Does the hockey stick "matter"?
November 14th, 2005Posted by: admin
Post by Steve McIntryre
Stefan Rahmsdorf and others (including Roger Pielke, the proprietor of this site) have taken the position that the Hockey Stick is irrelevant to the great issue of the impact of 2xCO2 on global climate. Even the originator of the Hockey Stick, Michael Mann, who received many awards and honors for its construction, ironically has taken the position that it doesn’t “matter”. (I do not believe that he has not returned any of the honors.) I’m inclined to agree that, for the most part, the Hockey Stick does not matter to the great issue of the impact of 2xCO2. However, I believe that it matters (or should matter) to IPCC, to governments that relied on IPCC and to climate scientists who contributed to and supported IPCC and to people who may wish to rely on IPCC in the future.
The Hockey Stick was not, as sometimes portrayed, an incidental graphic, buried in IPCC TAR. Nor was it an icon resurrected by sceptics purely to torment poor Michael Mann. It could almost characterized as the logo for IPCC TAR. Figure 1 below shows Sir John Houghton, at the press conference releasing IPCC TAR, standing in front of the Hockey Stick. The graphic was used repeatedly in IPCC TAR and was one of the most prominent graphics in the Summary for Policymakers. Some governments (and, the Canadian government in particular) relied upon it in their promotion of Kyoto policy even more than IPCC. In the lead-up to adopting Kyoto policy, Canadians were told by their Minister of the Environment that “1998 was the warmest year of the millennium and 1990s the warmest decade”. So even if the Hockey Stick did not “matter” to the scientific case, it mattered to the promotion of the scientific case. Scientists may want to “move on”, but institutions cannot, if they want to maintain any credibility. If the Hockey Stick was wrong, it would be as embarrassing as the failure to find WMD in Iraq. In both cases, the policy might well be justified on alternative grounds, but the existence of the alternative grounds does not mean that responsible agencies should not try to isolate the causes of intelligence failure and try to avoid similar failures in the future.
The issues surrounding the MBH Hockey Stick are complicated by IPCC TAR statements and decisions, which, in retrospect, seem misguided, although there is little to suggest that IPCC AR4 is taking to steps to avoid similar potential problems. The most questionable IPCC statement about the Hockey Stick is that the MBH98 reconstruction had “significant skill in independent cross-validation tests”. I added bold to highlight the plural—a second level to the misrepresentation contained in this claim. The statement appears to have been written by Michael Mann about his own work. It is now known that the MBH98 reconstruction in the controversial 15th century portion failed the majority of cross-validation tests, including the standard R2 test [McIntyre and McKitrick, 2005a]; the source code provided to the Barton Committee shows that the adverse cross-validation R2 statistics were calculated, but not reported. It is also now known that the MBH98 reconstruction does not live up to its warranty that it is robust to the presence/absence of all dendroclimatic indicators, as the reconstruction depends on the inclusion of bristlecones, a series known to be potentially contaminated as a temperature proxy. Again, this adverse information was known to the authors and not reported.
If I were in Houghton’s shoes, I would be mad as a boil about all this. Since Houghton has a sincere belief that the impact of 2xCO2 is the great issue of our times, then, if I were Houghton, I would be particularly angry at being placed in a position where I used this logo and wasn’t fully informed about adverse information pertaining to it. I also wouldn’t be leaving it up to some probably adversarial committee like the Barton Committee to sort this out. I’d be all over the problem so that my community, the community of climate scientists, was not further embarrassed and so that government institutions would be able to rely confidently on the opinions of IPCC.
