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GWPF optimism on climate sensitivity is ill-founded

Posted on 10 March 2014 by dana1981

The UK anti-climate policy advocacy group Global Warming Policy Foundation (GWPF) has published a report written by Nic Lewis and Marcel Crok claiming “the IPCC hid the good news” regarding climate sensitivity.  Lewis is an amateur researcher and retired financier who has published a few papers estimating climate sensitivity, and Crok is a freelance science writer.

GWPF asked climate scientist Judith Curry to write the Foreword to the report, presumably to lend it more credibility.  However, Curry has no publications or expertise in this area, and once said that the global equilibrium climate sensitivity could fall anywhere between 0 and 10°C for doubled CO2.  This comment is totally incompatible with the body of climate sensitivity research, and also with the GWPF report.

The report itself is essentially a commentary and includes no new information.  It boils down to Lewis and Crok trying to make the case that climate sensitivity is on the lower end of the IPCC estimated range.  In the report, they find reasons to dismiss the many studies and varying approaches that arrive at higher climate sensitivity estimates, and fail to discuss the shortcomings of the lower sensitivity studies that they prefer.  In short, it’s a very selective and biased review of the scientific literature on the subject.  Recent papers by Gavin Schmidt and Drew Shindell at NASA GISS, not considered in the GWPF report, entirely contradict its conclusions, for example.

There are a few main methods to estimate the global climate sensitivity; Lewis and Crok focus on three of these and present their case for why each should be considered valid (when yielding low sensitivity results) or disregarded (when yieliding moderate or high sensitivity results).  Here we’ll look at each, including evidence the GWPF report failed to consider, and show that their conclusions are not supported when the full body of research is considered.  As climate scientist Steven Sherwood described it,

"The report is standard cherry-picking.  It offers no new evidence not already considered by the IPCC, relying very heavily on a few strands of evidence that seem to point toward lower sensitivity while ignoring all the evidence pointing to higher sensitivity.

It relies heavily on the estimate by Forster and Gregory, which was an interesting effort but whose methodology has been shown not to work; this study did not cause the IPCC to conclude that sensitivity had to be low, even though both Forster and Gregory were IPCC lead authors and were obviously aware of their own paper."

However, the good news is that the report is consistent with the 97 percent expert consensus on human-caused global warming. It acknowledges that global warming will continue as long as humans continue increasing the greenhouse effect, and merely suggests that future warming will be toward the lower, slower end of the IPCC estimates.  As climate scientist Ed Hawkins at the University of Reading also noted,

"Remarkably for a report published by the GWPF, the authors agree with mainstream climate scientists that significant further warming is expected ... It is great to see the GWPF accepting that business-as-usual means significant further warming is expected. Now we can move the debate to what to do about it."

Paleoclimate Studies

Paleoclimate studies attempt to estimate climate sensitivity based on the forcings and temperature responses from climate change events in the geologic record.  The most robust study of this type was done by the PALEOSENS team, published in Nature in 2012.  This study evaluated past climate changes over the previous 65 million years, considering nearly two dozen investigations of many different geological time periods. 

The study estimated with 68 percent probability that the equivalent equilibrium climate sensitivity is between 2.2 and 4.8°C for a doubling of CO2, generally consistent with IPCC estimates, and inconsistent with the lower estimates preferred by GWPF.  The 95 percent confidence range in this study was between about 1 and 7°C equilibrium sensitivity, so very low and very high climate sensitivities could not be ruled out, but are relatively unlikely, based on the historical record.  Additionally, the GWPF report uses 68 percent confidence ranges throughout, so the 2.2 and 4.8°C PALEOSENS paleoclimate estimate is inconsistent with the GWPF low sensitivity conclusions.

adapted fig 3a

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

The GWPF report has very little discussion of paleoclimate sensitivity estimates.  They just say that these studies don’t tightly constrain the possible climate sensitivity range, and past climate states are different than current and future climate states, so “little weight can be put on the palaeoclimate estimates.”  While there is some truth to these critiques, entirely disregarding the results of these studies is simply not justifiable. 

In summary, paleoclimate studies provide one line of evidence that supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is weak.

General Circulation Models

Climate models (general circulation models or GCMs) provide another method by which to estimate climate sensitivity.  The physics of the climate system are input into very detailed climate models, which can then estimate how the global temperature will respond to various forcings.  The results can give us projections of future global warming under a variety of scenarios, and also give us an estimate of the global climate sensitivity.  Most GCM equilibrium climate sensitivities range between 2 and 4.5°C (average 3.2°C in GCMs used in IPCC AR5).  This range is consistent with paleoclimate estimates.

Lewis and Crok make the following argument.

"Between the Fourth and Fifth [IPCC] Assessment Reports the best estimate of the cooling effect of aerosol pollution was greatly reduced. That necessarily implies a substantially lower estimate for climate sensitivity than before. But the new evidence about aerosol cooling is not reflected in the computer climate models. This is one of the reasons that a typical climate model has a substantially higher climate sensitivity than would be expected from observations: if a model didn’t have a high climate sensitivity, its excessive aerosol cooling would prevent it matching historical warming."

However, according to climate modeler Gavin Schmidt of NASA GISS, this is incorrect.

“Their logic is completely backwards. Climate model sensitivity to a doubling of atmospheric CO2 is intrinsic to the model itself and has nothing to do with what aerosol forcings are. In CMIP5 there is no correlation between aerosol forcing and sensitivity across the ensemble, so the implication that aerosol forcing affects the climate sensitivity in such 'forward' calculations is false ... The spread of model climate sensitivities is completely independent of historical simulations.”

