climate sensitivity relates explicitly to the earth's surface temperature rise AT EQUILIBRIUM per doubling of atmospheric CO2, this should give us the more accurate analysis. Unfortunately there are uncertainties due to the uncertainties in the paleodata.

    (ii) The second is to eschew equilibrium measurements and monitor the temperature response to enhanced greenhouse forcing as the temperature rises TOWARDS it's new equilibrium temperature. Simplistically (and Schwartz has used this simplisitic approach) one considers that there will be a hyperbolic rise to a new equilibrium temperature. A regular hyperbola is characterized by its MAXIMUM VALUE at equilibrium (in this case the climate sensitivity) and a TIME CONSTANT that characterises the rate at which equilibrium is achieved. Obviously if the maximum (equilibrium) hasn't been reached one needs to estimate the parameters defining the "shape" of the response.

    Schwartz makes several errors and unrealistic asumptions that are outlined in some of the links in John's top post. He uses detrended time series of 20th century temperature trends, makes an unrealistic assumption that the climate system response can be characterised by a single exponential (one time constant), and comes up with a time constant of 5 years. Unfortunately his detrending smooths out the longer time constants that alomst certainly apply to the system.

    In essence he's attempting to pursue the conclusion that we've had much of the warming due to the increased forcing already. However if more realistic multiple time constant series are used (i.e. his data are heavily biased towards the very rapid time constant(s) for atmospheric warming in response, for example, to volcanic eruptions, the less rapid time constant for ocean surface warming and so on), his climate sensitivty value increase rather markedly. And since the oceans are a repository of thermal inertia limit a rapid re-equilibration of the earth's surface temperature, it's rather more valid to include long time constants when considering the EQUILIBRIUM response of the Earth's temperature to greenhouse warming.

    In fact if a rather more realistic time constant of 15 years is used, Schwartz's climate sensitivty becomes near 3 oC per doubling of atmospheric CO2, pretty much equivalent to all of the other published data.

    It's pretty clear from reading Schwartz's paper that he realises that his analysis is extremely oversimplified.

    I'm not sure what you consider to be the relevance of the fact that warming leads rising atmospheric CO2 during the glacial-interglacial transitions for this analysis. The aim of all of these analysis is to determine the best estimate of the climate sensitivity to rising atmospheric CO2 levels. The source of the rise in atmospheric CO2 is not really relevant for these analyses...



    PS: Having read what I've written before posting it, I realize I've said quite a bit of what John Cook has already said in his top article !
  6. Response to comment 62 (Quietman) in "It's volcanoes (or lack thereof)":

    "You still assume AGW is a big player and I do not. See the sensitivity thread. "

    No, I don't assume, I conclude. There isn't much more I could say about that, after:

    http://blogs.abcnews.com/scienceandsociety/2008/07/tropical-storm.html

    http://blogs.abcnews.com/scienceandsociety/2008/07/global-warming.html#comments

    http://blogs.abcnews.com/scienceandsociety/2008/09/nature-is-not-a.html

    (PS in the first of those, before I realized "Science and Society" deletes website addresses, I had posted some links to some RealClimate posts where I had some comments. Might go back and repost those RealClimate sites here later... I remember one was about basic thermodynamics.)

    Relavant to climate sensitivity and time constants, I had made some comments in the last of those three "Science and Society" sites above, where it was found:

    For a linearized approximation of climate behavior, in terms of global average surface temperature, with constant effective heat capacity and climate sensitivity:

    If the time constant is also the e-folding time for exponential decay to equilibrium (that's my understanding of the term 'time constant'):

    Time constant (or e-folding time) = heat capacity (average per unit area) * climate sensitivity.

    For example, for a climate sensitivity of 0.7 K/(W/m2) and the globally-averaged effect of the heat capacity of the top 100 m of ocean, I found an e-folding time of about 6 years. From this website, the time constant may be more like 15 years or so (for a prolonged climate forcing change, which can then penetrate below the top of the ocean), so with the same climate sensitivity, the effect heat capacity may be more like that of the top ~ 250 m of ocean (but this is only a mathematical equivalence - the warming signal would penetrate more or less deeply at different locations depending on the circulation patterns).

    If it turns out that

    1.
    solar effects outside of TSI(multiplied by 0.7/4 to get climate forcing) and enhanced solar UV (I'm assuming that's taken into account in at least some models, since it is understood that a greater portion of solar variation is in UV and UV affects the upper atmosphere (stratosphere, etc.) in particular), such as magnetic effects and solar wind,

    2.
    variations in ocean tidal forcing, it's effect on ocean circulation and mixing.

    3.
    geomagnetic changes outside of effects of solar changes on magnetosphere

    4.
    volcanism or whatever

    ...
    if any of these turns out to have been more important than previously judged, the conclusion would not be that AGW is not important but that the climate forcing has been bigger than it was thought and so the climate sensitivity must not be as great -

    OR

    perhaps the climate sensivity is what we thought but we have underestimated the cooling effect of our aerosol emissions...

    Etc.

    PS of 1-4 above, ... well I'm skeptical. I'm interested in learning more about 1-3, though, although I'm doubtful of the effectiveness of proposed mechanisms for 1 and 3 (though there was one article you had mentioned some time ago that looked intriguing... about variations in transmissivity of clear sky atmosphere due to electromagnetic/ionospherice/solar wind/related stuff...); for 2, I'm doubtful as to the strength of the variations over the time periods pertinent to AGW (and not clear that it was really asserted that 2 would account for some significant portion of what has been attributed to AGW forcing)
  7. Some general info: "The Physical Science behind Climate Change Why are climatologists so highly confident that human activities are dangerously warming Earth? Members of the IPCC, the 2007 peace winner, write on climate change By William Collins, Robert Colman, James Haywood, Martin R. Manning and Philip Mote" link  "Simple Question, Simple Answer… Not" Real Climate, I have comments here (numbers of comments that were mine:) Real Climate (11,86,98,109,132,138,141) Real Climate (104,105,111) Real Climate (59,123,147,152,153,159,160,171,175,189,193,195,197,218,234,236,239,251,257,265,266,267,268(repeated an error regarding $700 billion, oops!),271,273,285,294, NOTE ALSO these comments that are not mine: 102,226 and the the responses to 81,166,201) Real Climate (9,10,11,49,50,60,138,142,161,166,170,171,175) HERE IS THE COMMENT ON THERMODYNAMICS (and some other comments): Real Climate (69,73,74,75)

    Response:

    [RH] Fixed links that were breaking page format.