If I were Houghton, one line of argument that I would not accept is that the other “independent” studies all say similar things. It was the Mann study that I stood in front of. If there are serious problems in it, which were known ahead of time and I didn’t know about them, I would carve everyone involved a new you-know-what. Now for public purposes, I’d feel a lot happier if I could at least retreat to the safe haven of other studies that showed something at least similar to the Mann study. But I’d be pretty worried about them on a couple of counts and I’d want them torn through from top to bottom. The first thing that would worry me is that the studies were not really “independent”. The coauthors all seem to swap places: you see Mann, Jones, Briffa, Bradley, Cook, Schweingruber – all well-known scientists, but all having coauthored together. I’d be worried about a monoculture and want a fresh set of eyes. The second thing that would worry me is that the same proxies are used over and over – the bristlecones, the Polar Urals etc. I’d be worried about systemic problems. I’d be worried that no one seemed to have gone through these other studies like M&M had gone through the MBH studies. Maybe there are more time-bombs. I wouldn’t just passively wait for them to go off.
If I were Houghton, I would be enraged at the public refusal by IPCC authors to show their data and methods. When I read in the Wall Street Journal that Mann had said that he would not be “intimidated” into showing his algorithm, I’d have taken immediate action; I’d have told Mann to stop acting like a prima donna, to archive every line of code and data used in MBH98 and stop fighting a pointless battle that simply embarrassed IPCC and the entire field of climate science. I’d have done more than that. I’d have notified everyone contributing to IPCC that we did not expect the same kind of nonsense any more, that anyone contributing to IPCC would have to ensure that their archives of data and methodology were complete or else we couldn’t use their articles. I’d have done so before I heard from some redneck Republicans.
I would also review how we were checking studies in IPCC AR4. If our very logo for IPCC TAR blew up on us, then something was wrong with our procedures for review. I wouldn’t go around patting ourselves on the back and telling everyone that this was the most “rigorous” review procedure in the history of science, since we’d goofed on such a prominent issue. I’d want to know why we goofed and how to avoid it in the future, or at least, how to minimize the chances of a recurrence. So when some redneck tried to use the Hockey Stick fiasco against IPCC, I’d at least have an answer.
A final thing that I’d ask myself: if this damn chart is “irrelevant” to the great issue of 2xCO2, why did we use it at all? And why did we rely on it so much in our sales presentations? Why didn’t we just talk about the issues that were important and stay away from little irrelevant stuff? Maybe I’d find out, when I investigated, that someone had decided that this was merely for sales promotion – the climate equivalent of a sexy girl sitting on a car. If that were the case, I wouldn’t necessarily be happy about it, but at least I’d understand it. Then I’d want to make sure that we were also selling steak as well as sizzle. I’d sure want to make sure that we’d really done a good job on the issue which Ramsdorff and others now say was the “real” issue: climate sensitivity to 2xCO2.
Here I’d be bothered by how little guidance we actually gave to policymakers interested in an intermediate-complexity analysis of whether 2xC02 will lead to a temperature increase of 0.6 deg C or 2.6 deg C or 5.6 deg C. When I re-examined the TAR, I’d notice that we’d virtually skipped over these matters. I’d think: it’s not enough just to list all the results of different models; let’s try to figure out why one model differs from another, what are the circumstances under which a model gives a low sensitivity and what are the circumstances that a model has high sensitivity – if that’s the “real issue”. When I saw that we’d barely touched this sort of analysis in IPCC TAR, I’d be pretty embarrassed. I would certainly vow that in AR4, we would not repeat the mistake of ignoring the “real issues” in favor of hood ornaments.
The other thing that I wouldn’t do is simply ignore the problem and hope that it goes away of its own accord. I wouldn’t rely on the assurances of Mann and similar protagonists that the various alleged defects do not “matter”. No corporation would do so in similar circumstances and IPCC shouldn’t either. I would long ago have got some independent statistician to see if there really was a problem that I should be worried about. I wouldn’t have stood still for this water torture. I’d tell Mann to co-operate with the investigator and request McIntyre to cooperate. I’d try to get the parties to sign off on an exact statement of points and issues that everyone agreed on and ones that were in dispute. Once I saw what was in dispute, I’d ask for what would be involved to determine once and for all who was right on specific issues. I would long ago have gotten tired of barrages from both sides, where I couldn’t be sure that they were not at cross-purposes.