Climate scientist Kevin Trenberth also notes that the change in the estimated aerosol forcing is mainly associated with indirect aerosol effects, but half of GCMs don’t include these indirect effects, and those that do actually tend to simulate less warming.

“Some other models like CESM1 did include microphysics and an indirect aerosol effect, and had slightly lower 20th Century warming than observed … yet its climate sensitivity is higher than for [some other models that don’t include the indirect aerosol effect] … the [GWPF] comment presumes that models have been tuned to reproduce the 20th Century temperature record, but this is mostly not true”

This point was also made by Schmidt et al. (2014), which additionally showed that incorporating the most recent estimates of aerosol, solar, and greenhouse gas forcings, as well as the El Niño Southern Oscillation (ENSO) and temperature measurement biases, the discrepancy between average GCM global surface warming projections and observations is significantly reduced.  This approach also accounts for the previously underestimated volcanic aerosol forcing, demonstrated by Santer et al. (2014), but not included in the GWPF report.

Schmidt 2014 adjustments

GCM mean (dark blue #1) and envelope (lighter blue) range of global surface temperature projections vs. HadCRUT4 (red #1) and Cowtan & Way (red #2) global surface temperature instrumental estimates.  The GCM mean result incorporating changes in ENSO and updated solar and aerosol forcings (blue #2 and #3) are also shown.  Adapted from Schmidt et al. (2014) by Kevin Cowtan.

In summary, GCMs provide another line of evidence that generally supports an equilibrium climate sensitivity between about 2 and 4.5°C, and the GWPF justification for dismissing these estimates is incorrect.

‘Instrumental’ Estimates

The method preferred by the GWPF report, and that which Lewis has used in his own papers, involves estimating climate sensitivity using a combination of recent instrumental temperature data (including ocean heat content data), less complex climate models, and statistics.  A few studies using this approach since about 2012 have begun yielding lower climate sensitivity estimates.  In their report, GWPF cite Ring et al. (2012), Aldrin et al. (2012), Lewis (2013), and Otto et al. (2013) as all yielding central equilibrium climate sensitivity estimates between 1.76 and 2.00°C.  However, the GWPF report only references the “main results” of Aldrin et al. (2012), whose study actually estimated equilibrium climate sensitivity of about 2.5 or 3.3°C when accounting for cloud and indirect aerosol effects.  Aldrin et al. wrote,

“Thus, the estimate from our original analysis should be interpreted with care. In further work, the uncertainty of the cloud lifetime effect should also be taken into account.”

The GWPF report, however, did not interpret their estimate with care.  It simply used the result that was convenient for their argument, and left out the cloud uncertainties.

As for Lewis (2013), it's not without its own red flags.  As documented at And Then There's Physics, when using data up to 1995, the method yields an estimated climate sensitivity range of 2.0–3.6°C, but incorporating an additional 6 years of data reduces the estimate approximately 33 percent, to 1.2–2.2°C.  Climate sensitivity is a relatively constant parameter; if adding just 6 years of data changes the result so dramatically, one should really question the method being used.  Instead, Lewis argues that it's the only reliable method for estimating climate sensitivity.

The challenge with this 'instrumental' method of estimating equilibrium sensitivity is that it’s based on transient instrumental measurements.  There is currently a global energy imbalance, and reaching a new equilibrium state will take over a century.  Therefore, estimating equilibrium climate sensitivity based on measurements of a climate that’s out of equilibrium requires making some significant assumptions, for example that feedbacks will remain constant over time.  However, several recent studies have suggested that these assumptions may not be correct.  For example, Armour et al. (2013),

"Time-variation of the global climate feedback arises naturally when the pattern of surface warming evolves, actuating regional feedbacks of different strengths. This result has substantial implications for our ability to constrain future climate changes from observations of past and present climate states."

Rose et al. (2014),

“Results imply that global and regional warming rates depend sensitively on regional ocean processes setting the [ocean heat uptake] pattern, and that equilibrium climate sensitivity cannot be reliably estimated from transient observations.”

Olson et al. (2013),

“We demonstrate that a single realization of the internal variability can result in a sizable discrepancy between the best [climate sensitivity] estimate and the truth. Specifically, the average discrepancy is 0.84°C, with the feasible range up to several °C. The results open the possibility that recent climate sensitivity estimates from global observations and [intermediate complexity models] are systematically considerably lower or higher than the truth, since they are typically based on the same realization of climate variability.”

Trenberth and Fasullo (2013) also note that ocean heat content (OHC) variability can strongly impact the 'instrumental' climate sensitivity estimates (emphasis added).

"Climate sensitivity estimates are greatly impacted by such variability especially when the observed record is used to try to place limits on equilibrium climate sensitivity [Otto et al., 2013], and simply using the ORAS-4 estimates of OHC changes in the 2000s instead of those used by Otto ... changes their computed equilibrium climate sensitivity from 2.0°C to 2.5°C, for instance. Using short records with uncertain forcings of the Earth system that is not in equilibrium does not (yet) produce reliable estimates of climate sensitivity."

None of these papers or concerns with ‘instrumental’ climate sensitivity estimation methods are mentioned in the GWPF report.  Instead, the report argues that this approach provides the only reliable method for estimating climate sensitivity, and that all other methods that produce higher estimates (e.g. paleoclimate and GCMs) are wrong.

