Tag Archives: Skeptical Science

A new LGM reconstruction, with implications for climate sensitivity

LGM Ice Sheet Extent from Clark et al., 2009

First off, it’s important to note that the paper has only appeared in CPD, it still has to pass review. However, I’m going to comment on the results for two reasons. Mundanely, I have a sliver of free time now, and I don’t know that the same will be true after the paper’s (presumed) eventual publication. More importantly, however, I think it’s safe to say that its results will be misinterpreted to the same or even a greater extent than Schmittner et al., 2011 (hereafter S11) was. The mainstream press largely ignored some potential reasons to be skeptical of that paper’s results (discussed by RealClimate and Skeptical Science among others, as well as by one of the paper’s authors in an interview with me at Planet 3.0). And of course the denialist echo chamber distorted the results ludicrously, going so far as to erase an entire portion demonstrating them to be consistent with the larger body of evidence on climate sensitivity (e.g. Knutti and Hegerl, 2008) and inconvenient to dismissals of the danger posed by unchecked GHG emissions.

With the throat clearing out of the way, here’s how things stand. Fyke and Eby (2012) offered some criticisms of S11. They objected to some of the proxy data used, and more importantly, pointed out that the model used (a version of the UVic model, which is more akin to simplified EMICs than GCMs) simply couldn’t produce realistic behaviors of key atmospheric processes which caused it to underestimate ECS:

[T]o explore the potentially large dependence of Schmittner et al.’s results on the choice of climate model, we carried out a new model simulation with the most recent version of the UVic ESCM in which the atmospheric latitudinal profile of heat diffusion varies in response to the global average atmospheric temperature anomaly (the “Mod” simulation in Fig. 2). This functionality gives a new model with much improved fit to both Antarctic and Arctic LGM temperatures as recorded by ice cores, yet still retains an excellent fit to low-latitude temperatures. Notably, and most importantly, we found that this model ranks very well with respect to the relative RMSE test, but with a much higher ECS (3.6°C) than similarly ranked models in (1). As suggested in (1), the lack of dust forcing in our LGM model may lower the equivalent ECS by ~0.3°C, but this is still well above the median ECS estimate of 2.3°C in (1).

Fyke and Eby’s revised LGM-derived ECS was quite similar to other LGM-based studies, such as Holden, et al. (2010). Criticism that the UVic model used had an atmospheric component that was perhaps insufficient to fully capture the climate state at the LGM was echoed in the RealClimate discussion as well as by coauthor Nate Urban in our interview.

Schmittner, et al. (2012) responded to Fyke and Eby by largely disagreeing with their discarding of some proxy records, but conceding that their model choice may well have led to underestimating ECS and uncertainty in their reconstruction:

This tentatively supports the conclusion in (1) that structural model uncertainties (in particular, formulations of atmospheric heat transport) may have led to systematic underestimation of ECS2xC in (2). Further study with new ensemble model experiments, including the modified heat flux formulation and LGM dust forcing, are necessary to quantify the effect of heat flux uncertainties on the best ECS2xC estimate.

Schmittner, et al. go on to suggest that further modeling be done to try to better test the effects of using more realistic models with their approach.

Several groups are doing that, or something very similar. One is Tamsin Edwards, who has teased her experiment but not revealed its results (yet). Another is Jules Hargreaves and James Annan, who discussed S11 and also teased their experiment some months back but likewise did not discuss their results.

Which brings us to today (or, technically, Wednesday). Annan and Hargreaves, 2012 (hereafter AH12) has been submitted to Climate of the Past – Discussion, and their results are now available. They used almost exactly the same proxy data as S11, but used a different model (in fact, an ensemble of the GCMs used in the PMIP2 project) and methodology to constrain the difference in climate between the present and the LGM. Their results share some similarities to S11 but also contain some differences.

AH12 use pseudo-proxy data to validate their reconstruction. Their fit to the proxy data is improved relative to S11 (correlation of 0.73 vs S11′s 0.53).