So does the Hockey Stick matter? Yes, if you’re a climate scientist that believes that the IPCC is an important institution whose opinions should be valued. Mann now thinks that the Hockey Stick does not matter. As so often, life is full of ironies.
November 15th, 2005 at 5:52 pm
Hi Steve,
You write, “I’m inclined to agree that, for the most part, the Hockey Stick does not matter to the great issue of the impact of 2xCO2.”
I think you’re neglecting two issues that are even greater than that:
1) The issue of above-2xCO2 nonsense, and
2) Drastic **under**prediction (not over-prediction!) of world per-capita GDP growth in the 21st century.
I’ll discuss each of these issues below:
1) ABOVE 2xCO2 NONSENSE
The more-or-less unchanging estimate of climate sensitivity to 2xCO2 is 1.5 to 4.5 degrees Celsius. Let’s take the absolute highest value there, and assume that all warming of approximately 0.7 deg C from 1880 to date has been due to GHGs (no solar, no land changes, nothing else). That that means that the total warming from today to 2xCO2 would be 4.5 – 0.7 = 3.8 deg C.
Well, the quoted IPCC TAR range is up to 5.8 deg C…a full 2.0 deg C higher! Where does most of that come from? It comes from the ridiculously high atmospheric methane concentrations, CO2 emissions and resulting CO2 concentrations assumed in the most ridiculous “scenarios.” (Better known to children as fairy tales.) Take the A1F1 and A2 fairy tales:
http://www.grida.no/climate/ipcc_tar/wg1/030.htm
No one I know of is ignorant enough to think those have a chance of coming to pass. In fact, even James Hansen (who allows his name to be associated with such ridiculous article titles as “Can we defuse the global warming time bomb”?) basically acknowledges that they are nonsense ***in that very same article.*** Here is the quote: “Also the IPCC predilection for exaggerated growth rates of population, energy intensity, and pollution calls into question the realism of their results.”
http://www.sciam.com/media/pdf/hansen.pdf
So we basically have an IPCC TAR primary author admitting the exaggeration of, and dismissing the validity of, the IPCC’s work!
And it is entirely proper that he is dismissive of the IPCC TAR’s projections. Apparently, the highest projections were added AFTER PEER REVIEW, right before publication of the report:
http://www.sepp.org/NewSEPP/IPCC-Ex_PoliticalScientist.htm
Note from that article how even Stephen Schneider (who in no way could be accused of being a “sceptic”) apparently wrote in Nature:
“This sweeping revision depends on two factors that were not the handiwork of the modelers: smaller projected emissions of climate cooling aerosols; and a few predictions containing particularly large CO2 (carbon dioxide) emissions.”
Don’t you think that is a blatant example of fraud…to add the most radical high projections AFTER peer review?
November 15th, 2005 at 5:53 pm
Continuing my comments (the computer wouldn’t post the longer version):
2) DRASTIC UNDER-PREDICTION OF WORLD PER-CAPITA GDP
I’ve commented about this in depth on your blog. Basically, I am extremely confident that the IPCC dramatically UNDER-predicts world per-capita GDP in the year 2100. This is an area where, I am very sorry to say, Ian Castles and David Henderson may end up HURTING the cause of accurate assessment of the 21st century, rather than helping it. Please do NOT misunderstand me…I agree completely with their conclusion that the IPCC OVER-estimates CO2 emissions. But the IPCC over-estimates CO2 emissions because they are so completely clueless (I think a more accurate assessment is ‘dishonest”…see discussion above) about the carbon intensities that are likely to occur as the 21st century progresses.
I am very confident that the world per-capita GDP in the year 2100 will be well ABOVE the IPCC’s highest estimate of $129,000 (in 1990 dollars).
http://markbahner.typepad.com/random_thoughts/2004/09/second_thoughts.html
In contrast to your debate about the “Hockey Stick,” even lay persons can understand that it is morally wrong for people who are making $5,000, $10,000, $20,000 or $40,000 a year to sacrifice for people who are making over $100,000 per year. (My actual prediction is over $1,000,000 per year.)