However, an important new paper just published by Drew Shindell at NASA GISS reconciles the difference between the climate sensitivity estimates in these varying approaches, but not in the direction advocated by the GWPF report.  Shindell notes that the ‘instrumental’ approach studies preferred by the GWPF report assume that the global mean temperature response to all forcings is equal.  His study investigates this assumption by comparing GCM temperature responses to greenhouse gases with their responses to aerosols and ozone.

Shindell, who was a co-author on Otto et al. (2013), notes that “forcing in the NH extratropics [above 30° latitude] causes a greater global mean temperature response than forcing in the tropics”; a result noted by Hansen et al. (1997):

“A forcing at high latitudes yields a larger response than a forcing at low latitudes. This is expected because of the sea ice feedback at high latitudes and the more stable lapse rate at high latitudes

The forcing from aerosols and ozone isn’t globally uniform, but instead focused more in the northern hemisphere extratropics.  Hence it results in a relatively larger temperature response than an equivalent forcing from greenhouse gases, which are well mixed throughout the atmosphere.

When assuming equal sensitivity to all forcings, Shindell estimates the transient climate response (TCR) at 1.0–2.1°C, most likely 1.4°C, which is almost identical to the Lewis GWPF report estimate (1–2°C, most likely 1.35°C) and also similar to the estimate in Otto et al. (2013).  However, when Shindell accounts for the higher sensitivity to the aerosol and ozone forcings, the estimated TCR range rises to 1.3–3.2°C, most likely 1.7°C.  Compared to the IPCC estimated TCR range of 1–2.5°C, and the range in climate models of 1.1–2.6°C, Shindell's results give a low probability for the low end of the range and higher probability for the high end; the opposite of the GWPF report.  Given the strong correlation between TCR and equilibrium climate sensitivity, Shindell’s results also suggest that the lower climate sensitivity estimates are unlikely to be accurate.

Accounting for Cloud and Water Vapor Observations

The GWPF report also notes that changes in cloud cover in a warming world are a key to determining climate sensitivity.  On this topic the report merely claims “Observational evidence for a positive cloud feedback is weak, at best.”  However, there have been several studies comparing observed changes in cloud cover to cloud simulations in climate models.  For example, Fasullo and Trenberth (2012) used satellite data from the NASA Atmospheric Infrared Sounder (AIRS) and Clouds and the Earth's Radiant Energy System (CERES) to examine the relationship between seasonal changes in relative humidity (RH) in the dry subtropics and the Earth's albedo via cloud cover.

"...the results strongly suggest that the more sensitive models perform better, and indeed the less sensitive models are not adequate in replicating vital aspects of today’s climate. The correct simulation of the vertical structure of RH and clouds should be a prerequisite for developing confidence in projections for the future."

Sherwood et al. (2014) built on the work of Fasullo and Trenberth by looking at the way that various climate models handle the cloud feedback.  They found GCMs with a low climate sensitivity were inconsistent with observations.  It turns out that these models were incorrectly simulating water vapor being drawn up to higher levels of the atmosphere to form clouds in a warmer world.  In reality (based on observations), warming of the lower atmosphere pulls water vapor away from those higher cloud-forming levels of the atmosphere, and the amount of cloud formation there actually decreases.  The diminished cloud cover leads to greater warming (a positive feedback), and is better reproduced in the GCMs with higher climate sensitivities.

These studies were omitted from the GWPF report, but they provide yet another line of evidence for high and against low climate sensitivity.

Climate Policies are Insufficient in Any Case

The GWPF report concludes by complaining that by not emphasizing the lower climate sensitivity estimates, the IPCC has “inadequately informed” policymakers about the state of the science.  However, from a policy standpoint, we're not doing nearly enough to reduce emissions even in the best case scenario.  As Myles Allen, co-author on Otto et al. (2013) noted of the GWPF report,

“Their prediction of 1.35 degrees C [TCR] is, even if correct, only 25% lower than the average of the general circulation models used in the IPCC 5th Assessment.  A 25% reduction in TCR means the warming we might have expected by 2050 might take until the early 2060s instead.”

Moreover, as detailed above, the full body of scientific evidence suggests that climate sensitivity is relatively high.  Even if you believe the GWPF report is right, there's a good chance it's not.  Proper risk management therefore mandates that we must take action to mitigate the threat of dangerous climate change.

But in any case, the full body of evidence is firmly against the conclusions of the report.  The authors merely dismiss or ignore the research that doesn’t support their desired conclusion, and overlook the shortcomings of the research that does.

Note: the discussion of the Shindell results has been incorporated into the Advanced rebuttal to the myth 'climate sensitivity is low'.

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Comments 1 to 46:

  1. There's a great point in one of Richard Alley's lectures where he talks about the CS graph Dana has included here. He says, when the study that is 5th from the bottom came out he spent two days answering phone calls from reporters. All the other CS studies, nothing.

    Tells you a lot.

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  2. Dana,

    You say Lewis has published a "few" papers on climate sensitivity.  I am aware of only two papers by him in the peer-reviewed literature, and on one of those he was a co-author on a paper temperatures over western Antarctica. Did I miss a couple?  For now Lewis lies squarely in the climate "hobbyist" desgnation, and his fellow fake skeptics are bending over backwards to try and boost his impact.

    Curry's foreward was entertaining to read, nothing more.  That she is falling over herself to praise this report is hardly surprising given that she has declared that she supports the (ideological and political) objectives of the GWPF lobbyists.

