RealClimate logo


Technical Note: Sorry for any recent performance issues. We are working on it.

The AR4 attribution statement

Filed under: — gavin @ 29 January 2012

Back in 2007, the IPCC AR4 SPM stated that:

“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

This is a clear statement that I think is very well supported and correctly reflects the opinion of most climate scientists on the subject (and was re-affirmed in two recent papers (Jones and Stott, 2011;, Huber and Knutti, 2011)). It isn’t an isolated conclusion from a single study, but comes from an assessment of the changing patterns of surface and tropospheric warming, stratospheric cooling, ocean heat content changes, land-ocean contrasts, etc. that collectively demonstrate that there are detectable changes occurring which we can attempt to attribute to one or more physical causes.

Yet, in a paper just out in BAMS (Curry and Webster, 2011) this statement is apparently evidence that IPCC is unable to deal with uncertainty. Furthermore, Judith Curry has reiterated on her blog that the term ‘most’ is imprecise and undefined. For instance:

Apart from the undefined meaning of “most” in AR4 (which was subsequently clarified by the IPCC), the range 50.1-95% is rather imprecise in the context of attribution.

However, Curry’s argument is far from convincing, nor is it well formed (why is there a cap at 95%?). Nor was it convincing when I discussed the issue with her in the comments at Collide-a-Scape last year where she made similar points. Since the C&W paper basically repeats that argument (as has also been noticed by Gabi Hegerl et al who have a comment on the paper (Hegerl et al.)), it is perhaps worth addressing these specific issues again.

Let’s start with what the statement actually means. “Most” is an unambiguous adjective (meaning more than half), and ‘very likely’ in IPCC-speak means that the statement is being made with between 90 to 99% confidence (i.e. for every 10 such statements, the scientists expect 9 or more to pan out). Given that some people have found this confusing, it may help somewhat if the contents of the statement are visualised:



Figure 1: Two schematic distributions of possible ‘anthropogenic GHG contributions’ to the warming over the last 50 years. Note that in each case, despite a difference in the mean and variance, the probability of being below 50, is exactly 0.1 (i.e. a 10% likelihood).

The figure shows two Gaussian distributions, both of which have the probability of x being less than 50 at 0.1. i.e. P(x<50)=0.1. If either of them had been the distribution of the estimated increase in global temperatures due to anthropogenic greenhouse gas increases relative to the observed increase, the IPCC statement would have been almost exactly correct (i.e. if x=100*trend_caused_by_GHG/actual_trend). These distributions show a number of key issues that need to be appreciated. First, the actual increase of temperatures purely due to the rise in GHGs is not precisely known (and therefore there is a distribution of potential values). Note that we are presuming that there is a single ‘true’ answer, so the distribution is a measure of our ignorance, not a claim that the answer itself is a random variable.

Second, the IPCC statement is not a declaration about what the most likely value of ‘x’ is. It states merely that P(x> 50%) is at least 0.9. In the two figures, one has the mean value of x at 80%, while the other has the mean value at 100%. Both fit the IPCC statement equally well. Some people have interpreted the IPCC statement confusing the likelihood of the statement with the actual relative trend (i.e. that the 90% refers to the expected attribution), but that would be a big misreading of the text.

Third, there is certainly a potential for the increase in temperatures due to anthropogenic GHG changes to be greater than the observed trend because we know that there have been both natural (volcanic and solar) and human-caused (reflective aerosols, land use change) factors that are expected to have lead to cooling over the post-1950 period (therefore there is no cut off at 95% of the actual trend). The actual trend will be a function of the warming factors, balanced by the cooling factors. And of the warming factors, the well-mixed greenhouse gas (CO2, CH4, N2O, CFCs) changes are the dominant term (about 75% of the increase in warming factors from 1950, the rest is related to black carbon effects, ozone etc.).

Fourth, the statement clearly encompasses many different estimates of what the actual trends are being driven by and is not therefore a particularly strong conclusion. Myles Allen (Allen, 2011) points out that during the drafting, the text was changed from ‘contributed substantially’ to ‘most’, and focused on greenhouse gases rather than the total anthropogenic effect specifically in order to have a more quantitative conclusion and more justifiable statement.

Now let’s put some real numbers in here. Attribution is fundamentally a modelling task, and the principal models that can be used are the coupled GCMs – at least to start with. What do they estimate the warming trend from the well-mixed GHGs to have been over the last 50 years? The figure below shows this for some of the GISS CMIP5 models (more model data can be downloaded from CMIP5 portal):



The 50 year trends (here, from 1956 to 2005, 5 ensemble members), are 0.84ºC (range [0.79,0.92]) for just greenhouse gas forcing. and 0.67ºC (range [0.54,0.76]) for the all-forcings case (in CMIP3, the envelope of the all-forcing trends is [0.4,1.3], or equivalently 0.74 +/- 0.22ºC (1 sigma spread) using 55 individual model simulations – the wider spread reflecting structural variations in the models and forcings). As in the more recent model simulations, the GISS CMIP3 50 year trends using only well-mixed GHG forcings is around 0.1ºC more than the ‘all-forcing’ case (data here).