Unfortunately, I don’t think that the economics community will recognize the likelihood of extremely high economic growth for another 10-20 years:
http://markbahner.typepad.com/random_thoughts/2005/11/why_economic_gr.html
By then, who knows how many billions of dollars will have been wasted on (deeply flawed) climate change analyses?
Summary: I hope you will take the time to inform yourself on both of these matters…Item 1 particularly. The simple fact is that people are listening to what you say. As I think they should.
Best wishes,
Mark
November 16th, 2005 at 9:28 am
I believe Steve has heard my Bahner Warning Klaxon (BWK) before…I have turned the volume down and merely pass on the information as text, so as to not disturb the conversation in the other thread.
D
November 16th, 2005 at 2:11 pm
Two short comments:
(i) The sentence ‘bristlecones, a series known to be potentially contaminated as a temperature proxy’ has been proposed by McKitric and MacIntyre (MM), but this claim has not, to my mind, been backed up by convincing evidence. Is it really known? How certain is this?
(ii) Wouldn’t it be right to subject MM to the same rigorous examination that Mann, Bradley and Huges had to endure, in order to ensure objectivity? In other words having a committee (for instance consisting of John McCain and Joe Lieberman) going through all papers, funding history, etc, (same demands as put upon MBH). That way, the ordeal is not just one-sided.
November 16th, 2005 at 10:34 pm
(1) Rasmus, look at our paper in E&E for a discussion of bristlecones. Graybill and Idso, who published the chronologies which dominate the MBH98 PC1, stated that their 20th century growth pulse was not due to temperature. Even Hughes has said that the growth pulse is a “mystery” – a position hardly consistent with attributing them as a unique detector of world climate history.
(2) Much more effort has gone into analyzing and attempting to refute our papers than ever went into considering MBH – consider the comments von Storch and Zorita; Huybers; submissions by Wahl and Ammann; submission by Ritson; Rutherford et al, and even Mann (rejected by Climatic Change) and at realclimate. In contrast, I am unaware of ANY serious effort to analyze details of MBH98 prior to our work.
I posted up answers to Barton questions at climateaudit.org on the basis that I was asked the same questions. Barton did not ask MBH to create data archives if they did not already exist; only for the location if they existed. If the authors routinely archived results, they would presumably also index the archives. It’s interesting that Ray Bradley falsely claimed that he had archived his various Arctic datasets, when he has only archived a little bit from the early 1990s. Given the publicity that Rafael Palmeiro received for providing incorrect answers to a House Committee, you’d think that they would have been careful to provide correct answers.
November 17th, 2005 at 7:03 am
Statement: ‘Graybill and Idso, who published the chronologies which dominate the MBH98 PC1, stated that their 20th century growth pulse was not due to temperature.’
Question: what is the evidence for this – in your own words if you please. Why? To just cite Graybill and Idso is not much more reassuring than blaming the Flying Spaghetti Monster…
The phrase ‘a series known to be POTENTIALLY contaminated …’ doesn’t say it IS contaminated, does it. A suppose all pealoclimatic proxies can potentially be contaminated by other factors. So, are there ANY evidence suggesting that the bristle cones are LIKELY contaminated? How would this, if this is the case, take place, and would it really matter if the bristle cone is influenced by temperature. Normally, the regression analysis ought to be able to destill the temperature signal from other factors, to some degree. If this is the case, then ‘being potentially contaminated’ may not be so important.
Regarding evaluation/validation, there is a lengthy discussion on the RealClimate.org site.
One could argue that the MM paper has become important – not in scientidfic sence, but in political sense, judging from the aftermath of the discussion. Perhaps it serves to falsly smear the IPCC? But, I am not convinced that the debate about the hockey stick is that important in terms of climate science and the ongoing global warming.
