ipcc wgi third assessment report - (tar ... - …

(PCC WG( TAR Gov/Expert Review (Expert Comments) Chapter 12 21111113

IPCC WGI THIRD ASSESSMENT REPORT - (TAR)GOVERNMENT/EXPERT REVIEW - APRIL-JUNE 2000

COLLATED EXPERT COMMENTSCHAPTER 12

General CommentsThere is some ambiguity: the entire chapter 12 stresses the uncertainties about the causes of thewarming. (See specific comments below).Walter Dragoni, Dept. Scienze Terra- Perugia University, ITALY, (Exp.)

General commentsI am basically happy with the chapter, particularly the conclusion. Removal of "the balance ofevidence" seems correct. The evidence is remarkably consistent that an anthropogenic effect can bedetected, though the level and character of the proof that this can be done varies. The introduction of anumber of new and imaginative analysis techniques is a strong factor in strengthening the famousconclusion of the SAR.

I am not so happy that section 12.6 is a repeat of the synopsis at the beginning. There are several waysof overcoming this; one is to remove the synopsis on page 5 and retain section 12.6.

The chapter has a fault that is not easy to overcome. Because so much of the work is similar there is arepetitiveness about the chapter that leads to the danger of boredom. This is not helped by the fact thatsome parts are not easy to fully understand. These problems might be reduced if the text wassomewhat reduced in length, offset by one or two extra clarifying diagrams.Chris Folland, Hadley Centre, UK, (Exp.)

General CommentsSince submitting my December review of the first draft of the TAR chapter 12, Myles Allen, GabiHegerl and myself have engaged in an ongoing discussion, both directly and bye-mail relative topoints raised by this review. Myles has kept other Chapter 12 authors appraised of our discussions. Ithas been very helpful to receive, over the last months, weeks and days, many of the key unpublishedreports used in the preparation of chapter 12. The discussions and reports have served to clarify andprovide focus for this present review. It would be redundant in this review to repeat, or again raisedetailed issues that already have been considered or are under consideration. Therefore this currentreview will be confined to a few major issues that, if not adequately addressed will surely impact on thecredibility and acceptance of Chapter 12 by the scientific community. These issues are now discussed:

1) The Chapter 12 text switches bewilderingly between 'attribution', 'consistency', and 'attributionconsistency'. It provides a poor and, in a number aspects misleading, account of the use the Bayesian,as opposed to probabilistic, statistics (cf for example point 2 below). Section 3 of the tech-report ofAllen, Gillett, Hegerl, Schnur, Stott, Boer, Delworth,Jones, Mitchell, and Barnett, May 17,2000 (anupdate of Allen 2000 a) provides substantially better and more accurate discussions of these issues.Certainly material from Section 3 should be incorporated into, or replace text in the current chapter 12.

2). The current chapter 12, and its source material, are in large measure derived from an overlysimplistic view of the methodology of 'optimal detection', as outlined in its Appendix 12.1 "OptimalDetection is Regression." Unqualified use of this methodology is only proper when both the functionalforms for signal-patterns and the error-covariance matrix (describing the background noise) are welldetermined from simulation, experiment or theory. For many problems these inputs are known, and forthem the results obtained using this methodology are directly applicable. Not so unfortunately for'climate change'. Hegerl2000a has clearly demonstrated the wide disparities for both the signal-patterns and the unforced correlations found by different simulation models, and particularly markedfor aerosol response-patterns. Climate change analysis therefore belongs in a class of problemsrequiring considerable care in formulation and evaluation.

Chapter 12, page 24 line 6 notes that "The detection of the net anthropogenic signal is robust, but thethe simultaneous detection of its separate components is very sensitive to differences in model

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responses". However Chapter 12 , without any qualifications, later proceeds to directly determinesignificance levels from the 'confidence ellipsoids' (equ. A12.1.4) based on the separately determinedgreenhouse gas and "very sensitive" sulphate aerosol signal amplitudes. This by any standards isimproper, and a robust detection/attribution procedure should have based results on 'robustly' definedquantities such as the net amplitude formed by summing the separate greenhouse gas and aerosolsignal amplitudes. Given the data currently available to the authors, ie the error covariance matrix ofthe signal-amplitudes, it would be easy to reformulate the results in terms of robust quantities. Chapter12 should be revised accordingly.