    This report though is also another example of how the fake skeptics fail to present a coherent and physically plausible alternative hypothesis to the theory of AGW.  Some of them deny it is even warming, others claim anthropogenic global warming (AGW) is a hoax, others claim that there is some magical negative feedback that will result in virtually no warming, others like Lewis cherry pick literature to delude themselves into thinking that climate sensitivity is low, while others are convinced that an ice age is imminent ;)

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  3. 'Few' is vague.  I'm pretty sure it's 2, but I said 'few' in case I missed any.  I think he's just got Lewis and Otto though.

    The Curry Foreword is kind of interesting from a psychological perspective.  First, why did GWPF invite her to write it?  She has no publications and no expertise in sensitivity research, as her comments on the subject make crystal clear.  I can only guess they wanted a 'climate scientist' to write something since neither of the authors is really a climate scientist.  Basically to try and make it seem more credible.  And I suppose they couldn't think of many climate scientists who would be willing to endorse that report, for obvious reasons.

    And then why would Curry agree to write the Foreword?  It totally undermines her claimed role as the 'bridge builder', as GWPF is an anti-science, anti-policy, politcal advocacy group.  Perhaps it's naîveté about what GWPF is and does.  Perhaps it's that she views her role as amplifying 'skeptic' voices.  That's basically how she explained it on her blog.  But it's pretty hard to maintain the perception of a bridge-building open-minded skeptic when you're writing material for a group like GWPF.

    In any case, we shouldn't turn the comments into a psychoanalysis of Judith Curry.  More important is that the report itself is a totally biased, cherry picked misrepresentation of the full body of climate sensitivity research.

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  4. Albatross, Dana, Nic Lewis has been a co-author of two papers, and sole author of a third that I am aware of:

    Improved Methods for PCA-Based Reconstructions: Case Study Using the Steig et al. (2009) Antarctic Temperature Reconstruction

    Energy budget constraints on climate response

    An Objective Bayesian Improved Approach for Applying Optimal Fingerprint Techniques to Estimate Climate Sensitivity

    On a side, note, as I was doing my google searches to find exact titles and links for the above, I came across a new paper by Craig Loehle also estimating low climate sensitivity (1.99 C per doubling).  His trick appears to be to attribute much of the recent warming to the PDO, but I would need to buy a copy of the paper to determine the exact details.  Publication date is March 24th, but now available online, so we can expect to see that cited a lot in the near future.

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  5. Let me see ... looks like stage 3)1)it's not happening2)it's not us3)it's not bad4)it's too hard5)it's too latealtho i suppose 4 and 5 could be rolled together
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  6. The Shindell paper is breathtakingly simple. It just takes two completely well known facts, puts them together and points out the trivial and inevitable conclusion:

    1. The NH responds more quickly to changes in forcing than the SH, because the SH has more water.

    2. Most of the aerosol cooling effect occurs in the NH.

    Since the cooling is happening in a place where is has a more rapid effect, the impact is greater than if it were happening uniformly. So we see less warming than we would expect from a model which treats the whole globe uniformly.

    This leads to the second conclusion: If we use a uniform forcing over both hemispheres, such as in Otto et al or my own n-box model, then we will conclude that TCR is lower than it actually is.

    I think there must be a few climate scientists slapping their foreheads over this one. The only thing left to do is to confirm the size of the effect.

    (My model produces more mainstream TCR values only becuase I haven't adopted the latest aerosol forcing estimates. Thus it contains two errors which just happen to roughly cancel out.)

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  7. In a larger context, at the point AR5 was finalised, there were in my view 3 big, new and unexpected problems in climate science.

    1. Could the apparent slowdown in warming be explained?

    2. Why was observed warming lower than the model projections?

    3. Why did simple models give lower sensitivity estimates than GCMs and paleoclimate.

    I think now we've got tentative answers to all three. Coverage, forcings and ENSO address (1) and (2) and Shindell addresses (3).

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  8. Praise be !! The gospel according to Lewis & Crok is now available in all its glory without having to hand out your e-mail to the GWPF gatekeepers.

    The 44 page GWPF Report 12 linked in the post is but an edited version of the longer 72 page GWPF Report 13. (The shorter version was "written for the lay reader, and summarises [the] longer, more technical document.") Both reports are not solely the work of Lewis & Crok but also incorporate the "help & comments" of Annan, Curry, Henderson, McKitrick and Montford. Curry also wrote the forward for both documents, strangely both times providing "this report" with a word-for-word identical eulogy.

    The question to be asked of Lewis & Crok is why they feel any serious scientific study should be presented to the world as a GWPF Report. These documents (along with GWPF Briefing Papers) have such an excellent track record of misrepresenting the evidence and presenting untrustworthy analysis that they manage to bring all UK charities and the UK Charity Commission into disrepute. Frankly I cannot consider a less appropriate method of publication (unless you consider a web-page on such planets as Wattsupia constitutes 'publication').

    Surely, if this gospel according to Lewis & Crok were worthy of anything other than the rubbish bin, they would publish elsewhere, perhaps within a journal which peer reviews its content. Or perhaps Lewis & Crok believe the "help & comments" of Annan, Curry, Henderson, McKitrick and Montford are adequate to eliminate any embarrasing errors, a belief I fear that itself constitutes an embarrasing error.

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  9. In the acknowledgements the Loehle paper thanks " Nic Lewis for helpful suggestions." Very interesting timing of the paper and report.