The actual observed trend depends a little on the dataset used, but is around 0.6 +/- 0.05ºC (1 sigma uncertainty in the OLS fit). If we then estimate the percentage (as illustrated above), assuming a 0.2ºC sigma in the model spread, ‘x’ is roughly 140% +/- 35% (1 sigma). If we interpreted that range as a Gaussian distribution (not really a good idea, but simple enough for illustration), we’d estimate that P(x<50%) would be less than 1% (even less likely than the IPCC AR4 statement allowed for).

There are good reasons why the IPCC assessed that the probability was not as low as suggested by the models or any individual attribution paper. Specifically, the overall assessment must take into account potential structural uncertainties that don’t come into the straight model analysis. For instance, the models may systematically be overestimating the GHG-driven trend, they may be underestimating the internal variability, and they may be undersampling the structural uncertainty in making models themselves. The first kind of error would cause an overestimate in the mean of the distribution, while the other factors would cause an underestimate in the variance of the trends – all would increase P(x < 50%). On the other hand, the net forcing is almost certainly less than the effect of anthropogenic GHGs alone and so that biases the mean of the ‘all-forcings’ trends low, and some of the spread in the trends is related to different models having different forcings (biasing the spread wide). These elements can be quantified during the attribution (using fingerprint scaling, monte-carlo emulators etc.), but when they are all taken into account, the difference is less than one might think (it turns out that structural uncertainty likely isn’t being underestimated and the internal variability in models comfortably spans the range inferred in the real world (Yokohata et al., 2011; Santer et al., 2011)).

Curry and Webster specifically bring up two issues that, they claim, lessen the confidence one should have in the IPCC statement: that the history of solar forcing is uncertain in scale, and that aerosol forcings have a huge error bar. These two statements are true as far as they go – the scale of solar forcing is not tightly constrained prior to about 1960, and the total aerosol forcing and it’s variation in time is uncertain. But C&W’s specific complaint is that the attribution studies used in AR4 used solar forcing that was too large compared to more recent studies. However, reducing any warming trend associated with solar actually makes the attribution statement more likely which somewhat undercuts their point.

With respect to aerosols, the key thing to remember that regardless of the magnitude of the change, the sign of the forcing is almost certainly negative (i.e. the net aerosol effect has been one of cooling). The dominant anthropogenic aerosols are sulphates (derived from the SO2 emitted during the burning of sulphur-containing fossil fuels), which are reflective, and hence cooling. Other aerosols (black carbon, organic carbon, nitrates) are more uncertain, but have a net effect that is smaller.

Now, the statement in AR4 specifically states that the effect of greenhouse gases is more than half of the observed trend, which is actually independent of the effects of aerosols. But with the high probability of aerosols being a net cooling, this increases the ratio of the GHG-driven trends to the actual forced trend.

The final issue is whether the internal variability of the system on multi-decadal timescales has been properly characterised. For instance, it is possible that all the models grossly underestimate the internal variability, in which case any expected trend due to GHGs would be drowned out in the noise. But there is no positive evidence for this at all – as Hegerl et al point out, the estimates of multi-decadal variability in the models and observational records all overlap within their (substantial) uncertainties (arising from the shortness of the record, and the difficulty in estimating internal variability in the presence of multiple forcings). So while it is conceivable be that there is a bias, it is currently undetectable, which implies it can’t be that large.

In summary then, the IPCC AR4 statement was a fair, even conservative, assessment. There is an unfortunate tendency to reify the particular statements made by IPCC, since there were clearly other correct statements that could have been made. For instance, it might well have been worthwhile to add a statement about the likely range of the anthropogenic trends (i.e 80-120% of the actual trend or similar), so that a better picture of the appropriate distribution could be given (see Huber and Knutti, 2011) for examples). But claims that the statement was unsupported, or that it demonstrated that IPCC was ignoring uncertainty are simply untenable.

The next iteration (IPCC AR5) is now underway, but given the early results of the CMIP5 models (which are on the whole very similar, as discussed at fall AGU), and more recent literature on this issue (see refs below), I see no reasons in the recent literature why the conclusions in AR5 will be much different. But if anyone still finds the assessment confusing, they have an opportunity to make their points via the IPCC review process, and the resulting conclusions will likely be clearer because of them.