Figure 5: a) Validation with GCM-Generated Pseudo-Proxy Data and b) Fit to Proxy Data

One of the criticisms of S11 was that it found an LGM globally-averaged surface temperature that seemed awfully warm (areas where proxy data were available averaged a mere~2°C colder than more modern temperatures) relative to other estimates, which show an LGM nearly three times that cold (e.g. von Deimling et al., 2006). This warmer LGM was necessarily responsible for much of the difference in their ECS value vs. the “canonical” estimate of 3°C. The authors attributed much of this difference to the use of warmer MARGO SST data vs. older (and cooler) data, but that explanation might appear somewhat insufficient, as the PMIP2 models that best fit the MARGO data themselves had ECS estimates closer to 3°C (Otto-Bliesner et al., 2009). Another odd result of S11 was the large discrepancy between their land only and ocean only results.

AH12 find an overall cooling at the LGM of ~4°C. Their land only and ocean only data are somewhat different, but are much closer than S11′s and are consistent within their uncertainties:

Figure 1: LGM Surface Air Temperature Reconstruction

Figure 2: LGM SST Reconstruction

In some ways, this represents a validation of S11: it’s certainly warmer than previous estimates, and the warm SSTs do arise from the MARGO data rather than some problem with S11. In other ways, however, it’s a contradiction of S11 and a validation of consensus estimates: the IPCC AR4′s estimate for LGM cooling was 4-7°C, consistent with AH12 but not S11.

AH12′s LGM-derived ECS is where I anticipate the greatest amount of well-meaning misunderstanding as well as outright misrepresentation. Why? Because it’s low: 1.7°C (1.2-2.4°C).


One of the criticisms of S11 I raised with Nate Urban in our interview was the problem of the asymmetry of climate sensitivity during different climatic states- i.e. climate sensitivity itself may be smaller at colder times than it is during warmer times. So hypothetically a perfect estimate of equilibrium sensitivity derived from data from the LGM might be significantly lower than a perfect estimate of ECS in a doubled-CO2 future due to the non-linearity of certain feedacks. While this asymmetry is by no means an unquestionably real phenomenon, there are some very good reasons to suspect it to be true (e.g. Crucifix, 2006; Hargreaves et al., 2007; Yoshimori et al., 2011). In fact, the authors of the MARGO SST data used by S11 themselves go out of their way to warn against mistaking an LGM-derived ECS as being comparable to 2xCO2 ECS for precisely this reason (Waelbroeck et al., 2009).

AH12 note this explicitly:

However, such a simplistic estimate is far from robust, as it ignores any asymmetry or nonlinearity which is thought to exist in the response to different forcings… The ratio between temperature anomalies obtained under LGM and doubled CO2 conditions found in previous modelling studies varies from 1.3… to over 2…

Therefore, a more apples-to-apples comparison (taking into consideration the asymmetry issue) of their findings to a doubling of CO2 might look more like 2.8°C, with a range of 1.56-4.8°C.

[All I've done is apply the average of asymmetry values (1.3-2) cited by AH12 to their central value of 1.7°C, while applying the low and high end asymmetry values to their lower and upper 95% CI values respectively. This is obviously meant to be illustrative of the difference taking asymmetry into account makes for 2xCO2 vs. LGM values rather than a rigorous quantitative exploration.]

This puts the 2xCO2 ECS inline with consensus estimates such as the IPCC AR4 GCM-only estimate of 3±1.5°C. Interestingly, some of the S11 authors, using the same UVic model but with instrumental rather than LGM paleo data, found broadly similar results for ECS (Olson et al., 2012).

I’m not claiming to show what AH12 “really” says about ECS, but rather making a general point that often gets overlooked in discussions of ECS estimates derived from colder climates. And it’s certainly possible that my not-even-back-of-the-envelope extrapolation of their LGM ECS into a 2xCO2 ECS is horribly misguided for some reason that I am as of yet unaware- but I’ve inquired, and will dutifully revise this post if there is.