  8. Patrick

    William Collins, Robert Colman, James Haywood, Martin R. Manning and Philip Mote"

    "Drivers of Climate Change
    Atmospheric concentrations of many gases—primarily carbon dioxide, methane, nitrous oxide and halocarbons (gases once used widely as refrigerants and spray propellants)—have increased because of human activities."

    True IF based on high sensitivity.
  9. Re: "Simple Question, Simple Answer… Not"

    Interesting comments from John Mashey:

    [[ John Mashey Says:
    8 September 2008 at 2:31 PM
    “These people, typically senior engineers, get suspicious”.

    Please, can we get deeper than “senior engineers” - that really isn’t improving insight. If we want to do that, we need to probe a lot deeper than just “senior engineers”.

    Let me offer a speculation, although not yet a serious hypothesis:

    1. SPECULATION
    Amongst technically-trained people, and ignoring any economic/ideological leanings:

    1) Some are used to having
    a) Proofs
    OR
    b) Simple formulae
    OR
    c) Simulations that provide exact, correct answers, and must do so to be useful
    d) And sometimes, exposure to simulations/models that they think should give good answers, but don’t.

    2) Whereas others:
    a) Are used to missing data, error bars,
    b) Complex combinations of formulae
    c) Models with varying resolutions, approximations, and that give probabilistic projections, often only via ensemble simulations.
    d) Models that are “wrong”, but very useful.

    My conjecture is that people in category 1) are much more likely to be disbelieving, whether in science, math, or engineering.

    2. ANECDOTAL EXAMPLES:
    1) In this thread, a well-educated scientist (Keith) was convinced that climate models couldn’t be useful, because he was used to models (protein-folding) where even a slight mismodel of the real world at one step caused final results to diverge wildly … just as a one-byte wrong change in source code can produce broken results.

    See #197 where I explained this to him, and #233 where light dawned, and if you’re a glutton for detail: #66, #75,l #89, #1230, #132, #145, $151, #166 for a sample.

    2) See Discussion here, especially between John O’Connor & I. See #64 and #78. John is an EE who does software configuration management. When someone runs a rebuild of a large software system, everything must be *perfect*. There’s no such thing as “close”.

    Also in that thread, Keith returned with some more comments (#137) and me with (#146), i.e., that protein-folding was about as far away from climate modeling as you could get.

    3) Walter Manny is a Yale EE who teaches calculus in high school. He’s posted here occasionally (Ray may recall him :-), and participated in a long discussion at Deltoid, and has strong (contrarian) views. In many areas of high school/college math, there are proofs, methods known for centuries, and answers that are clearly right or wrong.

    4) “moonsoonevans” at Deltoid, in #21 & #32 describes some reasons for his skepticism, #35 is where light dawns on me. He’s in financial services, had experienced many cases where computer simulations done by smart people didn’t yield the claimed benefits. In #35 I tried to explain the difference.

    All this says that if one is talking with an open-minded technical person, one must understand where *they* are coming from, and be able to give appropriate examples and comparisons, because many people’s day-to-day experience with models and simulations might lead them to think climate scientists are nuts.

    3. A FEW SPECIFIC DISCIPLINES & CONJECTURES

    1) Electrical engineers (a *huge* group, of which only tiny fraction are here)

    Many EEs these days do logic design, which requires (essentially) perfection, not just in the design, but (especially) in simulation.

    Design + input =>(logic simulator) => results

    At any step, the design may or may not be bug-free, but the simulator *must* predict the results that the real design would do given the input, exactly, bit for bit. Many test-cases have builtin self-checks, but the bottom line is that every test-case yields PASS or FAIL, and the simulator must be right.

    Many people buy simulators (from folks like Cadence or Synopsys), and run thousands of computers day and night simulating millions of test-case inputs. But, with a million test-cases, they’re not looking for an ensemble that provides a distribution, they’re looking for the set of test-cases to cover all the important cases, and for EVERY one to pass, having been simulated correctly. This has some resemblance to the protein-folding problem mentioned above.

    Now, at lower levels of timing and circuit design, it isn’t just ones and zeroes (there’s lots of analog waveforms, probabilistic timing issues, where one must guarantee enough margin, etc). When I’d tease my circuit designer friends “Give me honest ones and zeroes”, they’d bring in really ugly, glitchy HSPICE waveforms and say “so much for your ones and zeroes”. (This is more like the molecular “docking” problems that Keith’s colleagues mentioned.)

    At these levels, people try to set up rules (”design rules”) so that logic designers can just act at the ones-and-zeroes level.

    If one looks at EEs who worry about semiconductor manufacturing, they think hard about yields, failure attribution, and live with time-series. (Standard answer to “We got better yield this month, how do you think it looks?” was “Two points don’t make a trend.”

    2) Software engineers

    Programs often have bugs, but even a bug-free program can fall apart if you change the wrong one byte of code, i.e., fragile. (I don’t recall the source, but the old saw goes something like: if skyscrapers were like software, the first woodpecker would knock them over.)

    Configuration management / software rebuilds are fairly automated these days, and they must be correct. One cannot include the wrong version of code, or compile with incompatible options.

    Performance engineering and benchmarking tend to be more probabilistic-oriented, and although a lot of people want to believe in one number (once the mythical “MIPS” rating), we’ve (mostly) fixed that over the last 20 years. Good performance engineers have always given relative performance ranges and said YMMV (Your Mileage May Vary).

    3) Mechanical engineers

    This, I expect, varies. In some cases, closed-form equations work pretty well. In other cases, one is using big structural dynamics and computational fluid dynamics codes to obtain “good-enough” approximations to reality before actually building something. For example, automobiles are extensively modeled via “crash codes”.