References

  1. G.S. Jones, and P.A. Stott, "Sensitivity of the attribution of near surface temperature warming to the choice of observational dataset", Geophysical Research Letters, vol. 38, pp. n/a-n/a, 2011. http://dx.doi.org/10.1029/2011GL049324
  2. M. Huber, and R. Knutti, "Anthropogenic and natural warming inferred from changes in Earth’s energy balance", Nature Geosci, vol. 5, pp. 31-36, 2011. http://dx.doi.org/10.1038/ngeo1327
  3. J.A. Curry, and P.J. Webster, "Climate Science and the Uncertainty Monster", Bulletin of the American Meteorological Society, vol. 92, pp. 1667-1682, 2011. http://dx.doi.org/10.1175/2011BAMS3139.1
  4. G. Hegerl, P. Stott, S. Solomon, and F. Zwiers, "Comment on “Climate Science and the Uncertainty Monster” J. A. Curry and P. J. Webster", Bulletin of the American Meteorological Society, vol. 92, pp. 1683-1685, 2011. http://dx.doi.org/10.1175/BAMS-D-11-00191.1
  5. M. Allen, " In defense of the traditional null hypothesis: remarks on the Trenberth and Curry WIREs opinion articles ", Wiley Interdisciplinary Reviews: Climate Change, vol. 2, pp. 931-934, 2011. http://dx.doi.org/10.1002/wcc.145
  6. T. Yokohata, J.D. Annan, M. Collins, C.S. Jackson, M. Tobis, M.J. Webb, and J.C. Hargreaves, "Reliability of multi-model and structurally different single-model ensembles", Clim Dyn, vol. 39, pp. 599-616, 2011. http://dx.doi.org/10.1007/s00382-011-1203-1
  7. B.D. Santer, C. Mears, C. Doutriaux, P. Caldwell, P.J. Gleckler, T.M.L. Wigley, S. Solomon, N.P. Gillett, D. Ivanova, T.R. Karl, J.R. Lanzante, G.A. Meehl, P.A. Stott, K.E. Taylor, P.W. Thorne, M.F. Wehner, and F.J. Wentz, "Separating signal and noise in atmospheric temperature changes: The importance of timescale", Journal of Geophysical Research: Atmospheres, vol. 116, pp. n/a-n/a, 2011. http://dx.doi.org/10.1029/2011JD016263

160 Responses to “The AR4 attribution statement”

  1. 51

    #49–

    Not very useful, but solidly certain allright. Like: temperatures will not fall back to pre-1970s level over the coming decade. Or over the following one. Or the following. Or at any forseeable time — unless we learn to stop emitting, and start actively pulling CO2 down from the atmosphere.

    Well, that’s potentially a very useful prediction indeed. :-(

  2. 52
    Ray Ladbury says:

    Doug Proctor,
    Perhaps part of you confusion arises from misunderstanding where the idea of anthropogenic global warming comes from. It is not, in and of itself, a theory. It is, rather, a prediction of the consensus theory of Earth’s climate. In this theory, increasing greenhouse gasses cause warming–independent of how that increase occurs. The prediction that anthropogenic greenhouse gasses from fossil fuel burning would warm the climate dates from Svante Arrhenius in 1896.

    The consensus model of Earth’s climate actually has a long list of impressive successes:

    http://bartonpaullevenson.com/ModelsReliable.html

    There are certainly uncertainties in the model, but they do not invalidate the basic structure of the model–which performs well for the most part–and they certainly do not invalidate what we think we know with high confidence about the planet’s climate. Certainly, the so-called skeptics present no positive alternative interpretations that have exhibited any predictive power whatsoever.

    So, my quesiton would be this: Given that we have a model that has demonstrated significant predictive power(17 successful predictions), that no serious and irremediable problems have been found and no unambiguous predictive failures seen, and given that it exhibits behavior that looks like what we see on planet Earth and that there are no alternative models with anywhere near the success of the current consensus model, why the hell wouldn’t we rely on it to give us guidance precisely where it is most reliable (e.g. the response of the climate to changes in greenhouse gasses)?

  3. 53
    grypo says:

    Curry believes it is possible there could be global cooling over the next decades – say 20 to 30 years.

    [Response: I wonder what odds she'd need to bet on that? - gavin]

    No odds, of course.

    We don’t know what the climate will be for the next several decades, there are a number of reasons to expect the continue flat trend for the next several decades. In terms of when global warming will come “roaring back”, it is possible that this may not happen for the first half of the 21st century.

    I am being asked what is meant by “possible”. More than 50%? Well the whole situation is too uncertain to put probabilities on, IMO. Hence my previous suggestions for looking at a “possibility distribution” rather than trying to guess at probabilities. And for generating scenarios via scenario falsification.


    post

    The emissions scenario is already factored into the different predictions. So it down to solar forcing (which she says may be low for 90 years) having a large impact, volcanoes, and oscillations. She doesn’t mention that the further we get out, the possibility of cooling factors becomes vanishingly small, nor is there any mention of what happens if these factors all don’t work out well.

  4. 54
    SecularAnimist says:

    Doug Proctor wrote: “If AGW is not a theory, not up to the level of hypothesis …”

    Anthropogenic global warming is neither a “theory” nor a “hypothesis”.

    It is an empirically observed fact.

  5. 55
    Martin Vermeer says:

    > It is an empirically observed fact.

    It is also a specific prediction by a theory that has very broad application, like explaining why the Earth surface has the temperatures it has (making the Earth inhabitable), why mountains often have snow tops (Tyndall was a mountaineer!) — but also such esoteric astronomy gems like why the Solar spectrum (and stellar spectra) and the brightness of the Solar disc look the way they do, etc.

  6. 56
    Hank Roberts says:

    > … global warming will come “roaring back”, …
    > this may not happen for the first half of the 21st century.

    “I believe it is peace for our time… Go home and get a nice quiet sleep.”

    http://eudocs.lib.byu.edu/index.php/Neville_Chamberlain%27s_%22Peace_For_Our_Time%22_speech

  7. 57
    Ray Ladbury says:

    [edit - please stay substantive]

  8. 58
    dhogaza says:

    why mountains often have snow tops (Tyndall was a mountaineer!)