3). The main results of Chapter 12, that readers will take away with them, are contained in Figures12.9 and 12.10. Figure 12.9 provides results for attributed anthropogenic warming with associated' 5to 95%' error bars that correspond to RMS errors at one-sixth of the signal level. The implication ofthis figure is that, not only has anthropogenic warming been established 'beyond doubt', but that it isnow known quantitatively to good precision. This precision is reaffirmed on page 25 lines 31-33 whichstate that detection results "continue to hold even when the the variance of the control simulation isinflated by a factor of four". Such precision is better than a factor two higher than anything that hasappeared in the published literature to date.

The May 17, tech-report of Allen et al (updating Allen 2000a) provides full details of the analysisesfrom which the chapter 12 figure 12.9 derives. Certainly the spatio-temporal attribution algorithm usedin this study are the best to date, and in many other aspects this is an excellent report. However a basicunderlying premise of the tech-report is fundamentally flawed.

Page 14 of the report states" .. no further signals are detectable beyond the GS signal." and "Thismeans .... that recent climate change is attributable to GS influence." . What is being referred to areattributions based on single-signal GS attribution. Based on this presumption it follows that thecontribution from natural variability to this centuries climate change can be neglected and the fullobserved warming is attributable to GS. Further with this presumption it is no longer necessary toinclude uncertainties in natural variability into a calculation of the significance to be attached to thefinal result. Based on the above premises one indeed obtains the results displayed in figures 12.9 and12.10. The fallacy in this argument is easily demonstrated.

Consider an the extreme case where two signals A (anthropogenic) and B (natural) have comparable(closely degenerate) response patterns and comparable forcing strengths. The regression results of asingle A-signal pattern analysis would show an A signal double what is actually present. A secondregression analysis on the fit residuals would find a negligible B signal. Using the above Allen et alpremise, the A-signal amplitude would be double what is actually present, the B-signal amplitudewould be negligible, and now, with no need to include uncertainties in the B-signal amplitudes, theapparent measurement precision would be very good. Of course, given our prior knowledge, none ofthe above is true.

This example illustrates the general principle that stepwise regression analysis should not be usedwhen the objective is to provide physically meaningful results. To provide physically meaningfulresults requires a 'parallel' multi-signal analysis based on all known or suspected signals, which cannotbe demonstrated on an 'a priori' basis to be negligible. Of course if prior knowledge providesconstraints on the values of any given signal that knowledge should be folded into the multi-signalanalysis as a constraint(s) (cf our next comment (4)).

Figure 12.10, shows the projected climate change to 2040, derives from Fig 12.9 and therefore thepreceding comments on Fig 12.9 are equally applicable to results in figure 12.10. Allen 2000b providesthe basis for the extrapolation results contained in figure 12.10. Fig 3 of Allen 2000b also provides asubstantially better representation of the extrapolation and associated uncertainties than is containedin figure 12.10. This reviewer recommends substituting for for figure 12.10 with the Allen, 2000b,figure 3.

4) Page 15 lines 53 to 54 states "This is fundamentally different from the classical viewpoint in whichprior expectations only enter indirectly". This is simply incorrect. The introduction of prior constraintsinto probabilistic statistical determinations is a widely used technique. For instance the high energyexperimental physics field uses regression techniques to determine the parameters of events observedin colliding beam experiments. Quite independent prior information is provided by accelerator

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IPCC WGI TAR Gov/Expert Review (Expert Comments) Chapter 12 21111113

characteristics that restricts the vertex or origin of the fits to a restricted region of space. Routinely suchprior information is incorporated into the fits to provide 'vertex-constrained fits'. Maximum-likelihoodconstrained fitting has been used in innumerable applications, routinely and over the full span of yearsof my professional career.

The above comments have dealt with areas where this reviewer believes changes, corrections ormodifications are needed. However the most important, and potentially destructive problems in thecurrent Chapter 12 draft lie in its major omissions.

In a field such as climate change, the literature provides many divergent viewpoints and the success orfailure of an overview is how well it has succeeded in presenting a well-balanced view to its readers.

Our comment 3 discussed the 'summary' figures 12.9 and 12.l0, showing strikingly 'precise'attribution and extrapolations into the future. It would have been appropriate to have presented other'less sanguine' results, illustrative of inherent problems faced by current attribution studies. For thispurpose I would suggest:

a) The inclusion of Hegerl et al (2000 a) figures 10 showing the evolution over time of their detectedgreenhouse gas and sulphate aerosols amplitudes from 1929 through to 1995. Hegerl's figure 10illustrates the high sensitivity of attribution results to choice of start-time for the analysis.

b) A 'box D' added to the current chapter figure 12.7 that displays results from Hegerl (2000a) for thesecond half of the century. Box D should contain the Hegerl (2000a) figure 11 results for the earlierhalf ofthe century. This would enable readers to assess both the internal consistency of the Hegerlattribution methodology and to assess correspondence with the analysis ofTett et aI1999).