November 17th, 2005 at 11:53 am
Rasmus, Graybill collected the bristlecone samples, was a highly respected dendrochronologist at the University of Arizona. This is hardly the Flying Spaghetti Monster. He carried out a variety of statistical comparisons between bristlecone growth and local and regional climate variables. In the case of the important Sheep Mountain and Campito Mountain sites, a weather station operated very close by for nearly 30 years. The lack of correlation between temperature and ring width was first reported by Lamarche, Fritts, Graybill and Rose (a blue chip dendro panel) for Sheep Mountain in Nature [1984]. Graybill carried out a more comprehensive collection, reported in Graybill and Idso [1993]. The phenomenon of direct CO2 fertilization was acknowledged as potential problem in IPCC 2AR. I have carried out my own comparisons of bristlecone ring widths to gridcell temperature – see our recent Reply to von Storch and Zorita – which also show no connection between the 20th century growth spurt in bristlecones and gridcell temperature. Hughes has said that the bristlecone growth spurt is a “mystery”. It would be helpful if you read our article in E&E, which reviews the issues in some depth.
The specialists explicitly said that the series is not a temperature proxy in the 20th century. One of the reasons for the acceptance of MBH98 was the belief that its proxies were carefully selected as temperature proxies. In face of the explicit specilaist statements, the onus is on Mann to show that these proxies are valid.
Perhaps more importantly, Mann did studies without the bristlecones (see the BACKTO_1400-CENSORED directory) and the PCs so calculated did not have a hockey stick shape. He did not report these results and even went so far as to claim that their results were robust to the presence/absence of all dendroclimatic indicators – which again was a factor in its widespread acceptance. Now realcliamte argues that any calculations without bristlecones amount to “throwing out data”. Well, if the results are “robust” to all dendroclimatic indicators, they should be robust to bristlecones without any huffing and puffing about “throwing out data”.
2) I googled “realclimate evaluation verification” and was unable to locate the promised discussion of evaluation/verification. I noticed a curious discussion by realclimate coauthor William Connolley at his own blog, where he seemed to take some pride in not knowing the difference between an R2 and a RE statistic.
November 17th, 2005 at 7:20 pm
is your site down steve?
November 17th, 2005 at 9:23 pm
The site maxed out its disk space and crashed. The server host has been impossible to get hold of to restore things. I’m pretty irritated.
November 18th, 2005 at 5:19 am
climateaudit is back
November 18th, 2005 at 8:26 am
Need to keep a remote backup copy of the site. This is a standard BBS thing…
November 21st, 2005 at 5:16 am
Hi Steve,
Again, it’s not the single PC shapes that matter, but the final regression results. I think it is easy to get confused by focusing on just ONE PC. PCA can sometimes give you different first modes if the leading eigenvalue is not well-separated from the next. Thus, by removing some series can result in a different leading principal component, and there is nothing suspicious about that. The regresion step finds the best wieghted combination of N PCs, and the important question is therefore how many PCs were included and what fraction of the variance these account for.
Most of the recent upturn is due to instrumental data. The real ‘hockeystick has an upturn at the end, but this is small compared with the superimposed instrumental data. The fact that the end of the ‘hockey stick’ matches the beginning of the warming seen in the instrumental data so well, and the fact that the slow decline before this matches the orbital parameter theory, is interesting (this too is an independent validation…). In fact, the hockey stick may serve as a counter evidence to your proposition – that the bristle cone data are not representative of the temperature, is false.
But, the way I see it, all paleoclimatic proxies are uncertain, and may be influenced by other factors (eg moisture, light, etc), and one must be careful to put too much confidence in such ‘reconstructions’. IPCC TAR states only that it is LIKELY (66-90% chance) that the 1990s have been the warmest dacade and 1998 the warmest year of the millennium (p. 3). I think this statement is fair, but also that this does not pu too much importance on the proxy.