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  10. Thanks Dana for a great post and also Kevin for comments as well.

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  11. Tom @4,

    Thank you for the info on the three Lewis papers.

    We at SkS are aware of the new Loehle paper, how shall I say this...? There are several critical issues that undermine the premise of his paper and his results. I will leave it at that for now.

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  12. @Tom Curtis

    if you email me at I can forward you a copy of the Leohle paper

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  13. I agree that this "report" from the GWPF contains nothing new. It's just a reshuffle and redeal of existing literature. The only thing I find noteworthy is that it further reinforces the point that there is no scientific consensus on a best estimate for equilibrium climate sensitivity, which is entirely in agreement with the IPCC's statement in AR5 WG1 SPM: "No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies."

    Tangentially, I'm not sure how to square Gavin Schmidt's statement above, with the findings in Keihl (2007).

    Schmidt: "Climate model sensitivity to a doubling of atmospheric CO2 is intrinsic to the model itself and has nothing to do with what aerosol forcings are. In CMIP5 there is no correlation between aerosol forcing and sensitivity across the ensemble, so the implication that aerosol forcing affects the climate sensitivity in such 'forward' calculations is false."

    Keihl (2007): "These results clearly illustrate a strong inverse correlation between total anthropogenic forcing used for the 20th century and the model’s climate sensitivity. Indicating that models with low climate sensitivity require a relatively higher total anthropogenic forcing than models with higher climate sensitivity....

    These results explain to a large degree why models with such diverse climate sensitivities can all simulate the global anomaly in surface temperature. The magnitude of applied anthropogenic total forcing compensates for the model sensitivity...

    What is the major reason for the large uncertainty in total anthropogenic forcing? Figure 2 shows the correlation between total anthropogenic forcing and forcing due to tropospheric aerosols. There is a strong positive correlation between these two quantities with a near 3-fold range in the magnitude of aerosol forcing applied over the 20th century."

    • Is it somehow mathematically possible that if A and B have a "strong negative correlation" and B and C have a "strong positive correlation" that A and C can have "no correlation"?
    • Are the latest generation of GCMs somehow entirely different, rendering Keihl's observations out-of-date?  Has Schmidt actually back-tested the CMIP5 ensemble in the same way that Keihl did with earlier models and found no correlation?

    Also, could someone please link to a source for Gavin Schmidt's quotation?  I couldn't find it on realclimate or anywhere else, except for here and in Dana's column in the Guardian (which linked back to here).

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  14. Russ R., you persist in your factually incorrect interpretation of the "no best estimate" quiote as meaning "we have no idea."  That statement instead means they are unwilling to make a point estimate of the value, but certainly are willing to make a range estimate--the range they refer to in that same sentence as the "assessed lines of evidence and studies."  And as is clear from other sentences, they judge the point value most likely is in the middle of that range than at either end.

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  15. Climate sensitivity is also the topic of How much hotter is the planet going to get? by Michael LePage, New Scientist, Mar 9, 2014. LePage's article parallels Dana's OP with respect to recently published papers about climate sensititvity.   

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  16. Russ @13 - the Schmidt quote is from a personal communication.

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  17. Tom Dayton,

    You say:

    "they are unwilling to make a point estimate of the value, but certainly are willing to make a range estimate"

    Yes... and the range they give is "Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)."

    So how exactly does Lewis & Crok's "best estimate for ECS of 1.6–2.0°C" contradict this?

    You continue:

    "And as is clear from other sentences, they judge the point value most likely is in the middle of that range than at either end."

    Which other sentences?  Citation please.

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  18. It is in examining GWPF Report 13 that Lewis & Crok's objections to the 'effective' TCR within GCMs can be fully assessed - within in the shorter GWPF Report 12, their explanation is cut down to incoherence.
    The problems the IPCC GCMs exhibit, allegedly, is that they do not perform properly. The GCM 'effective' TCR values are far higher than even GCM 'actual' TCR values. This is what causes the GCMs to project their alarmingly high future temperatures (as opposed to the reassuringly low ones suggested by Lewis & Crok).

    The 'effective' TCR of course can be determined by using "the observational-TCR based formula" as derived by those clever analysts Lewis & Crok.

    ΔT(2012-2100) = (ΔF(2012-2100) x OTCR)/3.71 + 0.15ºC

    :where OTCR is derived (as described in GWPF Report Note 86) not just from ΔT(1951-2010) attributions but actual ΔT(1951-2010) as measured by real thermometers and these compared not just to ΔF(1951-2010) but also to ΔF calculated from dF/dt(1951-2010), both of which yield the exact same result (TCR=1.4ºC excluding a minor adjustment) which only goes to show how accurate this TCR derivation is!

    Not only that, Lewis & Crok manage to manipulate this highly sophisticated and complex model to yield GCM 'effective' TCR not just for 2012-2100 but also for 1850-2000.
    This is why Lewis & Crok find without even wielding a single error bar that, "in the case of climate sensitivity and TCR, arguably the most important parameters in the climate discussion" (and few would disagree with that), the IPCC AR5 "failed" to provide an "understanding (of) the scientific basis of risk of human-induced climate change." And if (unlike Lewis & Crok) you link to the source document laying out the role of the IPCC, you will find that Lewis & Crok are accusing IPCC WG1 of being in contravention of the Principles Governing IPCC Work.