The second draft (and first draft) does an excellent (and much needed) job of reviewing the highlydiverse approaches employed in climate attribution studies, both published and in progress towardspublication. However readers will be look in vain to find which methodologies are now solely ofhistoric interest, which, while academically interesting, do not reflect reality, and which are thepresently most significant. Most importantly readers will look in vain for evaluations of thecomparative strengths and weaknesses of these approaches, and whether differing results areunderstood or reconcilable. Finally, on the basis of the above, the reader will legitimately expect to findrealistic 'bottom-line' attribution results with attached confidence levels. Realistic of course meansthat the full range of known or suspected sources of uncertainty have been taken into account.

What Chapter 12 lacks is the final section that pulls together all the divergent threads into a coherentwhole. Without such a section Chapter 12 in like a runner who, after running an excellent race, sitsdown just before the finish-line. The talent is there, please take the final steps and cross the finish-line.ProfD Ritson, Varian Physics Dept, Stanford University, USA, (Exp.)

General comments.In industry a number of people are sceptical with respect to the enhanced greenhouse effect. There foreI advice to add at the beginning of Chapter 12 (Detection of Climate Change and Attribution of Causes)an introductory remark that there exists strong evidence for the effects of anthroponic emissions on alocal and on regional scale. Thus, this proposed addition has psychological reason, rather than ascientific reason.

For the local scale I refer to extensive research on the heat island effect. A good recent article is:Robert Bomstein and Quinglu Lin in Atmospheric Environment: "Urban heat islands and summertimeconvective thunderstorms in Atlanta: three case studies, 34 (2000), pp. 507-516.

For the regional scale strong evidence has been publicised by Daniel Rosenfeld: "Suppression of Rainand Snow by Urban and Industiral Air Pollution", Science Vol. 287, 10 March 2000, pp. 1793-1796.His Figure lC would be a very good illustration of how air pollution influences cloud droplet sizes andthus rainfall on a scale of hundreds of kilometres.H. Visser, Kema, The Netherlands, (Exp.)

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General commentsOverall comment: The uncertainty in present knowledge of forcings summarized in Chapter 6 andespecially Figure 6.7 is quite large. The total forcing over the period 1850-2000 might be as great as 3W m-2; might be as small as 1 W m-2; might even be negative if one considers the extremes inuncertainties in aerosol forcing. Regionally, at least in areas affected by aerosols, the uncertaintiesmust be even greater. So I think there is a great burden on the detection and attribution community tosquarely face the consequences of these uncertainties before they can claim [page 5, line 47] "Weconclude that there has been a discernible human influence on global climate". I am not saying thatsuch a conclusion cannot be reached. Rather that I don't think the authors have adequately explored theconsequences of the uncertainties in forcing in reaching that conclusion. To be sure, there arenumerous references to the uncertainties, e.g., page 4 line 51 to 5 line 14. But to my thinking what islacking (and crucial) is systematic examination of the implications of these uncertainties on thedetection and attribution process. How could that be done? I would say by brute force errorpropagation: Repeat the exercise at the upper and lower bounds of the uncertainty range and examinethe outcome. If the detection "signal" remains, then one has confidence in the assertion of detection.And a much more powerful statement to that effect. Without this exercise, one is limited to ratherweak statements such as page 5, line 49 "This work suggests that anthropogenic greenhouse gases are asubstantial contributor to the observed warming over the past 50 years." I am concerned that the verb"suggests" will be pounced upon by those who would wish to undermine the conclusions of thechapter, which is a crucial chapter to the report. I really think that the only way to firm up such aconclusion is by the error propagation approach.

For the detection exercises reported, whenever accounts are given of a given study with a given forcingthe magnitude of the forcing that was employed is rarely specified, despite the fact that the forcings arehighly uncertain. The magnitude of sulfate direct forcing given in chapter 6 was uncertain to a factorof two either way. The indirect effect due to sulfate is likely of comparable magnitude and even moreuncertain. In this context it seems crucial if statements are being made that one sees that sulfate forcingis or is not detectable, that the magnitude of the forcing be specified, let alone the further statement asto whether the forcing was detectable at the lower and upper limits of the sulfate forcing given inchapter 6.