    4) Petroleum engineers

    It’s been a while, but certainly, people who do seismic analysis and reservoir modeling *start* with data from the real world, analyze it to make decisions, so ought to be a little more accustomed to probabilistic analyses.

    5) Financial engineers (Google: financial engineering)

    Not having physics to constrain simulations yields some wild results, although at least, some people are very comfortable with risk, uncertainty, and ensemble projections. I especially like Sam Savage’s Flaw of Averages”.
    On the other hand, when Nobel Economists lose $B (LTCM), I’m not surprised there is skepticism about climate models.

    4. CONCLUSIONS

    That’s a speculative start. I do *not* think lumping a large group together as “senior engineers” helps progress, because I have at least anecdotal evidence that the sources of skepticism tend to be attached to the kinds of models and (maybe) simulations that someone does day-by-day. The problem is that many people tend to generalize from their discipline to others, and especially if they have trouble getting useful models, they tend to be suspicious of others’.

    At one extreme, people have long-established mathematical proofs, and answers that are clearly right or wrong.

    At the other extreme, people have to make decisions based on the best approximations they can get, and if their discipline has good-enough approximations, they tend to think one way, and if the approximations aren’t so good, they may think another about equations and climate models. ]]

    Since both 2 and 3 apply personally I can relate.
  10. Patrick
    Science and Society is a good blog but Disney owns ABC and therefore the site and does not allow links. That is why an argument on AGW is better done here. John allows links and actually prefers hyperlinks. But he has asked us to keep the discussion pertinent to the thread and not post "lists of links" so I try to break up my comments for readability.

    PS
    I looked at your remark on THERMODYNAMICS but skipped most of the other links purely because I don't care to go to that web site. Articles and Papers are much more appreciated as links (less opinion and more facts).
  11. I decided to repost some comments at http://blogs.abcnews.com/scienceandsociety/2008/07/global-warming.html#comments - The comments about Monckton's paper, at least one (big) one of which was deleted from that website, hence this repost:

    ---

    MORE ARTFULLY WRITTEN CRITIQUES FOUND AT RealClimate: "Once more unto the bray", DELTOID: "Monckton's Triple Counting"

    MONCKTON'S PAPER "Climate Sensitivity Reconsidered":

    (Monckton has a prior record of demonstrating an apparent lack of accuracy in this subject matter.)

    INTERNAL VARIABILITY

    --"[G]LOBALLY-AVERAGED land and sea surface absolute temperature TS has not risen since 1998 (Hadley Center; US National Climatic Data Center; University of Alabama at Huntsville; etc.). For almost seven years, TS may even have fallen (Figure 1). There may be no new peak until 2015 (Keenlysideet al., 2008)."
    "The models heavily relied upon by the Intergovernmental Panel on Climate Change (IPCC) had not projected this multidecadal stasis in “global warming”; "

    Of course they haven't, because:

    1.the timing of specific cases of such short term interannual to decadal variability is not so important to longer term climate trends, and even to the extent that models may reproduce the general characteristics of variability, such variations get averaged to near-zero in combining multiple runs of models.

    2. 1998 was extra warm because of the El Nino. There will be, in any given period of sufficient length, some number of warmer years and cooler years relative to any longer trend. One can't conclude there is/has been a 'stasis', especially a 'multidecadal stasis', in the warming.

    ________

    --"nor (until trained ex post facto) the fall in TS from 1940-1975;"

    what 'trained'?

    --"nor 50 years’ cooling in Antarctica (Doran et al., 2002)"

    Ozone hole has a regional effect there, and I can't take Monckton's word for it that the Antarctic has cooled for as long as 50 years. Anyway, at least a part of it have warmed.

    --"and the Arctic (Soon, 2005);"

    The arctic has been warming.

    --"nor the onset, duration, or intensity of the Madden-Julian intraseasonal oscillation, "...

    Models can't reproduce all aspects of internal variability yet. So they aren't perfect. That doesn't invalidate all of what they can do.

    --"(oceanic oscillations which, on their own, may account for all of the observed warmings and coolings over the past half-century: Tsoniset al., 2007);"

    Then where has all the warming from greenhouse gases gone?

    The rest of that paragraph - short term variability, so what. Medieval Warm Period - perhaps mainly a Northern Hemisphere or European phenomenon; not so big globally. Warming on other planets - Pluto's temperature response to the eccentricity in it's orbit lags the forcing due to thermal inertia; what warming on Jupiter? (recent circulation changes may be an internal variability - which, I believe, was predicted by a model!); Mars' albedo is affected by Dust storms, and is the warming global? Earth's climate history is understood generally better than those of other planets. Solar Grand Maximum - is it grand enough?

    ________

    Reproduction of Hansen's graph from 1988: - it's a mischaracterization; for the emissions scenario we've most closely followed, the temperatures have followed quite closely thus far.

    ________

    CLIMATE VS WEATHER

    --"The climate is “a complex, non-linear, chaotic object” that defies long-run prediction of its future states (IPCC, 2001), unless the initial state of its millions of variables is known to a precision that is in practice unattainable,"

    He's confusing climate with weather and ignoring different timescales.

    --"combined with a heavy reliance upon computer models unskilled even in short-term projection, with initial values of key variables unmeasurable and unknown, with advancement of multiple, untestable""

    so what? This isn't weather prediction.
    ________

    DEFINITIONS

    --"consequent increase in aggregate forcing (from Eqn. 3 below) of ~0.26 W m–2, or <1%. That is one-twentieth of the value stated by the IPCC. The absence of any definition of “radiative forcing” in the 2007 Summary led many to believe that the aggregate (as opposed to anthropogenic) effect of CO2 on TS had increased by 20% in 10 years. The IPCC – despite requests for correction – retained this confusing statement in its report."

    Anyone who's confused could have looked at the other numbers given by the IPCC to figure it out. In the context of climate CHANGE radiative forcing is often discussed as the CHANGE from some reference level.