November 21st, 2005 at 11:53 am
Rasmus
I am bemused by your logic. How is it possible at all “that the bristle cone data are … representative of the temperature” ?
The only thing that can do that is a comparison of the local temperature from the bristlecone sites, and the growth rates of the bristlecones. Graybill did this study, and published it, as M&M reference. MBH accept this to be true – see MBH’99.
Where we know the relationship of temperature to bristlecone growth, the relationship is poor. How can data where we don’t know the temperature validate that ?
what is more, I am bemused by your approach to Graybill’s data. Graybill published; his data and interpretations are a matter of public record. You can go look at that data and form your own view. McIntyre has paraphrased what Graybill said- what is wrong with agreeing with the authors ?
I am suprised you do not wish to form a view on the relationship between temperature and bristlecone growth.
yours
per
November 22nd, 2005 at 11:05 am
Why else does this matter: http://www.newscientist.com/article.ns?id=mg18825265.400&feedId=online-news_rss20 has a story about how “green” fuels are causing problems. Of course, all this needs to be taken in context. This story also is a great example of the law of unintended consquences. I suspect most of what we do to “save” the earth from AGW (if it is occuring) will not have the expected results.
November 23rd, 2005 at 12:57 pm
Rasmus,
you say: “PCA can sometimes give you different first modes if the leading eigenvalue is not well-separated from the next.” This is true, but not relevant here. The pattern of the bristlecones is quite well-separated.
But you’re missing the point:
1. Mann et al. said that their PC1 was the “dominant component of variance” – they didn’t say that it was just the bristlecones.
2. Mann et al. said that their reconstruction was robust to dendroclimatic indicators in total. A fortiori, it should be robust to the presence/absence of bristlecones.
3. The bristlecones are stated not to be a temperature proxy by the specialist authors and a”mystery” by coauthor Hughes. The MBH reconstruction should NOT depend on such controversial and tainted proxies.
4. Mann et al. should have disclosed all this and not made the claim about robustness to the presense/absence of all dendroclimatic indicators.
You can’t just do another data mining step at the regression step as Mann et al. do. If the bristlecones are not a tmeperature proxy, then you have to check for spurious regression results. Did MBH check the Durbin-Watson statistics as one way of minimizing chances of spurious regression? If they did, they ignored the results.
I don’t have time right now to parse IPCC application of the hockeystick, but I disagree with the suggestion that the HS was presented as an incidental argument in IPCC and that its usage was consistently nuanced and qualified.
November 27th, 2005 at 5:01 am
Steve,
If it is true as you say, that the Bristlecone is not correlated with temperature or whatever predictor, then a multiple regression wouldn’t attach much weight to it. Of course, previous claims about the Bristlecone may be wrong. Moreover, a stepwise multiple regression ought to pick up the ’signal’ and discard the noise. The fact that the end of the reconstructed proxy yields such a good resemblance to the instrumental data, and the agreement between long slow decline before that and the Milankovitch theory, point provide independent evidence for the curve’s validity. Now, these large-scale features in the MHB-curve are also in good agreemetn with other reconstructions, that is within reasonable bounds of uncertainty. (These paleoclimatic reconstructions are generally associated with a high degree of uncertainty compard to instrumental data, due to sparse information.) When I do a Google on ‘Bristlecone temperature’, I get 27500 hits, but when I include ‘flawed’ in the searxch string, I only get 297 (your name keeps cropping up…). Why such a difference if your claim about their quality were true?
Cheers!
Rasmus
November 28th, 2005 at 1:54 pm
Rasmus writes: “Why such a difference if your claim about their quality were true?”
Perhaps the null hypothesis should be that the document frequency of “flaw”, “flawed”, etc is ~1/1000.