Examples: Page 23, line 13. "representation of sulphate aerosols .... The authors find significantevidence for a greenhouse-gas-plus-aerosol fingerprint."Line 24 "greenhouse gas plus direct sulphate".Line 38-39, "combined greenhouse gas plus (direct) sulphate fmgerprints ... are not detected".Line 50 "greenhouse gas fingerprint and an additional sulphate aerosol fingerprint.... The modelamplitudes of these signals, and of the combined greenhouse gas and sulphate signals were consistentwith observations."

It is not clear what the magnitudes of the sulfate forcings were in the several studies, absolute orrelative to the greenhouse gas forcings. Was the fact that the sulfate signal was detected in someinstances and not in others for the same forcing or for a different one? One way in which the forcingsmight be succinctly summarized is in Table 12.1; the "Signals" colunm could specify the global meanvalue of each of the forcings examined.Stephen E. Schwartz, Brookhaven, USA (Exp.)

General comments.Throughout the document virtually the only aerosol mentioned is "sulphate aerosol". It is almost as ifChapter 6 which pointed out the several other aerosol forcings considered to be significant over theindustrial period did not exist. My hunch is that the reason for this is that the detection and attributionwork reported here took place over the last several years in parallel with the aerosol forcing work thatis reported in chapter 6. So at least with respect to aerosols, this chapter lags by almost a full IPCCreport cycle behind the forcing chapter. (One might expect the detection and attribution chapter of thefourth IPCC assessment report to include studies incorporating other aerosol species.) For whateverreason, I think it is important that this chapter at least state the reason D&A studies reviewed here dealonly with sulfate aerosols and discuss the implication of that in the context that all forcings have beenacting on the planet over the time for which the observational data on climate change used in the D&Astudies pertain.Stephen E. Schwartz, Brookhaven, USA (Exp.)

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General comment.Too often the document reads like a progress report since the second assessment report. Example page3, line 8-9, first para of exec summary: "results of the research carried out on these uncertainties since1995 is summarized below." Page 18, line 53: "In this section we assess new studies ...". Page 28, Line18 "In summary some progress has been made ..." In my opinion the document should be a statementof the present state of understanding that takes into account new work in the context of older work, nota summary of progress.Stephen E. Schwartz, Brookhaven, USA (Exp.)

General comment.The document is sprinkled with the phrase "reasonably well" e.g. page 8 lines 37 and 41. It is not atall clear what is meant by that rather vague phrase.Stephen E. Schwartz, Brookhaven, USA (Exp.)

General comment.GENERAL STYLE COMMENT: Use of first person plural. In general throughout the entire TAR thefirst person plural is ambiguous. Chapter authors (which change from chapter to chapter)? IPCCcollectively? The scientific community? This ambiguity should be avoided, by explicitly stating whois meant or by eliminating altogether. Examples. Page 3, line 15 "We also have a betterunderstanding." Probably the scientific community. Page 8, line 36: "Here we assess" pretty clearlythe chapter authors. But line 22 "as good as our knowledge of the forcings ..." probably refers to thescientific community; better "as good as knowledge ..." Line 25. "strengthens our qualitativeassessment." Not at all clear whose assessment is meant. Page 9, line 51: "we require estimates ... "Who requires? Page 20, line 4 "our knowledge of the physical climate system. "Whose knowledge?Page 26, line 54 "our confidence" Whose? Page 27, line 1 "our knowledge" Whose? 1 Caption"by which we have to multiply"; just use passive voice "by which amplitude must be multiplied". Isuggest some general policy for the TARas a whole and then careful search and appropriate replace.Stephen E. Schwartz, Brookhaven, USA (Exp.)

General CommentsThis chapter will be very carefully scrutinized and I believe could be improved by a simple device: acareful list of arguments against the idea that greenhouse warming has been detected and attributed andwhat it would take to deal with each of the arguments in future assessment reports.

Sec 12.5 , Remaining Uncertainties indicates that detection has been proven and some remaininguncertainties exist. I think the basic idea that detection is probabilistic and only a probability of ahypothesis being true should guide the chapter and that the essence of the chapter, and future IPCCAssessment reports, should be how to better and better increase the probability of detection.Policymakers should be given to understand that detection will never be a yeslno thing nor is anypolicy in the real world based on such yeslno criteria.Dr Edward S Sarachik, University of Washington, USA, (Exp.)

Specific Comments

Page 3, line 6.5 years of data may not make a big difference but five year of understanding probably has. i.e. we havebetter experimental design and lots of different groups have compared their model simulations withthose observed.Simon Tett, Hadley Centre, UK, (Exp.)

Page 3 line 13.Clearer if "globally" inserted after "warmest".Chris Folland, Hadley Centre, UK, (Exp.)