    OT but this is a great tidbit, something I was unaware of (Tyndall being a mountaineer).

    [Response: This is true. However, it is also true that Tyndall was specifically looking for a way to understand the apparent history of earth's temperature changes, due to geologic evidence about ice ages, which at the time went against everything physicists thought they knew about the sun. He says so in his original paper. Geology begets physics, in other words! --eric]

  9. 59
  10. 60
    Hank Roberts says:

    > ignoring the eminently more predictable (and larger magnitude)
    > future forcing of CO2, is just plain silly.

    I agree completely, Eric.

    What I mean is that even if we got a magical grace period from the sun’s variability, it seems we’d fail to take advantage of the opportunity any such little delay gives us.

    Take a different case — China and India experiencing 1800s British coal smoke smogs, 1960s US acid rain, and contemporary mercury pollution by burning coal with no air pollution controls.

    That’s increasing the world’s longterm problems from more CO2 in the air and oceans.

    It’s also given some brief minor reduction in warming due to sulfates.

    Or say we had another mega-volcano causing a few cold years.

    Would we be smart enough to use any opportunity to move away from carbon fuels?

    Doesn’t seem like we’re that smart.

    “Delay is the deadliest form of denial.”

  11. 61

    # 53 grypo

    I’ve said this before, and it is very simple to understand.

    - An extended solar minimum would remove on avert around 0.1 W/m2
    - Relative thermal equilibrium for the Holocene is 0.0 (zero)
    - Current estimates of positive forcing are around 1.8 W/m2
    - Subtract 1.8 W/m2 – 0.1 W/m2 = 1.7 W/m2

    In other words, it certainly looks like we will continue warming even with a quiet sun, based on the known evidence.

    This understanding of course does not account for the unknown factors. There may be an alien race in a cloaked spacecraft that will soon aim their cooling ray guns at our oceans to save us.

    Personally I’m a bit skeptical about the possibility of aliens trying to help us out though.

  12. 62
    Marcel Kincaid says:

    Rather you are positing that despite the currently known forcings explaining the situation well, there must be some factor which negates all of that, and an additional unknown forcing or forcings that has exactly the same net effectt.

    This was pointed out to Alex repeatedly at Deltoid. The problem seems to be the employment of two different epistemologies, one in which propositions are given provisional probabilistic truth status based on the degree to which they are supported by evidence, and the other in which all propositions that have not been formally proven are given equal epistemological status. The first is the epistemology of science and all rational empirical inquiry; human beings could not function without it in their daily lives, let alone science. The second is a misapplication of deductive standards of proof from logic and mathematics to empirical matters.

  13. 63
    Marcel Kincaid says:

    Let’s see what and when things will happen.

    One could say the same, with equal wisdom, about cigarette smoking, laying out in the sun until your skin peels, or eating three hamburgers a day and never getting off the couch.

    You can’t reasonably have it both ways: both settled/certain and impossible to predict.

    This is well known from Chaos Theory to be wrong: determinism does not imply predictability.

  14. 64
    Marcel Kincaid says:

    In terms of when global warming will come “roaring back”, it is possible that this may not happen for the first half of the 21st century.

    I am being asked what is meant by “possible”. More than 50%? Well the whole situation is too uncertain to put probabilities on, IMO. Hence my previous suggestions for looking at a “possibility distribution” rather than trying to guess at probabilities.

    Is English not Professor Curry’s native language? She seems to have no idea what these words mean.

  15. 65
    Rod B says:

    re Marcel Kincaid (22)
    OT but very insightful. I am neither a fan of Clinton nor Gore but it is instructive how fabled lore leaves truth in the dust, like your recounting of Clinton’s testimony. The lore has become a great sound bite and metaphor but entirely misconstrued. I’m sure there’s some connection with the AGW debate but none comes to mind at the moment. Same can be said of Gore’s invention of the internet tale.

  16. 66
    Rod B says:

    Hank, true, delay is the deadliest form of denial. It is also, in general, the greatest protection against rash behavior.

  17. 67
    RichardC says:

    The OP starts “Back in 2007, the IPCC AR4 SPM stated that:

    “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

    This is a clear statement that I think is very well supported”

    I think the statement is misleading and incorrect. It takes the net effects of GHGs, solar, aerosols, natural variability, etc and asserts that one cause, GHGs, very likely caused >50% but less than 100% of the net result. Why less than 100%? Because “most of” by common usage means “less than all”. However, if we expand the previously posted efforts which backed out ENSO, solar, and volcanoes, to also include aerosols, and perhaps also figure in the thermal inertia of the system and all other factors, I’m sure that the best estimate is that anthropogengic GHGs have caused greater than all of the observed increase in global average temperature.

    Would the moderators be kind enough to make their best guess as to a REAL percentage? I’m guessing that if they do they’d instead say something like, “It is probable that anthropogenic GHGs have caused 150% +-20% of the observed global temperature increase, some of which has been masked by aerosols, thermal inertia, and/or natural variability. It is almost certain that at least 100% of the observed increase is due to anthropgenic GHGs.”

    The 150%, 20%, and 100% numbers are made up, and the inclusion of thermal inertia is questionable, but you get the drift.