    ________

    --"non-Popper-falsifiable theories,"

    Give it time and we'll see. So far things are happenning that have been expected. But for the purposes of public policy, this could also be seen to a degree as an application of scientific knowledge already gained - we know A, B, C, so we expect if we do D we'll get E. etc.

    --"and, above all, with the now-prolonged failure of TS to rise as predicted (Figures 1, 2), raise questions about the reliability and hence policy-relevance of the IPCC’s central projections.""

    There has not been such a prolonged failure. We expect a few bumps and dips.

    ________

    feedbacks:

    Unless otherwise specified, the climate sensitivity to radiative forcing by a doubling (or whatever change) of CO2 is based on the radiative forcing of the doubling of CO2, whatever the source of CO2 is, including feedbacks.

    --"For this and other reasons, it is not possible to obtain climate sensitivity numerically using general-circulation models: for, as Akasofu (2008) has pointed out, climate sensitivity must be an input to any such model, not an output from it."

    Nonsense. Climate sensitivity is input only implicit - it is the evaluation of model output that can determine explicitly what the sensitivity is.

    ________

    Radiative forcing: This is basic physics; if the amount and the radiative properties are known, radiative forcing can be calculated. For CO2 and generally other gases, both are known quite well. Some things about clouds are known, but the feedbacks from changes in the amount of different types of cloud in different places is a source of uncertainty. In so far as external radiative forcing, I think aerosols have the greatest uncertainty, in part because of more complex effects.
    ________

    --"The signature or fingerprint of anthropogenic greenhouse-gas forcing, as predicted by the models on which the IPCC relies, is distinct from that of any other forcing, in that the models project that the rate of change in temperature in the tropical mid-troposphere – the region some 6-10 km above the surface – will be twice or thrice the rate of change at the surface (Figure 4):"

    That's a general tendency with any global warming - it isn't as apparent in the graph for the response to solar forcing because the solar forcing used is so small in comparison. The fingerprint of increased greenhouse gases is cooling of the upper atmosphere, which has occured, and is not expected from a positive solar forcing. Granted that the response to ozone changes is more similar to greenhouse gas forcing (though there are differences), but it wouldn't make sense to arbitrarily suppose we can't tell at all how much is from what, especially given knowlege of radiative forcing.

    --"However, as Douglass et al. (2004) and Douglass et al. (2007) have demonstrated, the projected fingerprint of anthropogenic greenhouse-gas warming in the tropical mid-troposphere is not observed in reality. Figure 6 is a plot of observed tropospheric rates of temperature change from the Hadley Center for Forecasting. In the tropical mid-troposphere, at approximately 300 hPa pressure, the model-projected fingerprint of anthropogenic greenhouse warming is absent from this and all other observed records of temperature changes in the satellite and radiosonde eras:"

    The radiosonde record may have a bias due to the nature of the devices used.
    There has been some disagreement but I think the satellite record turned out to reveal tropospheric warming and stratospheric cooling, as expected, though off hand I don't know about the distribution within the troposphere.
    The graph shown appears to indicate tropospheric warming and stratospheric cooling, which seems to support the idea of greenhouse-forcing induced warming.

    --"There are two principal reasons why the models appear to be misrepresenting the tropical atmosphere so starkly. First, the concentration of water vapor in the tropical lower troposphere is already so great that there is little scope for additional greenhouse-gas forcing."

    Interesting, but increasing water vapor higher in the troposphere would still have important effects.

    --"Secondly, though the models assume that the concentration of water vapor will increase in the tropical mid-troposphere as the space occupied by the atmosphere warms, advection transports much of the additional water vapor poleward from the tropics at that altitude."

    That likely only would happen if the water vapor amount increases in the tropics at that altitude. Increased water vapor in the tropics and increased advection of water vapor don't contradict the other - they should tend to go together.

    --"Since the great majority of the incoming solar radiation incident upon the Earth strikes the tropics, any reduction in tropical radiative forcing has a disproportionate effect on mean global forcings."

    Only in the wavelengths dominated by solar radiation (SW radiation; the greenhouse effect deals with LW radiation - it too can vary with latitude, but ...).

    --"On the basis of Lindzen (2007), the anthropogenic-ear radiative forcing as established in Eqn. (3) are divided by 3 to take account of the observed failure of the tropical mid-troposphere to warm as projected by the models –"

    This is nonsense. The radiative forcing being considered is clearly the external forcing. What is being asserted here is that the feedbacks are not the same as expected - distinctly different, even if it were correct. None of this discussion justifies dividing external radiative forcing by 3 or any number.

    -----------
    PS Lindzen himself, though not free of mistakes in general, did not make this same mistake in the work Monckton is citing. See DELTOID: "Monckton's triple counting"

    I didn't go into a lot of Monckton's statements over k (seems like a waste of time considering...) but see the DELTOID work just mentioned, which states:

    --"What Monckton is doing is double counting his (dubious) evidence that sensitivity is lower than the IPCC number. If he had two pieces of evidence that sensitivity is half the IPCC number he would multiply them together to claim that sensitivity is one quarter the IPCC number. This is not correct."

    PS this DELTOID writing is far more brief than what I wrote here.
    -----------


    --"it is simple to calculate that, in 2001, one of the IPCC’s values for f was 2.08. Thus the value f = 3.077 in IPCC (2007) represents a near-50% increase in the value of f in only five years."

    He's comparing one value to what? An average? That's an odd way to assert a 50 % increase.

    --"With these assumptions, ? is shown to be less, and perhaps considerably less, than the value implicit in IPCC (2007). The method of finding ? shown in Eqn. (24), which yields a value very close to that of IPCC (2007), is such that progressively smaller forcing increments would deliver progressively larger temperature increases at all levels of the atmosphere, contrary to the laws of thermodynamics and to the Stefan-Boltzmann radiative-transfer equation (Eqn. 18), which mandate the opposite."

    Well, that's odd, since IPCC temperature projections increase with increasing radiative forcing. I didn't bother to check whether Monckton's application of Stefan-Boltzman was correct or not.