More than anything, this is a place-marker in the event that the typical denialist spin cranks up as it has over papers in the past.


  • Annan, J. D., and J. C. Hargreaves (2012), A new global reconstruction of temperature changes at the Last Glacial Maximum, Climate of the Past Discussions, 8(5), 5029–5051, doi:10.5194/cpd-8-5029-2012.
  • Clark, P. U., A. S. Dyke, J. D. Shakun, A. E. Carlson, J. Clark, B. Wohlfarth, J. X. Mitrovica, S. W. Hostetler, and A. M. McCabe (2009), The Last Glacial Maximum, Science, 325(5941), 710–714, doi:10.1126/science.1172873.
  • Crucifix, M. (2006), Does the Last Glacial Maximum constrain climate sensitivity?, Geophys. Res. Lett.33(18), L18701, doi:10.1029/2006GL027137.
  • Fyke, J., and M. Eby (2012), Comment on “Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum,” Science, 337(6100), 1294–1294, doi:10.1126/science.1221371.
  • Hargreaves, J. C., A. Abe-Ouchi, and J. D. Annan (2007), Linking glacial and future climates through an ensemble of GCM simulations, Clim. Past3(1), 77–87, doi:10.5194/cp-3-77-2007.
  • Holden, P., N. Edwards, K. Oliver, T. Lenton, and R. Wilkinson (2010), A probabilistic calibration of climate sensitivity and terrestrial carbon change in GENIE-1, Climate Dynamics, 35(5), 785–806, doi:10.1007/s00382-009-0630-8.
  • Knutti, R., and G. C. Hegerl (2008), The equilibrium sensitivity of the Earth’s temperature to radiation changes, Nature Geoscience, 1(11), 735–743, doi:10.1038/ngeo337.
  • Olson, R., R. Sriver, M. Goes, N. M. Urban, H. D. Matthews, M. Haran, and K. Keller (2012), A climate sensitivity estimate using Bayesian fusion of instrumental observations and an Earth System model, J. Geophys. Res., 117(D4), D04103, doi:10.1029/2011JD016620.
  • Otto-Bliesner, B. et al. (2009), A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum, Climate Dynamics32(6), 799–815, doi:10.1007/s00382-008-0509-0.
  • Schmittner, A., N. M. Urban, J. D. Shakun, N. M. Mahowald, P. U. Clark, P. J. Bartlein, A. C. Mix, and A. Rosell-Melé (2011), Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum, Science, 334(6061), 1385–1388, doi:10.1126/science.1203513.
  • Schmittner, A., N. M. Urban, J. D. Shakun, N. M. Mahowald, P. U. Clark, P. J. Bartlein, A. C. Mix, and A. Rosell-Melé (2012), Response to Comment on “Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum,” Science337(6100), 1294–1294, doi:10.1126/science.1221634.
  • von Deimling, T. S., A. Ganopolski, H. Held, and S. Rahmstorf (2006), How cold was the Last Glacial Maximum?, Geophys. Res. Lett., 33(14), L14709, doi:10.1029/2006GL026484.
  • Waelbroeck, C. et al. (2009), Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum, Nature Geoscience, 2(2), 127–132, doi:10.1038/ngeo411.
  • Yoshimori, M., J. C. Hargreaves, J. D. Annan, T. Yokohata, and A. Abe-Ouchi (2011), Dependency of Feedbacks on Forcing and Climate State in Physics Parameter Ensembles, Journal of Climate, 24(24), 6440–6455, doi:10.1175/2011JCLI3954.1.

Skeptical Science is now an iPhone App

Image shamelessly lifted from Skeptical Science


Most readers probably don’t need to know any more than that before racing off to download it. For those of you who may not know about Skeptical Science, a bit of background:

Skeptical Science is a fantastic blog run by John Cook, which I link to often. A physicist by training, John has approached the climate issue from an educated layman’s perspective, and seeks to address common “skeptic” complaints, myths, and attacks on climate science by doing something they never seem to- examining what the science actually says! To that end John tracks down relevant studies [full disclosure- I occasionally send papers I think might be helpful] and synthesizes their main points in a way that is both easy for the general public to digest and doesn’t sacrifice nuance or detail.