  18. 68
    Jim says:

    Well, forgive me while I go badly off topic but there’s an interesting story here regarding Tyndall and mountaineering. Tyndall was indeed a mountaineer and a pretty good one. He made two of the early attempts on the Matterhorn before the infamous first ascent (with fatalities) by Edward Whymper et al. His second attempt reached the west ridge, a couple hundred meters below the summit, the highest point reached at that time, and later named for him as Pik Tyndall.

    Mt Tyndall in California’s Sierra Nevada is also named for him, the summit of which is just a few meters higher than the point he reached on the Matterhorn. This peak in turn has its own interesting history, being first climbed by Clarence King then of the California Geological Survey (and later the first director of the USGS), from which he identified and named Mt. Whitney, the highest peak in the continental U.S. However, when he attempted Whitney’s first ascent several years later (1871), he mistakenly climbed what is now Mt. Langley instead. Langley, as it turns out, was named (later) for Samuel Langley, who actually did climb Mt Whitney (in 1881), making the temperature and infrared radiation measurements subsequently used by Arrhenius in his first calculations of the greenhouse effect.

  19. 69

    #68–OT, of course, but Tyndall’s “signature” climb was the first solo ascent of Monte Rosa, August 17, 1858. It was an impromptu affair:

    “After breakfast I poured what remained of my tea into a small glass bottle, an ordinary demi-bouteille in fact; the waiter then provided me with a ham sandwich, and, with my scrip thus frugally furnished, I thought the heights of Monte Rosa might be won…”

    http://en.wikipedia.org/wiki/Monte_Rosa#First_ascents

    [Response:I think Tyndall would probably beg to differ. The Matterhorn is the much more difficult climb--which is why its first ascent was a full 10 years after Monte Rosa.--Jim]

  20. 70
    Hank Roberts says:

    > the greatest protection against rash behavior

    That would be science.

    Regrettably there wasn’t enough of that available when we got into this handbasket and started down our current path.

    The delay changing our behavior, with what we know now, is — say, doesn’t this language have a word for short term profit by intergenerational theft? Hmmmm, there ought to be a word for that.

  21. 71
  22. 72
    Ray Ladbury says:

    Rod B. says, “delay is… also, in general, the greatest protection against rash behavior.”

    You know, there was a guy on I-66 the other day that had that motto. They scraped what was left of him off of a semi’s back bumper.

  23. 73
    Hank Roberts says:

    Re the Sciencemag link, based on a few model runs out of who knows how many.

    Sounds like describing only one or strands of the spaghetti chart.

    Here, let me fiddle with that Sciencenow language a bit. I’m
    speculating that what they meant to write was something like:

    Out of _____ model runs,

    … sea ice

    in at least some model runs
    southward along the east coast of Greenland….

    in at least some model runs,
    melting made surface waters less salty,

    in at least some model runs,
    reducing ocean mixing and

    in at least some model runs,
    chilling the waters that return to the Arctic.

    the colder water

    in at least some model runs,
    completed a feedback loop by

    in at least some model runs,
    encouraging the formation of more sea ice

    that

    in at least some model runs,
    maintained an icy chill

    in at least some model runs,
    directly upwind of Europe

    for
    in at least some model runs,
    centuries.

  24. 74

    “[Response:I think Tyndall would probably beg to differ. The Matterhorn is the much more difficult climb--which is why its first ascent was a full 10 years after Monte Rosa.--Jim]”

    Certainly–Tyndall himself didn’t make much of the climb, as I think the quote I gave suggests.

    Nevertheless, it still seems to me that the Monte Rosa climb is (so to speak) ‘most cited.’

  25. 75
    RichardC says:

    54 SA says, “Anthropogenic global warming is neither a “theory” nor a “hypothesis”.

    It is an empirically observed fact.”

    I agree that warming of the lower troposphere is an empirically observed fact, but I think you step out of bounds by including attribution.

  26. 76
  27. 77

    #75 RichardC

    Ever heard of cause and effect?

  28. 78

    This iconic attribution statement seems to understate the human contribution, esp because it doesn’t specify the range or most likely value, but only (an underestimate of) the lower limit.

    Besides specifying a range, it would have been useful to also focus on the net anthropogenic contribution rather than solely on the greenhouse gas contribution.

  29. 79
    Martin Vermeer says:

    I agree that warming of the lower troposphere is an empirically observed fact, but I think you step out of bounds by including attribution.

    Richard I think it is you who is stepping out of bounds. Note that any observation process is also a modelling process: a thermometer only gives proper temperatures if it is properly calibrated, i.e., you apply a calibration model to the raw values. This is a trivial example: the model behind the use of the satellite MSU data is pretty complex. Processing surface station data into a global average is somewhere in-between. Yet, we call all these “empirical observations”.

    Also the fact that it is mostly CO2 etc. that is behind the current warming is an empirical observation, in the same sense. We know based on physical theory what increasing CO2 concentrations should do, and what we now see happen was predicted well before it started happening. You can point a $50 radiometer at the sky and see the back radiation for yourself — and more of it the more CO2 (and H2O) molecules there are along your line of sight, going down to the horizon. Satellites have done that too looking down, and yes, the thermal infrared spectrum has changed in pretty much the way expected.

    http://www.skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse-effect.htm

    If you want to disbelieve this, not only do you have to provide an alternate cause for the warming (perhaps not evidently absurd to a non-expert), you also have to explain why observed changes in the outgoing IR spectrum do not do what common sense tells they are doing.