    --"the feedback-sum b cannot exceed 3.2 W m–2 K–1 without inducing a runaway greenhouse effect. "

    I think he's misunderstanding something here. I suspect the IPCC listing of feedbacks are in total after taking into account their responses to each other.

    --"Figure 7" "Fluctuating CO2 but stable temperature for 600m years"

    The sun increases in brightness over hundreds of millions of years, so the same CO2 level 500 million years ago would allow a cooling relative to now. CO2 forcing is roughly logarithmic to changes in concentration within a certain range; An increase from 280 ppm to 7000 ppm is 4.64 doublings. 7000 ppm is one of the higher estimates of CO2 for that time period. Not shown is a possible dip in CO2 in the late Ordivician. The temperature graph is unrealistic and cartoonish (which may be fine for some purposes, but not here) and no one can say that global average surface temperature has not exceeded 22 deg C during the time span shown.


    --"The Bode equation, furthermore, is of questionable utility because it was not designed to model feedbacks in non-linear objects such as the climate."

    Yes, that's what the climate models are for.

    --"since CO2 occupies only one-ten-thousandth part more of the atmosphere that it did in 1750"

    That's over 30% relative to CO2 concentration in 1750. Which is enough for CO2.



    ________________________________________________________


    The other (opposing) paper "A Tutorial on the Basic Physics of Climate Change":

    A decent brief overview for introductory purposes, although they leave themselves open to not taking into account convection and variations in optical properties over wavelength. But the IPCC, etc. findings certainly do take into account these things.

    For more, see my attempt to explain it qualitatively but clearly at "Tropical Storm Bertha" I mention there a particularly useful book (draft copy available online) by Ray Pierrehumbert: "Principles of Planetary Climate".



    _____________________________________________



    CLARIFICATION/CORRECTION:

    --"Since the great majority of the incoming solar radiation incident upon the Earth strikes the tropics, any reduction in tropical radiative forcing has a disproportionate effect on mean global forcings."

    I wrote: "Only in the wavelengths dominated by solar radiation (SW radiation; the greenhouse effect deals with LW radiation - it too can vary with latitude, but ...). "

    Actually, I should have just said No. Because radiative forcing is what it is; that it might be a smaller percentage of the radiative fluxes in the tropics if it itself doesn't vary in latitude much is irrelevent for finding the global average.

    Actually, invariance over latitude invariance isn't the case - direct forcing by increases of CO2 and some other greenhouse gases is greatest in the subtropics and least in polar regions; the near surface climate response is opposite that pattern because of the distribution of feedbacks - albedo feedbacks of less winter snow (at latitudes that get winter sun) and summer polar sea-ice loss (open water stores solar heat and releases it in the dark polar winter, delaying ice formation), and greater evaporative cooling of tropical waters which reduces surface warming but contributes (upon condensation) to the warming of the middle-to-upper troposphere.




    -----------------------
    3.models can not reproduce every aspect of the weather and climate system exactly - this does not mean they are not very useful.

    The apparent reduced warming of the mid to upper low-latitude troposphere would be just as puzzling if the cause of warming were solar brightenning.
  12. Last comment posted before seeing your last comment -

    "John allows links and actually prefers hyperlinks. But he has asked us to keep the discussion pertinent to the thread and not post "lists of links" so I try to break up my comments for readability."

    Duly noted. (I know I wandered a bit from 'Volcanoes' in the prior segment of discussion... part of it was centered on tides and that naturally expanded to other things...)



    "PS
    I looked at your remark on THERMODYNAMICS but skipped"..." Articles and Papers are much more appreciated as links (less opinion and more facts)."

    That's fine - the rest of my RealClimate comments I listed above were for just in case you were interested (many covering topics we discussed back at Science and Society, but each time I write something I don't do it the exact same way, so... etc.).

    I did have some RC comments on radiative physics in the atmosphere in particular and when I finnish tracking those down I will post a reference to those and ONLY those.
  13. More on radiation transfer within and through the atmosphere: Regional Climate Projections (25,26*,55**,56**,57**,58**,83***,85***,104,110****,111,146,191) asterisks pertain more to visualizing greenhouse effect physics  (171-173,174*,175,176*,180~,181~,184*****,189****,192***,193,194,203,214,215,218 (*** near bottom),232**,235,238,245,246) Real Climate (105**,144**,168 (LTE) ,170** (and LTE),172 (mainly LTE),192**,229**,241**(ext),251***(BB),252***(BB),261**(ext),274*,275*,285*,289) LTE = local thermodynamic equilibrium BB = fundamentals of blackbody radiation and radiative physics ext = what happens in extreme scenarios

    Response:

    [RH] Fixed links that were breaking page formatting.

  14. Back to basics:

    Remind me again what it is that makes you doubtful of AGW?

    Because it occurs to me we're on the climate sensitivity thread now, and your position is not the climate is more stable but that it it is more stable specifically to greenhouse forcing and not to some various other factors...

    ---

    "Articles and Papers are much more appreciated as links (less opinion and more facts)"

    - understandable, but in matters of science (and some other things), good opinion writing includes reasoning and facts, and/or references to facts and reasoning.

    ---

    From comment 8 above:

    Your argument must be against the title of that section, "Drivers of Climate Change". Because climate sensitivity has nothing to do with how sure we are that it is human activity that is responsible for the recent changes in those things.
  15. (refering again to your comment, 8, above) The part you would want to argue against starts on p. 3:

    "Attribution of Observed Changes
    Although confidence is high both that human activities have caused a positive radiative forcing and that the climate has actually changed, can we confidently link the two? This is the question of attribution: Are human activities primarily responsible for observed climate changes, or is it possible they result from some other cause, such as some natural forcing or simply spontaneous variability within the climate system?"
  16. Patrick
    My argument is that the sensitivity is low (always has been) ie. I agree with Spencer. My agrument is for the earth itself combined with the sun are the primary drivers and while I do not actually argue against the Green House Effect, I feel that it is overstated. That the problem 1975-2007 was caused by high solar activity and vulcanism/tectonic activity. I have posted the most current articles in the volcano thread, the greenland glaciers thread and an abstract in the "Arctic sea ice melt - natural or man-made?" thread.
  17. But
    1. which of Spencer's arguments do you agree with?
    2.
    -A. if the climate sensitivity is low than how does one explain the recent changes being caused by the minor changes in solar forcing or likely small forcing of changes in geomagnetism (?) or likely very very small forcing if any of submarine volcanic activity (volcanic aerosols already having been accounted for by IPCC etc.)?
    OR
    -B. what values do you expect when quantifying changes in non-TSI (and non-UV/TSI, as I expect that the UV enhancement associated with TSI is already accounted for (?)) solar forcing, geomagnetic forcing, tidal forcing, etc, in terms of radiative forcing or some equivalent or direct 'temperature' forcing by circulation changes in the ocean? For example, that article you referenced some time ago back in Science and Society regarding effects on transmissivity in clear air due to heliospheric current sheet crossing or 'Forbush' - whatever those things were (PS could you explain that to me?).
  18. Patrick
    1: His idea that CO2 sensitivity is low.
    2.A: The concept that the solar wind does not have an effect on climate is wrong.
    3: Tectonics
  19. PS
    You apparently confuse ocean tides with tectonic tides.
  20. PPS
    I have a link posted here to Spencer's argument in comment 9.
  21. "You apparently confuse ocean tides with tectonic tides"

    No, but I do recognize that solid earth tides must respond to oceanic tides and vice versa, but except maybe for around the Bay of Fundy and that one place in France - well...

    "The concept that the solar wind does not have an effect on climate is wrong."

    Could you explain how it works or describe the observations supporting it? For example, from the article on Forbush and Heliospheric current sheet crossings etc. (PS you'd have to quantify the radiative forcing of that and then figure out a multidecadal trend...) Or does it affect circulation patterns directly via the E-region dynamo (within the base of the thermosphere) - ps mass of E-region dynamo is something like a millionth of the whole atmopshere, give or take a factor of ten (just because I can't look it up right now), but if you could find some mechanism to propogate a pressure perturbation or pattern downward while amplifying it...

    "I have a link posted here to Spencer's argument in comment 9. "

    I covered that very argument in some of our last comments in "Science and Society".
  22. Patrick
    Re: "I covered that very argument in some of our last comments in "Science and Society"."
    Actually you dismissed the idea rather abruptly, but that's OK. But it doesn't mean that I did.

    On the solar wind I will have to find the links again. I know I posted them on this site somewhere.
  23. "Actually you dismissed the idea rather abruptly"

    But I did explain why I was dismissing it (or perhaps more accurately, debunking it, or at least trying to debunk it). So what would really be helpful is if you could find errors in my reasoning - the gist of which was that Spencer was considering temperature response to cloud variability on short timescales, short enough that thermal inertia would limit the temperature response.

    And now I don't remember off hand, but I do wonder how it could be determined with such an apparently simple method to what extent the cloud variability caused the temperature and vice-versa.

    And the other problem - it seems to me cloud feedback is a part of climate sensitivity to other things, but there is little (on these short timescales) CO2 feedback to cloud 'forcing', and even less so feedback by changes in solar activity, etc.
  24. Patrick
    You did start explaining your logic but diverted to an unrelated subject. While I did find your argument very interesting, I was not convinced that you actually proved Spencer incorrect. Actually, I have not read any papers or articles that show him incorrect, only alternatives. I view the IPCC alternative as incorrect because it has not produced the promised evidence in degree ie. Spencer has shown that his numbers equal observation while the IPCC numbers are way off from the observations. This is why their models don't work.
  25. It's worth pointing out that Stephen Schwartz has recognised some of the essential errors in is analysis (see John Cook's top article and also my post #5), and has issued a correction in relation to the original analysis that everyone got very excited about.

    Now Schwartz considers that his analysis (still a rather weak approach - see Tom Cook's top post and my post #5), yields a climate sensitivity of 1.9 +/- 1.0 oC per doubling of atmospheric CO2. That's within the range of all the pukka science out there (a climate sensitivity near 3 oC +/- a bit), and if Schwartz were to relax his (rather arbitrarily determined) time constant for the earth's inertial respense to forcings a bit more, then he'd be smack on the value that everyone else gets.

    So another storm in a teacup! Sadly, while Schwartz's original paper was flashed all over the blogosphere, we can be pretty sure that Schwartz's reassessment will be left unmentioned...

    ..but not here!
  26. re #25:

    I forget to give the url to Schwartz's reassessment/correction of his original work:

    http://www.ecd.bnl.gov/steve/pubs/HeatCapCommentResponse.pdf
  27. Do you mean this paper?
    HEAT CAPACITY, TIME CONSTANT, AND SENSITIVITY OF EARTH'S CLIMATE SYSTEM by Stephen E. Schwartz
  28. please pay attention Quietman!

    We've already seen that paper (linked in John Cook's top post). My link is to the correction that Stephen Schwartz published in which he determined that the conclusions of the original version of the paper (your link), were incorrect and that his original climate sensitivity value was far too low.....
  29. chris
    I was asking if that was the original paper that you referred to. A little testy?
  30. CLIMATE SENSITIVITY
    "The sensitivity of the climate system to a forcing is commonly expressed in terms of the global mean temperature change that would be expected after a time sufficiently long for both the atmosphere and ocean to come to equilibrium with the change in climate forcing. If there were no climate feedbacks, the response of Earth's mean temperature to a forcing of 4 W/m2 (the forcing for a doubled atmospheric CO2) would be an increase of about 1.2 °C (about 2.2 °F). However, the total climate change is affected not only by the immediate direct forcing, but also by climate “feedbacks” that come into play in response to the forcing."