So visit Skeptical Science for more info, or just click the image at the top of this post to be taken to the iTunes store. And spread the word! This is an immensely handy resource and climate realists, proper skeptics, and science fans in general will love having it.

You can also follow Skeptical Science on Twitter.

Blog Action Day 2009 – Climate Change

Events seem to be conspiring against me getting a proper post out today, but I haven’t given up hope just yet. In the mean time, I’ll redirect interested parties to David Roberts’s welcome Seven reasons for optimism about the Senate climate Bill, Skeptical Science’s comprehensive Empirical evidence that humans are causing global warming, and the helpful (and new to me) Responses to Questions & Objections on Climate Change by economist Brett Paris.

Jonah Goldberg’s classic know-nothing, non-denial climate denial

Jonah Goldberg has a new op-ed in the LA Times on climate science, and it’s about as confused and asinine as you might expect. First Jonah wants to astound us with his grasp of paleoclimate science:

There was a Maunder Minimum! It had to do with sunspots! It was cold!:

During what scientist call the Maunder Minimum — a period of solar inactivity from 1645 to 1715 — the world experienced the worst of the cold streak dubbed the Little Ice Age. At Christmastime, Londoners ice skated on the Thames, and New Yorkers (then New Amsterdamers) sometimes walked over the Hudson from Manhattan to Staten Island.

Of course, it could have been a coincidence. The Little Ice Age began before the onset of the Maunder Minimum. Many scientists think volcanic activity was a more likely, or at least a more significant, culprit. Or perhaps the big chill was, in the words of scientist Alan Cutler, writing in the Washington Post in 1997, a “one-two punch from a dimmer sun and a dustier atmosphere.”

“What does the Maunder minimum have to do with anthropogenic warming?” you might ask, given that a mere 7 years of 2003-level CO2 emissions alone would make up for the lost radiative forcing. Jonah seems to believe that Revelations of Great Importance about sunspots may throw a monkey wrench in the whole anthropogenic warming scam:

Well, we just might find out. A new study in the American Geophysical Union’s journal Eos suggests that we may be heading into another quiet phase similar to the Maunder Minimum.

I’m not finding any “new studies” in EOS that claim this. Is Goldberg talking about Livingston and Penn’s article Are Sunspots Different During This Solar Minimum? If so, add “study” to the pile of things Jonah doesn’t understand the definition of. And again, given the effect of GHGs vs. a return to Maunder like conditions, Jonah’s barking up the wrong tree.

Meanwhile, the journal Science reports that a study led by the National Center for Atmospheric Research, or NCAR, has finally figured out why increased sunspots have a dramatic effect on the weather, increasing temperatures more than the increase in solar energy should explain. Apparently, sunspots heat the stratosphere, which in turn amplifies the warming of the climate.

Scientists have known for centuries that sunspots affected the climate; they just never understood how. Now, allegedly, the mystery has been solved.

Meehl et al.’s paper Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing is certainly interesting, but it’s by no means definitive, and it’s not really clear what impact it is supposed by Jonah to have on our understanding of anthropogenic warming- the paper purports to model the process by which known phenomena are amplified to produce known observations. Again, what point does Jonah hope to make by citing this?

Also, Milankovitch cycles have a significant impact on glaciation cycles! Something else climate scientists were ignorant of. Let Jonah blow your mind:

Last month, in another study, also released in Science, Oregon state researchers claimed to settle the debate over what caused and ended the last Ice Age. Increased solar radiation coming from slight changes in the Earth’s rotation, not greenhouse gas levels, were to blame.

The Clark et al. paper The Last Glacial Maximum helps pin down the timing of various forcings during the deglaciation ending the LGM (i.e. the lag vs. lead issue), identifying insolation as the initial driver. However, the paper confirms the significant amplifying effect of GHGs without which the magnitude of warming would not have been possible, and supports other evidence which shows that the West Antarctic Ice Sheet will not decay linearly but rather abruptly- not exactly a paper I’d want to cite in challenging the consensus on the need to mitigate.