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  19. There are several points of confusion here:

    1. The Kiehl paper was on the CMIP3 models.

    2. Forcings are dignosed from the models, they are not an input to the models. The input to the models is the external influences on the system, e.g. anthropoenic changes to atmospheric composition.

    3. Aerosol indirect effect, while it affects TOA energy balance, is technically a feedback rather than a forcing from the perspective of a GCM.

    Put 2 and 3 together and it should be clear that sensitivity and radiative forcing can be correlated without training. I don't know to what extent that is the case in practice.

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  20. Russ R.:  For example, read Box 12.2 (pp. 1110-1112).  Figure 1 shows the probability density functions.  And there is "On the other hand, AOGCMs show very good agreement with observed climatology with ECS values in the upper part of the 1.5°C to 4.5°C range."  Later there is this sentence: "comparisons of perturbed-physics ensembles against the observed climate find that models with ECS values in the range 3°C to 4°C show the smallest errors for many fields (Section"

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  21. dana1981 @16,

    Thank you for clarifying.


    Kevin C @19

    Has any similar analysis been done on the CMIP5 ensemble, to show the correlation (or lack thereof) between estimated ECS, and historical values for total anthropogenic forcing and aerosol forcing?


    Tom Dayton @20,

    Models are but one "line of evidence", as are observational studies. Currently, they point in different directions. Not hugely different... still within a likely range, but different enough to be cause for disagreement.

    I expect that over time as the observational record grows and the models improve, that difference will resolve and a consensus will emerge.  Until then, everyone will simply have to agree to disagree.

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  22. The paleoclimate report quoted above, with the exception of the PETM, deals with changes occurring within the slow feedback response time (deep oceans, weathering, etc). As such I believe they give lower sensitivity values than is applicable to our modern climate change. Carbon-belch events like the PETM, Permian, Triassic, Toarcian, even the Mid Miocene CRB/Monterey event, happen at timeframes faster than the slow feebacks can process them, causing CO2 to build up rapidly in the atmosphere and surface ocean. Warming in these events is much more severe due to the disequilibrium. Moreover the PETM timing in the above report is for a long-term event whereas one (albeit contoverial) paper recently put the initial CIE at 13 years. Modern climate change is not very comparable with glacial-interglacial slow changes, and much more comparable to the PETM, Triassic, Toarcian, and other rapid, massive carbon-belch events.

    0 0
  23. Of course its possible climate sensitivity may be towards the low side. It is also possible that a relatively small increase in temperature globally could be associated with much more extensive climate change than we have previously thought.

    Weather patterns recently suggest this may be the case. Nobody seems to have considered this possibility.

    0 0
  24. Russ R @ 13:

    "Is it somehow mathematically possible that if A and B have a "strong negative correlation" and B and C have a "strong positive correlation" that A and C can have "no correlation"?"

    I set up a spread sheet were A is a linear function plus a small random variable (Rand()*4), B is -A plus a smaller random variable (rand()*2), and C is a B plus a large random variable (rand()*16). Over a restricted range of values, I obtained in one instance correlations of A to B or -0.97, and of A to C of 0.06. Averaged across 21 trials, with a similarly restricted range of values the correlations were -0.59 for A to B, and 0.19 for A to C. So, in answer to your question - yes they can.

    Of course, as the range of values increases, the correlations of A to B, and of A to C rapidly approach -1. In the first example given, over a range of 100 values the correlations were -1, and -0.99 respectively. Averaged over 21 trials, they were -0.99 and -0.97 respectively.

    The key point here is that the range of values of climate sensitivity, and of aerosol forcing are small. Consequently this counterintuitive result is possible provided the variation the aerosol forcing is large (relative to that range), which it is.

    I am not saying that this is the basis for the apparent divergence between Kheil (2007), and Schmidt. That may be due to specific differences between the specifications of the CMIP3 and CMIP5 experiments, or of the actual CMIP3 and CMIP5 models. But your intuitive argument is not automatically valid regardless of specific reasons.

    0 0
  25. Russ R: This big paper from Shindel et al would seem to be key.

    They find that the correlation between aerosol forcing and ECS changes sign between CMIP3 and CMIP5 (also Andrews et al). However the CMIP5 models show no particular correlation between ECS and total forcing or effective aerosol forcing (which includes the indirect effect).

    0 0
  26. Russ R, I failed to include the B/C correlations in my response @24, and also made a small technical error on my spreadsheet.  Having corrected both errors, however, I find one example with correlations of:

    A/B: -0.62  B/C: 0.58  A/C: 0.01

    The twenty one trial averages are:

    A/B: -0.65  B/C: 0.33 A/C: 0.14

    Consequently, I believe the point I made still stands.

    Having said that, Kevin C's response is far more informative in this particular case.

    0 0
  27. Kevin C,

    Thanks, I'll give Shindel et al. a read.


    Tom Curtis,

    I tried the same type of spreadsheet as you.  My results also depended on how much random variable I inserted.

    I also looked at the 11 models in Kiehl's two charts.  Not having the data,  I estimated the values for CS, TF and AF as follows:

    Model /CS (°C) /TF (W/m-2) /AF (W/m-2)
    a /1.9 /2.04 /-0.6
    b /2.1 /2 /-0.63
    c /2 /1.7 /-0.7
    d /2.5 /1.68 /-0.74
    e /2.4 /1.6 /-1.15
    f /3.7 /1.48 /-0.62
    g /2.6 /1.24 /-1.26
    h /2.7 /1.21 /-1.42
    i /3.5 /1.2 /-1
    j /4.6 /1.16 /-1.3
    k /3.3 /0.8 /-1.15

    Between TF and 1/CS, ρ=0.78

    Between TF and AF, ρ=0.76

    Between 1/CS and AF, ρ=0.49

    So, to answer my own question, the inverse correlation between CS and aerosol forcing isn't as strong, but it's likely not zero.