    "As just mentioned, a doubling of the concentration of carbon dioxide (from the pre-Industrial value of 280 parts per million) in the global atmosphere causes a forcing of 4 W/m2. The central value of the climate sensitivity to this change is a global average temperature increase of 3 °C (5.4 °F), but with a range from 1.5 °C to 4.5 °C (2.7 to 8.1 °F) (based on climate system models: see section 4). The central value of 3 °C is an amplification by a factor of 2.5 over the direct effect of 1.2 °C (2.2 °F). Well-documented climate changes during the history of Earth, especially the changes between the last major ice age (20,000 years ago) and the current warm period, imply that the climate sensitivity is near the 3 °C value. However, the true climate sensitivity remains uncertain, in part because it is difficult to model the effect of feedback. In particular, the magnitude and even the sign of the feedback can differ according to the composition, thickness, and altitude of the clouds, and some studies have suggested a lesser climate sensitivity."

    Climate Change Science: An Analysis of Some Key Questions, pp 6-7,
    Committee on the Science of Climate Change
    National Research Council

    "Climate models calculate outcomes after taking into account the great number of climate variables and the complex interactions inherent in the climate system. Their purpose is the creation of a synthetic reality that can be compared with the observed reality, subject to appropriate averaging of the measurements. Thus, such models can be evaluated through comparison with observations, provided that suitable observations exist. Furthermore, model solutions can be diagnosed to assess contributing causes of particular phenomena. Because climate is uncontrollable (albeit influenceable by humans), the models are the only available experimental laboratory for climate. They also are the appropriate high-end tool for forecasting hypothetical climates in the years and centuries ahead. However, climate models are imperfect. Their simulation skill is limited by uncertainties in their formulation, the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature."

    Climate Change Science: An Analysis of Some Key Questions, p 15,
    Committee on the Science of Climate Change
    National Research Council

    Ref: The Real 'Inconvenient Truth' Some facts about greenhouse and global warming - JunkScience.com, Updated August 2007
  31. JunkScience is a place where truth goes to die. I haven't looked at the link, yet.

    But aside from that, I don't see anything in comment 30 that disagrees with my points or chris's or Philippe's or the host of this website, RealClimate, IPCC, The Weather Channel, Scientific American, James Hansen, Shakira (well actually I don't know what she specifically says on the matter but she did perform for Live Earth and one or more inaugural balls), ... etc.
  32. "We should note that devoid of atmosphere Earth would actually be a less-cold -1 °C (272 K) because the first calculation strangely includes 31% reflection of solar radiation by clouds (which obviously could not occur without an atmosphere) while ignoring that clouds add significantly to the greenhouse effect. Granted it's kind of a bizarre to include clouds in one half the calculation and not the other but that is the way it's commonly done, so, for simplicity, just stick with ~33 °C."

    The reason for including the LW effects and not the SW effects of clouds is because what is being discussed in the effect of greenhouse agents via LW radiation.

    If we really consider what would actually happen with the removal of all greenhouse agents, including SW effects, then yes, the cooling won't be so great, but it would still be enough to cause dramatic cooling by ice/snow albedo feedback.

    Thus far, no techical errors, but the distinction between a real greenhouse and a radiative greenhouse, and that greenhouse agents do not 'form a blanket' is rather nit-picky and besides the point. Real greenhouses don't have fabric blankets either - they have glass (or some other generally SW transparent material). I don't cover my self with glass when I get into bed in the winter!

    What they all have in common is that they slow the flow of heat from hot to cold - by inhibiting convection, reducing thermal conductivity, and/or increasing LW opacity - so that a greater temperature difference (between inside and outside, between my skin and the air in my room, between Earth's average surface temperature and the temperature of a blackbody that would emit the same LW power to space) is required to sustain a rate of heat loss to balance a given heat supply (the sun, my metabolism).
  33. (Thus far, no techical errors) - I haven't varified the temperature calculated for no albedo and no greenhouse effect. Actually though, there is some error if they reduced the albedo to zero, because there is some surface albedo and some backscattering to space from clear air.
  34. Some less trivial errors popping up now, such as this:

    "Greenhouse gases do not emit energy in the same bandwidth in which they absorb energy and thus emissions from carbon dioxide are not absorbed by carbon dioxide."

    That might be true (? - something about quantum mechanics) if there were no absorption-line broadenning by doppler and pressure effects, ... BUT IT IS MOST CERTAINLY NOT TRUE in the case of atmospheric greenhouse gases and clouds, etc.

    Height of tropopause: "10-50Km or 6-30 miles above the surface" - NO WAY! WAY OFF! 10 km is a typical midlatitude value, but it never gets even halfway to 50 km (I think it's somewhere around 15 to 18 km in the tropics - around the 100 mb level - see Holton, chapter 12). 50 km is actually about the height of the stratopause.

    "Sidebar:"

    Even absolute errors that are larger than projected changes are tolerable because ... well, you know I'll be taller if I stand on my toes than flat on my feet; you essentially only need to know the dimensions of my feet to calculate the difference (perhaps some feedback from posture changes...). Another way of looking at it - suppose the relative error in change is about the same as the relative error in absolute values. 10 % of 288 K would be HUGE, yet a 10 % error in 3 K is not too bad. Of course there is not a particular basis for arguing that the errors are related precisely that way, but ...

    Aside from that, I can bet what's coming up - greenhouse effect short-circuited by convection. Okay, but models take that into account! Remember it's tropopause level radiative forcing that tends to be important in driving surface and tropospheric temperatures - which are convectively coupled; this does not mean they don't respond to anything.

    I'm not going to bother with the link anymore; I'm quite sure there's nothing new there.
  35. "Of course there is not a particular basis for arguing that the errors are related precisely that way,"

    Actually they must not be that way; given that there is a correct value, the error range is quite sizable in projected changes. Too big, and yet not big enough.
  36. Okay, I skimmed the rest of it.

    There are some factual points that are true (and some that are true but irrelevant to anything), but the picture they paint is a worthless piece of trash. Forget the science, this is JUNK!
  37. "That might be true (? - something about quantum mechanics) if there were no absorption-line broadenning by doppler and pressure effects"...