Did you know that periodicity necessitates a long term upward trend, and that this self-evident fact is completely ignored by egg headed climate scientists? Gerald Meehl may think his team’s above referenced paper is only talking about a modest amplified response to the 11-year solar cycle, but Meehl’s actually in denial that he’s discovered a massive long term solar-induced warming trend. Jonah to the rescue:

“Global warming is a long-term trend, Dr. Meehl says. … [the Science 11-year solar cycle] study attempts to explain the processes behind a periodic occurrence.”

This overlooks the fact that solar cycles are permanent “periodic occurrences,” a.k.a. a very long-term trend.

Stupid, stupid Meehl… You have to look beyond what the evidence actually says and force yourself to see the evidence visible only to Jonah.

[Periodicity of course does not imply a long term trend either positive or negative. The existence or lack of any additional long term behavior superimposed on top of the 11-year periodicity is an altogether different kettle of fish, which again I'll get to later.]

It wouldn’t be a Goldberg piece without some requisite potshots at the deceptive Liberal Fascist Media:

For instance, when we have terribly hot weather, or bad hurricanes, the media see portentous proof of climate change. When we don’t, it’s a moment to teach the masses how weather and climate are very different things.

Of course we get the look-aren’t-I-reasonable non-denial:

No, I’m not denying that man-made pollution and other activity have played a role in planetary warming since the Industrial Revolution.

And you know what’s coming next- an enormous “but”, where non sequiturs and appeal to ridicule abound:

But we live in a moment when we are told, nay lectured and harangued, that if we use the wrong toilet paper or eat the wrong cereal, we are frying the planet.

Presumably the references to toilet paper and cereal aren’t meant to be taken at face value, but rather are to be “enjoyed” as Jonah’s trademark “aren’t enviros nutty little fascists” humor. If on the off chance they were an actual allusion to tropical deforestation, however, the warming effects of such due to increased atmospheric CO2 and decreased evapotranspiration are real enough.

Hey! Did you know that climate science has heretofore completely ignored solar variability in the context of climate change? Jonah breaks this scandal wide open:

But the sun? Well, that’s a distraction. Don’t you dare forget your reusable shopping bags, but feel free to pay no attention to that burning ball of gas in the sky — it’s just the only thing that prevents the planet from being a lifeless ball of ice engulfed in total darkness.

What’s priceless about this little argument from ignorance is that the sun is most certainly not, in Jonah’s words,  “the only thing that prevents the planet from being a lifeless ball of ice.” There’s a little something called the bloody greenhouse effect that warms the Earth by an additional ~33°C, keeping it from being a “lifeless ball of ice” whereas the sole influence of the sun does not.

Jonah then doubles (triples) down on the increased sunspot gambit, a la The Great Global Warming Swindle:

Never mind that sunspot activity doubled during the 20th century, when the bulk of global warming has taken place.

This is presumably where Jonah’s sunspot obsession has been leading all along. The ever-observant Jonah has noted that there seems to be a Correlation between sunspots/solar activity (which he Has Recently Learned causes warming, long term warming mind you) and what the enivro hippie fascist scientists would have us believe is the period of Allegedly Man-made Warming. Case closed, mirite?

Not so much. The correlation falls off completely in the last several decades:

This has been covered at length elsewhere (e.g. here, here, here).

Did you know that climate science predicts anthropogenic warming should be monotonic even though climate scientists actually never make that claim? Jonah, again, sets the scientific community straight:

What does it say that the modeling that guaranteed disastrous increases in global temperatures never predicted the halt in planetary warming since the late 1990s? (MIT’s Richard Lindzen says that “there has been no warming since 1997 and no statistically significant warming since 1995.”)