    0 0
  28. Lewis has a response to the Shindell paper here:

    1 0
    Moderator Response:

    [RH] This runs very close to the "no link only posts" rule. Please try to incluce some discussion of the issue when posting links.

  29. I enjoyed reading the thorough presentation of information that Lewis, Crok, Currie, Annan, Henderson, McKitrick, Montford, and Loehle would have had access to. It was very informative.

    If they were genuinely interested in improving the understanding of what is going on they would not be trying to get away with pushing out the glaringly inadequately investigated and justified information they have presented.

    Any media that gave their 'report' an iota of attention without a little basic fact-checking needs to be on an "Unreliable Sources" list. An honest and balanced media report would make reference to the report and its authors and include evidence of the clear inadequacy of their report.

    The gullibility of many in the current population, willing to accept whatever suits their deluded interest based on their experience immersed in the unsustainable and damaging mass-consumption socioeconomic systems, certainly makes attempts to best inform the general population about the unacceptability of how they want to 'enjoy their life' a battle, but it is worth fighting. As the unjustifiable claims become more glaringly incredulous, more and more people are will realize how easily impressed they had allowed themselves to become.

    People addicted to the unsustainable and damaging artificial industrial chemical-filled mass-consumption socioeconomic system need to admit to the harmful nature of their addiction. Then the changes to a better sustainable life can begin.

    1 0
  30. jwhite @28,

    It is a pity that Lewis chooses to keep airing his view and opinions at locations (such as the GWPF and CA) that habitually attack climate scientists and routinely try and undermine climate science by distorting the facts and data.  Where is Lewis on this thread?  He is undoubtably aware of it, yet he is posting on sites run by extremists and fringe elements. Him doing so does not bode well for his judgement being unbiased.

    His biases aside, I doubt very much that Lewis (a retired finacier) has the depth of knowledge, understanding and experience to speak to a complex issue such as this. Just as I would lack the same when speaking to models used to predict the financial markets-- you do not see climate scientists trying to argue that the economists and financeers methods are incorrect. But for some reason every contrarian out there feels that climate science is fair game and that they somehow know better.

    0 0
  31. I apologize for my post being so brief, but I don't have the technical background to make an informed comment on Lewis's post. I was hoping someone here would be able to do that. That's why I brought it to your attention. I respect and very much appreciate this site.

    0 0
  32. Hi jwhite @31,

    My gripe was not with your or the brevity of your post :)  Rather Lewis's inexperience, background and who he chooses to associate with.

    Thanks for notifying us about his CA post.  Someone at SkS will have a look.

    0 0
  33. It amazes me how much people enjoy focusing on splitting hair, all so they can avoid evaluating the full scope of evidence...  Well, setting up of impossible expectations doesn't do much for learning either.

    In any event, thank you for your stream of valuable information, stuff that we can actually learn from.

    And thank you for your Reposting Policy, I'm honored to be able to Repost such an excellent article at my little effort.

    1 0
  34. I do not understand how Lewis is given any deference.  The fact that 6 years of data results in his analysis dropping the TCR that much can not be right.  Nature does not change abruptly like that.  A CO2 model of natural availablity that uses SOI, volcanic aerosols, LOD, and TSI, and other natural cyclical terms can predict 50 years of rising temperature including the fluctuations, based on a training interval that only goes to 1960.

    CSALT model

    I suggest to keep on confronting Lewis.  He obviously doesn't like to be called on his cherry-picking work:

    "WebHubTelescope needs to check his facts. "


    0 0
  35. Albatross@30, 

    "Where is Lewis on this thread? He is undoubtably aware of it, yet he is posting on sites run by extremists and fringe elements."

    If I may offer some constructive advice, you might want to scan back through the history of comment threads here at SkS and look objectively at the "hospitality" that has been shown to visitors (by both regulars and moderators).

    If guests don't feel welcome, SkS won't find many guests coming to visit, and it will run the risk of devolving into an echo-chamber of groupthinkers nodding vigorously in agreement with one another while actual debate takes place elsewhere.

    You can judge for yourself whether or not it's already too late.  As it stands today, the only visitors SkS seems to attract are indistinguished nobodies like me.

    0 0
  36. Russ...  You may not have been around long enough to have noticed that we've had quite a few more prominent scientists posting here. In particularly contentious cases we assign a single SkS commenter to the discussion as to not overwhelm the guest. 

    What SkS does not have patience for are those who break the rules that are laid out in the commenting policy. Those get snipped or deleted or banded depending on the severity of the infractions.

    And, if you think it's bad here, you might take a day and pose as an advocate for action on climate change on sites like WUWT, JoNova, Curry, or any of the many other lightly or unmoderated "skeptic" sites. 

    0 0
  37. If we halted all CO2 emissions today, according to the AR5 Frequently Asked Questions 12.3, temperture would continue to rise for "several tenths of a degree" due to sulfate emission reductions.

    Then it would take several decades for methane emissions to return to pre-industrial levels.

    But we know that the loss of arctic sea ice between 1979 and 2012 has contributed a total of 25% of the total anthropogenic forcing.  We also know that this trend will continue through the next several decades.