    It doesn’t get any closer to ‘empirical observation’ than that.

  30. 80
    Dikran Marsupial says:

    From Prof. Curry’s paper “The third type [of uncertainty detectective] is the merchant of doubt (Oreskes and Collins, 2010), who distorts and magnifies uncertainties as an excuse for inaction for financial or ideological reasons”.

    Prof. Curry is a merchant of meta-doubt! Rather than simply magnify the scientific uncertainties, she attempts to undermine the ability of science to quantify the certainty using probabilistic terms.

  31. 81
    vukcevic says:

    Does anyone know where the GISS temperatures database
    http://data.giss.nasa.gov/work/gistemp/STATIONS//tmp. (station.number)/station.txt
    has moved to?

  32. 82
    J Bowers says:

    “I agree that warming of the lower troposphere is an empirically observed fact, but I think you step out of bounds by including attribution.”

    If you do audits, not if you do science.

    I like Michael Tobis’s (IIRC) take on AGW – the consequence of existing laws and theories.

  33. 83
    Alex Harvey says:

    Dear Gavin, #20:

    Thank you for your responses, but I remain interested in the stratospheric cooling.

    The Executive Summary of the AR4 WG1 report states,

    “Further evidence has accumulated of an anthropogenic influence on the temperature of the free atmosphere as measured by radiosondes and satellite-based instruments. The observed pattern of tropospheric warming and stratospheric cooling is _very likely_ due to the influence of anthropogenic forcing, particularly greenhouse gases and stratospheric ozone depletion.”

    Prof. Held seems to be saying that the existence of stratospheric cooling tells us nothing about the sensitivity of the climate to increasing greenhouse gases. If there were strong negative feedbacks that prevented the surface from warming, the stratosphere would cool just the same. If so, I fail to see why stratospheric cooling is even mentioned in the context of attributing global warming.

    I understand that all of the strands of evidence need to be looked at as a whole but at the same time it appears that three of the strands – stratospheric cooling, tropospheric warming, and the land-ocean constrasts – are not actually able to distinguish between hypothetical causes. These features all would occur whatever the cause of the observed warming at the surface and in the oceans. Or am I confused?

    By the way, I wonder if there have been modelling experiments using low sensitivity GCMs forced with increased solar radiation as well as with measured changes in CO2 and ozone depleting substances. In order for the stratospheric cooling to be relevant, I would want to know how the pattern of stratospheric cooling compares in this thought experiment would compare with the observed pattern. Would it differ in any way at all?

  34. 84
    Ray Ladbury says:

    Richard C., Really? What would it take for you to be convinced that CO2 was behind the majority of the warming? I mean we already know that CO2 is a greenhouse gas and that increasing greenhouse gasses lead to increased warming. We already know that other putative causes–e.g. heat from the oceans or increased insolation–won’t cut it. We know the stratosphere is cooling even as the troposphere warms. We know the warming matches quite well what is expected from greenhouse gasses. We know we’ve increased CO2 by ~40% over preindustrial times. This is just a beginning.

    I give up, dude. What’s missing.

  35. 85
    SecularAnimist says:

    RichardC wrote: “I agree that warming of the lower troposphere is an empirically observed fact, but I think you step out of bounds by including attribution.”

    This is like saying that we observe that apples fall from trees, and we know that gravity exists and causes unsupported objects to fall, but we “step out of bounds” if we attribute the fall of an apple to gravity.

    After all, that particular apple might be falling because of sunspots.

  36. 86

    #83–

    . . .stratospheric cooling, tropospheric warming, and the land-ocean constrasts – are not actually able to distinguish between hypothetical causes. These features all would occur whatever the cause of the observed warming at the surface and in the oceans. Or am I confused?

    Yes, Alex, you are confused. As Gavin told you previously, stratospheric cooling is diagnostic of the greenhouse effect ‘in action’–although it is true that lower stratospheric cooling also partially reflects ozone depletion.

    That means that, although it may in principle be possible for mysterious ‘negative feedbacks’ (which would be incompatible with the paleo-climate record) to mask surface warming while leaving the stratospheric cooling untouched, the stratospheric cooling actually observed is evidence that the greenhouse effect is now active–since there is no other cause for the total observed stratospheric cooling.

    And if we know that the greenhouse effect is acting to cool the stratosphere, then it is a more-than-reasonable inference that it also accounts for (some percentage of) the observed warming at various levels of the troposphere, including the surface.

    Hope that helps pull together some of the ‘strands of evidence’ for you.

  37. 87

    MS 34: Unfortunately, climate science does not have the luxury of input/output testing of the climate

    BPL: Sure it does.

    http://bartonpaullevenson.com/ModelsReliable.html

    Here goes another heroic attempt to get past RealClimate’s ReCaptcha…

  38. 88
    Dan H. says:

    BPL,
    I am not sure that you understand input/output testing.

  39. 89
    Rod B says:

    Ray (72), true, while delay is in general the greatest protection against rash behavior it can also cause some grief!