Cheers, — Jo
November 30th, 2005 at 5:04 am
Abstract from Lamarche et al (1984):
‘A response of plant growth to increased atmospheric carbon dioxide, which has been anticipated from laboratory data, may now have been detected in the annual rings of subalpine conifers growing in the western United States. Experimental evidence shows that carbon dioxide can be an important limiting factor in the growth of plants in this high-altitude environment. The greatly increased tree growth rates observed since the mid-19th century exceed those expected from climatic trends but are consistent in magnitude with global trends in carbon dioxide, especially in recent decades. If correctly interpreted, these findings have important implications for climate studies involving tree ring observations and for models of the global carbon dioxide budget.’
Graybill & Idso:
‘The growth-promoting effects of the historical increase in the air’s CO2 content are not yet evident in tree-ring records where yearly biomass additions are apportioned among all plant parts. When almost all new biomass goes into cambial enlargement, however, a growth increase of 60% or more is observed over the past two centuries. As a result, calibration of tree-ring records of this nature with instrumental climate records may not be feasible because of such growth changes. However, climate signals prior to about the mid-19th century may yet be discovered by calibrating such tree-ring series with independently derived proxy climate records for those times.’
These seem to concern CO2 and tree-rings rather than temperature and tree rings. Isn’t there a very big leap to say that this implies that tree rings are not of use for temperature construction? You need to prove each step of this chain of logic, before making strong arguments about the Bristle cone data. I mean, the fact that there are tree rings, is a consequence of seasonally varying temperature (precipitation), after all? Please eluciadate us…
Rasmus
December 2nd, 2005 at 11:03 am
(1) Rasmus said: “If it is true as you say, that the Bristlecone is not correlated with temperature or whatever predictor, then a multiple regression wouldn’t attach much weight to it.” This is not true. Spurious regressions occur. Google “spurious regression” and you’ll find many amusing examples. Graybill and Idso [1993[ said that anomalous bristlecone growth was uncorrelated to temperature; Hughes said that the growth was a "mystery" and the correlations of bristlecone growth to gridcell temperature are negligible. However, the trending MBH98 PC1 (and the Gaspe tree ring chronology) do have a highish correlation to the trending temperature PC1. However, the correlation of dot.com stock prices is just as high and dot.com stock prices plus white noise yield just as "good" a reconstruction Given the explicit views of the specialists, one has to allow for the possibility of a spurious regression - using spurious in the technical sense.
(2) you have not responded to the false MBH claim of robustness to the presence/absence of all dendroclimatic indicators when their reconstruction is not robust to the prsence/absence of bristlecones
(3) you have not repsonded to MBH withholding the CENSORED results without bristlecones;
(4) "The fact that the end of the reconstructed proxy yields such a good resemblance to the instrumental data". The cross-validation R2 of the 15th century proxy network is about ~0 - hardly a good resemblance. In the AD1820 step, MBH uses 112 proxies including actual instrumental data to "predict" 79 years of data, so a decent R2 is almost unavoidable.
(5) you have not addressed the MBH withholding of adverse cross-validation R2 data.
(6) the Milankowitch point is not argued in MBH and provides no evidence whatever;
(7) the "agreement" with other reconstructions is not particularly close; but the other studies are not independent. There is extensive overlap of proxies - 14 of 17 series used in Jones et al [1998] are used by MBH. Bristlecones, Polar Urals, Tornetrask are used over and over. The coauthors overlap. So similarity to these other studies means virtually nothing.
(8) You say: “When I do a Google on ‘Bristlecone temperature’, I get 27500 hits, but when I include ‘flawed’ in the searxch string, I only get 297 (your name keeps cropping up…). Why such a difference if your claim about their quality were true?” What a ridiculous point. Most of the hits have nothing to do with bristlecones as a proxy but with travel information or the like.
(9) MBH claimed that the proxies were carefully selected and people propose policies based on this. The onus is on them to show that the proxies are valid. I have raised questions from specialist literature. What is particularly disquieting is that MBH withheld information about the bristlecones and adverse cross-validation statistics, so previous scientists were not necessarily aware of the problems.