    In addition to the aerosol and albedo feedbacks that will occur during these next few decades,  the additional warming will produce carbon cycle feedbacks.  These feedbacks are,

    1.  The collapse of the Amazon rainforest
    2.  The rapid thaw of arctic permafrost, and
    3.  An additional slowdown of the Meridional Overturing Circulation

    All of these factors are not modelled in the AR5 scenario and will produce significant warming over the next several decades.

    0 0
  38. Russ R. - If you want examples of groupthink and echo chambers, take a look at WUWT, at Climate Audit, at JoNova, Bishop Hill, etc. Where the presentation of facts, of peer-reviewed papers, of statistical significance are all met with jeers, insults, ad hominems, etc. Persisting with facts on those venues tends to result in moderation. 

    On SkS, by contrast, jeers, insults, ad hominems, and sloganeering are met with facts, peer-reviewed research, and discussion of statistical significance. And commenters persisting in posting nonsense are moderated. Compare and contrast, note the distinction in content. Personally, I prefer fact-based discussions such as the ones here. 

    Your personal experience seems unhappy - I would point out (IMO) that you reap what you sow. 

    0 0
  39. Another near-term feedback is the reduction ocean-spray DMS aerosols that will contribute .2-.4C additional warming and the recent model results of decreased low-altitude cloud cover under warming scenarios (additional .2-.5)

    It is clear that we have already passed the 2C pre-industrial threshold, even if we halted all emissions today.

    0 0
  40. All: Russ R's most recent comment directed at Tom Curtis was deleted because it was inflamatory and insulting. Russ R may soon be recusing himself from posting on SkS.

    0 0
  41. Russ R.:

    If guests don't feel welcome, SkS won't find many guests coming to visit, and it will run the risk of devolving into an echo-chamber of groupthinkers nodding vigorously in agreement with one another while actual debate takes place elsewhere.

    For me, SkS is a resource for language and links I can use to counter pseudo-skeptic drivel elsewhere.  SkS authors can argue authoritatively, but as with other scientific disciplines, "actual debate" on climate-science issues does take place elsewhere: in the laboratories of working climate scientists, and the journals, conferences, and other professional venues where they expose their work to peer-review.  For the most part, scientific controversies are not resolved on blogs open to indistinguished nobodies like me and you.

    If you like to debate, Russ, you should consider presenting a paper at, say, the annual AGU meeting. Of course you'll need to be prepared for rigorous peer-review, your peers being the scientists best able to spot the flaws in your work because they know as much or more about your topic than you do.  Science at that level isn't for the faint-hearted, though. Scientists can seem "inhospitable" (even "vituperative") when critiquing arguments that aren't well thought out or well supported by evidence.  That's a good thing, because if your work can't survive the peer-review snakepit you're most likely fooling yourself.  Your feelings may be hurt, but your incorrect ideas won't make it in to the accumulating body of scientific knowledge, which is what really matters:

    Science doesn’t work despite scientists being asses. Science works, to at least some extent, because scientists are asses. Bickering and backstabbing are essential elements of the process.



    0 0
  42. I think it's fair to say that commenters on SkS can sometimes be a little hostile to commenters of the 'skeptical' persuasion.  But to be fair, there are a lot of 'skeptical' commenters who are flat-out trolls (look at the comments on my Guardian posts and how many the moderators are forced to delete for violating the guidelines, for example).  As a result, we're conditioned to assume that 'skeptical' commenters are probably trolls, because frankly they usually are (not directed at anyone here, just a general observation).

    In any case, Lewis' Climate Audit post was unconvincing.  His main argument was that the aerosol forcing is too uncertain to allow the Shindell method to tightly constrain climate sensitivity.  Sorry, but that aerosol forcing uncertainty is the same in the Lewis, Otto, etc. studies.  So why doesn't Lewis apply that critique to his own papers?  This is consistent with my point that Lewis is unwilling to consider the many, significant shortcomings in his own methods, because they yield the result he wants.  That's neither skepticism nor good science.

    In fact Andrew Dessler and other climate scientists have made this point, that the 'instrumental' method uncertainties are too large to tightly constrain climate sensitivity, because of the uncertain aerosol forcing, among othe reasons.  So we're still back to the body of reliable research being consistent with 2 to 4.5°C equilibrium climate sensitivity.

    1 0
  43. Russ R @42, I have responded in a more appropriate location.

    0 0
  44. Adding to Dana's @43, on at least one occassion when a noted "skeptical" climate scientist did comment at SkS, it became very clear that he was only interested in airing his talking points rather than engaging in genuine discussion.  I was not alone in being very frustrated by that.  It was frustrating because it meant he did not even respond to questions designed to elucidate his actual views, let alone engage in lines of enquiry that might challenge those views.  

    0 0
  45. Re Tom @44, what bothered me the most about our exchange with Pielke Sr. was that the comments were perfectly polite but challenged him on the science, and he would then run back to his blog and write a post about how mean we were being to him.  Similarly, contrarian commenters will often blatantly violate commenting guidelines, then cry censorship when those comments are deleted.  Classic victim complex.

    But now I'm getting off topic and verging on violating the commenting guidelines myself!  So I'll bring it back on topic by noting that ATTP has a good post on Shindell and Lewis.

    0 0
  46. But Dana, facts can be a very mean thing when they don't conform to one's predetermined outcome. They rattle the bars of the cognitive dissonance cage.

    0 0

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