  40. 90

    There’s no particular virtue in delay. A sensible person takes the action that his intelligence says is required. Doctors perform emergency surgeries when a delay would mean death.

    Given the information we have, we don’t have the time to burn all the coal and oil in the ground first. Which is the object of the delay. Not to spare us from some rash, foolish act.

  41. 91
    MartinJB says:

    Rod B (89): Let’s turn your premise on its head. When engaged in rash behavior (i.e. performing a global atmospheric experiment that gives every sign of going horribly awry), delaying the cessation of that behavior nothing but rash itself.

  42. 92
    Brian Dodge says:

    “After all, that particular apple might be falling because of sunspots.”

    Only if sunspots generated a force between masses proportional to the mass, and inversely proportional to the square of the distance between them, and counter the measurable gravitational force that isn’t sunspot dependent. (If one hangs two bowling balls at the ends of a beam with a long fine wire, and moves another bowling ball near one of the two suspended balls, the torsion caused by the attraction of the suspended and fixed masses can be measured – and it is independent of sunspots.)

    Likewise, any alternate cause of global warming has to warm the troposphere and cool the stratosphere, decrease the diurnal temperature variation, and some of the polar amplification (the amount due to the continuing radiative transfer effects of CO2 after sunset, whether daily or at the beginning of polar winter; visible wavelength albedo changes have no effect in the dark – which eliminates the sun, or cloud albedo, or sun-GCR-cloud albedo interactions.)

    “…I think you step out of bounds by including attribution.”

    What sort of Alice in Wonderland cause could mimic CO2 in causing the observed global warming changes, cancel the physics of CO2 IR absorption/emission in the atmosphere, but leave CO2 laser physics unaffected? Hytran calculations for triatomic H2O, triatomic CO2, triatomic O3, taking into account the appropriate masses, bond strengths, and molecular configuration don’t have different first principles; they give results that support AGW. What’s left – right wing, tea party, anti-science, Karl Rove “we create our own reality” fairy dust?

  43. 93
    Brian Dodge says:

    “BPL, I am not sure that you understand input/output testing.”

    Sure he does.
    Input – ~30Gt/a CO2 FF emissions:

    Intermediate step 1 – DpCO2/Dt ~1.5ppmv/a since 1962, ~1.8ppmv/a last 20 years, CO2 concentration has risen from 317ppmv to 392ppmv:
    Intermediate step 2 – DT(Best)/dt ~0.024degC/a, ~1.17 degC since 1962: (see this)

    Output(s) –
    sea ice melt”” and accelerating glacier loss”
    http://www.bom.gov.au/climate/current/statements/scs31.pdf Wettest March on record in Australia
    http://www.bom.gov.au/climate/current/statements/scs26b.pdf Record wet January brings unprecedented flooding to northwest Victoria
    http://www.bom.gov.au/climate/current/statements/scs18b.pdf Exceptional winter heat over large parts of Australia
    http://www.bom.gov.au/climate/current/statements/scs14.pdf Six years of widespread drought in southern and eastern Australia, November 2001 to October 2007
    http://www.bom.gov.au/climate/current/statements/scs10a.pdf Climate conditions preceding the December 2006 southeast Australian bushfires Issued 19th December 2006
    http://en.wikipedia.org/wiki/2003_European_heat_wave “The 2003 European heat wave is one of the hottest summers on record in Europe, especially in France. The heat wave led to health crises in several countries and combined with drought to create a crop shortfall in Southern Europe. More than 40,000 Europeans died as a result of the heat wave.”
    http://www.guardian.co.uk/environment/2005/jul/11/weather.france “Jul 11, 2005 … France is facing its worst water shortage since 1976″
    http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=18509 “It was not even officially summer in the Northern Hemisphere, but Pakistan was in the midst of a deadly heat wave when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured the top image on June 10, 2007. ”
    http://climateprogress.org/2010/06/01/record-heat-wave-may/ “‘Hellish heatwave’ in Pakistan sets hottest temperature in Asia’s history, 53.5°C (128.3°F); in India, hundreds die, death toll expected to rise as record temperatures soar up to 122°F – June 1, 2010″
    http://www.bloomberg.com/news/2010-08-10/russia-may-lose-15-000-lives-15-billion-of-economic-output-in-heat-wave.html “Russia’s record heat wave may already have taken 15,000 lives and cost the economy $15 billion as fires and drought ravage the country.”

    Not exactly what I would call Non Destructive Testing – YMMV.

  44. 94
    Alex Harvey says:

    Kevin McKinney, #86:

    You say I am confused although I don’t see where you have contradicted me. Everything you write appears to confirm my understanding. You write, “the stratospheric cooling actually observed is evidence that the greenhouse effect is now active – since there is no other cause for the total observed stratospheric cooling”. There are a number of observations that confirm the correctness of the greenhouse effect. There is likewise no other cause for spectroscopic observations of absorption of radiation than increasing CO2. Is there something special about stratospheric cooling or is it just another proof of the greenhouse effect?