    Actually, any macroscopic material in local thermodynamic equilibrium must have the same emissivity as absorptivity along any path length in any direction at any wavelength; otherwise, I've got some plans for a perpetual motion machine you might be interested in...
  38. Have a read: http://www.john-daly.com/forcing/review.htm

    which has commentaries on Hug & Barrett's presentation on climate sensitivity. You can access the original paper direst from the web page.
  39. Hello!
    You could add that Schwartz updated and corrected (in some aspects) his analysis. He now claims that climate response time is 8.5 ± 2.5 years. According to this climate sensivity is 1.9 ± 1.0 K. http://www.ecd.bnl.gov/pubs/BNL-80226-2008-JA.pdf
    Thus his estimate of climate sensitivity now is at the lower bound of the IPCC range.
    Response: Thanks for the link, I wasn't aware of Schwartz' response and have updated the article accordingly.
  40. So how about that Spencer & Braswell et al 2008?

    It has a few nice ideas about climate models and their feedbacks... and so far I haven't found anyone who has been able to debunk it. Has anyone found one?
  41. Sensitivity to cumulative carbon emissions is a new metric of sensitivity proposed in papers published in 2009 and discussed on the brand new blog Climate Physics Forums.
  42. Tom Dayton unintentionally reminded me that this is the right place to discuss the forthcoming Spencer paper on climate sensitivity. So this is in a sense a repost.
    For sure we need to wait to read the paper but one thing can already be said. From what Spencer himself says, he shows "monthly variations in the Earth’s net radiation [...] compared to similarly averaged tropospheric temperature". This is not wrong by itself but it should be clear that in this way he's looking just at the fast response component.
    From this work nothing can be said on the overall climate sensitivity which definitely includes components much slower than a month.
  43. The problem is that Spencer keeps repeating the same wrong conclusions in his blog ("These results suggest that the sensitivity of the real climate system is less than that exhibited by ANY of the IPCC climate models. ") so that they're still usefull for the skeptic community.
    I prefer not to comment on this attitude that he has in common with other (luckly few) skeptic scientists.
  44. Details about Riccardo's point about Spencer's papers are provided by chris in an excellent comment on another thread.
  45. Riccardo at 01:42 AM on 10 May, 2010
    I think it is a bit dangerous for any of us to comment on a paper that has not yet been published. However, on my reading of the article published so far, it does not appear that Spencer is claiming that he can calculate the equilibrium climate sensitivity from his approach. He is comparing the observed response against the statistics FROM THE MODELS OVER THE SAME RESPONSE PERIODS, and thereby suggesting that the models are overestimating the temperature/flux reponse. From this, one can PERHAPS validly draw the conclusion that the models are overestimating the equilibrium climate sensitivity (expressed in temp/flux). It is therefore not valid (or at least not valid until we have seen the paper) to say that "nothing can be said on the overall climate sensitivity".
  46. PaulK,
    even assuming that the comparison with climate models is appropiate, my point still stands. One cannot judge the sensitivy just looking at the first part of it, let alone deduce the long term behaviour of the climate system nor of the models. For example, what if the models miss just a damping factor? In the short term they produce larger postive and negative variations, but they will average out.
    But, as pointed out in my last comment, my guess is that Spencerr will not explicitly say anything like this in the paper, it's something just for his blog.
  47. I am not sure this paper has been mentioned yet. See

    Lin 2010.

    Obtains 3.1K for sensitivity, constraining the estimates a little to between 2.8K and 3.7K
  48. here is a more readable formatting of Lin 2010. ;)
  49. I think on this subject somewhere at the top of this page the paper(s) from Spencer should also be clearly addressed - while Schwartz was probably what started this debate I believe right now the focus of the deniers is moving to Spencer ...
  50. From the main article:
    A 2008 study led by James Hansen found that climate sensitivity to "fast feedback processes" is 3°C, but when accounting for longer-term feedbacks (such as ice sheet disintegration, vegetation migration, and greenhouse gas release from soils, tundra or ocean), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimatic (historical climate) data.
    Hansen et al estimate Earth System Sensitivity (ESS - that includes slow feedbacks) over Charney Sensitivity (CS - that only includes fast feedbacks) - or ESS/CS - at 2:
    Paleoclimate data permit evaluation of long-term sensitivity to specified GHG change. We assume only that, to first order, the area of ice is a function of global temperature. Plotting GHG forcing [7] from ice core data [18] against temperature shows that global climate sensitivity including the slow surface albedo feedback is 1.5°C per W/m 2 or 6°C for doubled CO2 (Fig. 2), twice as large as the Charney fastfeedback sensitivity. Note that we assume the area of ice and snow on the planet to be predominately dependent on global temperature, but some changes of regional ice sheet properties occur as part of the Earth orbital climate forcing (see Supplementary Material).

    pg. 220, James Hansen et al.(2008) Target Atmospheric CO2: Where Should Humanity Aim?,The Open Atmospheric Science Journal, 2, pp. 217-31
    http://pubs.giss.nasa.gov/docs/2008/2008_Hansen_etal.pdf
    A more recent estimate puts ESS/CS at about 1.4:
    As shown in Table 1, none of these assumptions greatly changes our estimate of ESS/CSacross all of the analyses presented in this article, the smallest value of ESS/CS we obtain is 1.3, and the largest is 1.5. Our combined modelling and data approach results in a smaller response (ESS/CS~ 1.4) than has recently been estimated using palaeo data from the Last Glacial Maximum, 21,000 years ago (ESS/CS ~ 2). This is probably due to the fact that transitions from glacial to interglacial conditions in the Quaternary involve large changes in the Laurentide and Eurasian ice sheets (see, for example, ref. 36), which result in a significant large-scale albedo feedback in these regions that is irrelevant for climates warmer that present.

    pg. 63, Daniel J. Lunt et al. (January 2010) Earth system sensitivity inferred from Pliocene modelling and data, Nature Geoscience, Vol. 3
    Either way the climate sensitivity that people have been talking about underestimates the warming that we can expect because by definition it omits the slow feedbacks -- which aren't necessarily that slow (e.g., the reduction in plant efficiency over the past decade, the saturation of some ocean CO2 sinks, Boreal forests in Canada, rising levels of methane emissions due to permafrost melt in Arctic tundra and Arctic shallow water continental shelves, e.g., near the coastline of Siberia.

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