Of course this has been covered many, many times elsewhere, notably by Robert Grumbine in determining climatologically significant temperature trends (and here), by Tamino in when to expect new record temperatures, and by RealClimate discussing what the IPCC models actually say. This issue is also explicitly addressed by Easterling and Wehner in their GRL paper Is the climate warming or cooling? that demonstrates (as does the RealClimate post) that climate models, contrary to Jonah’s claim they “guaranteed disastrous increases in global temperatures [and] never predicted the halt in planetary warming since the late 1990s” do in fact capture the kind of variability that can give the illusion of a period without warming despite a clear overall warming trend:

Jonah probably has, like Roger Pielke Jr., confused the IPCC projection of the forced component of climate over time with specific temperature predictions. However, the ensemble averaged projection isn’t even monotonic itself, and the individual modeling runs certainly are not:

Jonah asks:

What does it say that the modelers have only just now discovered how sunspots make the Earth warmer?

If it were remotely true, it would say quite a lot. Of course, there’s the pesky little issue of Jonah not knowing what the frack he’s talking about. Sunspots don’t themselves warm the Earth, but rather sunspots coincide with increased solar luminosity, and changes in solar forcing certainly have an impact on the climate system- something known for decades. While the precise mechanism by which this occurs may not have been modeled until now (and still might not), the magnitude of the effect is small enough so as to be irrelevant in the context of end-of-century projections of future warming.

Jonah’s own referencing of the Maunder minimum clearly demonstrates that the sunspot/solar variability/climate link is far from new information, and solar variability both pre-industrial and current are of course taken into account in modeling the climate. What does it say about Jonah that the implications of his own column debunks this ridiculous proposition?

And of course, we end with Jonah’s actual bottom line. All of this whinging about toilet paper and sunspots and cereal, and ignorance of the meaning of scientific “study”, what climate models actually say, etc. is merely the pretext for a political argument from ignorance in opposition to climate legislation:

I don’t know what it tells you, but it tells me that maybe we should study a bit more before we spend billions to “solve” a problem we don’t understand so well.

Although the scientific process is obviously deeply mysterious to Jonah, to the rest of us things aren’t so terribly opaque. While our understanding of the climate system increases, the broad strokes of the anthropogenic warming issue are well understood. This is why the national science academies of the US and other major science producing nations are united in their calls for significant reductions in greenhouse gas emissions. Jonah’s argument is the equivalent of  telling an HIV-positive patient that she should hold off on taking the recommended antiretroviral cocktail that will help her lead a relatively normal life because ‘we are still learning new things’ about the virus every day. It’s nonsensical, and more to the point grossly negligent advice.

The only meaningfully true statement Jonah makes in his entire column is this one:

What is the significance of all this? To say I have no idea is quite an understatement, but it will have to do.

Indeed. I believe in Jonah’s line of work we call that burying the lede.

This is what we can expect as blatant climate denialism becomes ever more ridiculed and politically untenable- the non-denial denialism. The lip service to anthropogenic warming that is ultimately revealed to be concealing the same old anti-mitigation arguments based upon economic/ideological opposition to the feared remedy of emissions reductions. This is what so many people who naively believe that “if only people were more accepting of science, they would be less opposed to preventing disastrous climate change” don’t get. The Jonah Goldbergs of the world are not, have never been, and will never be interested in what the science actually says. They are ideologically opposed to the implied solution of emissions reductions, and they shift their arguments accordingly.

Even if Jonah is made aware of the inanity of his specific complaints in this column in terms of policy relevance, he will simply move to a new but inevitably anti-mitigation position, be it geo-engineering, a gamed cost/benefit stance in the tradition of Jim Manzi, the Breakthrough fallacy of magical future clean energy, etc. Once those arguments are rebutted, we’ll still see a “But, but, China/India!” And by the time those arguments (or their successors, or their successors) are thoroughly debunked and untenable, there probably won’t be need of any further ones as the clock will have effectively run out on meaningful mitigation due to political and energy infrastructure inertia.

This is not, and will never be, about science for them. It’s about running out the clock on a perceived threat to economic interests.