  45. 95
    Hank Roberts says:

    scienceblogs.com/stoat/2011/09/why_does_the_stratosphere_cool.php

    with appropriate caveats, see the full post for those,

    “… the reason that the real atmosphere has a stratosphere is because of ozone absorbing UV, thereby warming that portion of the upper atmosphere;

    hence the stratosphere is considerably warmer than it would be under just longwave (LW, or infra-red, IR) forcing; and CO2 is only effective in LW frequencies;

    hence, increasing CO2 increases the stratospheres ability to radiate in the LW, but doesn’t substantially increase its ability to gain heat, because most of that comes from the SW;

    hence it cools….”

  46. 96
    Hank Roberts says:

    Or, of course, you can read the comments – one of the more interesting attempts at putting complicated physics into words.

    The key point for attribution is what Gavin says there later:

    “Stratospheric cooling at the same time as surface warming was predicted with the first 1-dimensional model published over 40 years ago – decades before the data came in and any trends could have been discerned. This is precisely how science gains credibility – observe, theorise, predict, confirm….”

    That doesn’t say why the prediction was made — it’s complicated, takes math and modeling. See Spencer Weart about why computer models were needed to begin to understand this stuff.

    The models don’t generate explanations in simple English.

    Got calculus?

  47. 97
    Anonymous Coward says:

    #83 Alex,
    Maybe you’ve missed the difference between evidence that can be used to determine sensitivity and evidence for a stronger greenhouse effect from CO2.
    Sensitivity is uncertain among other things because of the various effects clouds could be having on surface temperatures. Stratospheric cooling doesn’t tell you how clouds work.

    I guess stratospheric is special in that it’s clearer evidence than tropospheric heating on its own. But it’s only a piece of the puzzle.
    If you search the site, you’ll find lots of old comments from people struggling to understand stratospheric effects in detail.

  48. 98
    Chris G says:

    From what I’ve seen, Dr. Curry can be characterized as emphasizing uncertainty, but only talks about the side that says, “It might not be that bad.”

    If I were a clinician, I could say that the mind often retreats from information that it finds too painful. Mothers refuse to accept that a child has died, people refuse to accept that they have a terminal disease, etc. If there were a person too well trained in the physical sciences to tolerate the cognitive dissonance that would result from a belief in something not physically possible, but unable to accept the most likely outcome, they might retreat to the periphery of what is likely. Uncertainty and wide confidence intervals would be their friend. Their subconscious would keep them away from the other side that says, “It might be that bad or worse.”

    I’m not a clinician, and I’ve never met Dr. Curry, but that is my working model that explains her behavior, and allows me to preserve my belief that most people are honest and good, if perhaps a bit irrational.

    Eric,
    Nice explanation of a model. I sometimes explain that d = g*t^2/2, where g is 9.8m/s/s, is a model (distance traveled of a falling body) that can be proven wrong by anyone with the equipment to take accurate enough measurements of time and distance. That does not mean it is not useful.

  49. 99

    #94 Alex Harvey

    Is there something special about stratospheric cooling or is it just another proof of the greenhouse effect?

    Yes, there is something special about stratospheric cooling. The vertical profile of temperature change is one of the lines of evidence for human caused global warming.

    If it were solar forcing and not and increase in greenhouse gases such as CO2, the warming would be more evenly distributed through the vertical profile.

    But since we know we have thickened the CO2 sweater that keeps Earth warm, and we see that it is getting warmer near the surface of Earth and cooling in the stratosphere, that is a great indicator that the problem is an increase in CO2 in the troposphere that prevents heat from escaping, thus warming the lower atmosphere and cooling in the stratosphere due to the trapped heat in the troposphere.

    There are no other identified mechanisms that can do that… unless you’ve found something?

  50. 100
    Alex Harvey says:

    #95, #96, #97, #99:

    Thanks for all the responses.

    Hank Roberts, I see this one’s difficult even for the experts, but I do note that the models we are talking about are the radiative transfer codes – correct? These models are just models of the basic physics. We all know they give the right answer and do not contain the parameterisations and tunings that cause skeptics to doubt the GCMs. And the GCMs in turn agree with the radiative transfer codes simply because they have built in radiative transfer codes of their own. Right?

    AC, I haven’t missed the difference between evidence that can be used to determine sensitivity and evidence for a stronger greenhouse effect from CO2. That is precisely what I said: as added CO2 is known to lead to cooling in the stratosphere is a fundamental result of the physics of radiative transfer it does not help us to distinguish between competing hypothetical explanations for observed global warming. It simply tells us what we already know – that we are pumping CO2 into the atmosphere and the theory of the greenhouse effect is sound.

    John P. Reisman, you write “If it were solar forcing and not an increase in greenhouse gases such as CO2, the warming would be more evenly distributed through the vertical profile”. Yes, but are comparing apples and oranges. A model forced by solar that is NOT ALSO forced by CO2 and ODSs is not relevant. As I said in #83, in order for the observed stratospheric cooling to be useful in distinguishing between competing hypotheses you would need to build a chemistry climate model with low climate sensitivity (e.g. 1.5 K) and force that with increased TSI + observed GHG and ODS changes and THEN compare the modelled vertical heat distribution with observations.

    I bet no one has ever done this before? And I also bet that the reason no one has ever done it before, if I am right, would be that no one expects that such an experiment would reveal anything surprising.


Switch to our mobile site