Page 3, line 45.How sure are we that volcanic forcing has overwealmed solar forcing in the last two decades? There islittle discussion of volcanic aerosol in chapter 6 and no referenced work to support this.Simon Tett, Hadley Centre, UK, (Exp.)

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Page 3, line 48After "However, there is evidence for a detectable volcanic influence on climate and evidence thatsuggests a solar influence, especially in the early part of the twentieth century." Add a new sentence"However, theories that would explain the magnitude of the solar forcing remain speculative, and thepossibility remains that an unknown factor may be responsible for at least part of the warming seen inthe first part of the twentieth century."Ken Gregory, Centre for Business and the Environment, UK, (Exp.)

Page 4, line 2This appears to be an attribution statement (as opposed to the stated detection) that goes beyond thefinal conclusion? Why the conclusion might not go this far is that even though there have been manyadvances made (mentioned earlier in this paragraph), the paragraph neglects to mention all of theadvances that are known to be needed. The range of possible aerosol effects is an obvious one.Suggest putting this statement in the limited context in which models have been applied. (For example,models have not included effects or hypotheses that may explain features of the INDOEX and MSUdata.)Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 5, line 2.Note that enhancing variance removes the detection of the sulphate signal from Tett et al. Withoutdetection of sulphates (i.e. no evidence that they have has a significant cooling effect) one wouldconclude that observed and simulated climate change are inconsistent or that models were too sensitiveto changes in greenhouse gases.Simon Tett, Hadley Centre, UK, (Exp.)

Page 5, line 8Soot and biogenic aerosols are aerosols?Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 5 Line 8"The biggest uncertainty in anthropogenic forcing is associated with the effects of aerosols." I concur.But what are the implications of this uncertainty in detection and attribution? The authors of Chapter 6did not wish to add the several forcings nor to propagate the associated uncertainties. Yet if one isattempting to relate observed magnitudes and patterns of climate change to forcing, is it not obligatoryto consider the uncertainties in magnitudes and patterns of uncertainties, and to discuss the implicationsof these on the detection and attribution process? Next sentence: "Also the effects of otheranthropogenic factors including soot, biogenic aerosols, and changes in land use have not beenincluded ..." Presumably not included in the detection and attribution studies. But they _are_ includedin what has been driving climate change of the actual Earth over the industrial period. How can onearbitrarily exclude them from detection and attribution studies? That sentence continues "althoughtheir global effects may be relatively small." Yes, and what if they are not relatively small? What arethe consequences to detection and attribution? Next sentence: "Estimates of the size and geographicpattern of these effects vary considerably." Presumably the sentence refers to forcings, but maybe toresponse. In any event, is it not incumbent on detection and attribution studies to work with theseforcings within the ranges of their accepted uncertainties and see what the implications are on responseand on detection and attribution?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 5 Line 11"There are large differences in the response of different models to the same forcing." This should bequantified. Compare Chapter 8, page 21, line 1-2: "the radiative forcing associated with 20th centurygreenhouse gas increase along tends to overestimate the 20th century warming." This would seem toimply that the sensitivities of the several models are sufficiently narrow to make such a statement. Thisshould be reconciled, quantitatively.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 5, line 17Unambiguous detection and attribution of anthropogenic climate change signals will be accomplishedthrough the continued accumulation of evidence. The research since the SAR contributes to thisaccumulation.

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This means that at the moment the detection is ambiguous (i.e. not clear or unknown); so you cannotknow what the future accumulation of evidence will proof. I think this self-contradictory sentenceshould be deleted.Walter Dragoni - Dept. Scienze Terra- Perugia University - ITALY, (Exp.)

Page 5, line 22.My American Heritage dictionary defines "exceptional" as "Uncommon, extraordinary". I do notbelieve the current climate fulfills this definition. 15,000 years ago there were thousands of feet of iceover half of North America. The mean temperature of the planet for 97% of the last 300 million yearshas been higher than it is today. This word was chosen because it is rhetorically explosive, and IPCCexpects that to be the "headline" word. If any climate is "exceptional" , it is the Pleistocene and theHolocene.

Drop the word "exceptional". But--Kevin Trenberth already leaked it to The Washington Post in Aprilin an article by Curt Suplee.

A similar problem with "natural". I guess we're not a part of nature, right? This speaks volumes aboutthe philosophical bias of IPCC.

What's wrong with" the observed warming over the last 100 years is likely to contain a substantialhuman-induced component".Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 5, line 28Add extra words at the end of the sentence "although the possibility remains that an unknown factorthat could have been responsible for at least part of the warming seen in the first part of the twentiethcentury might also be influencing climate in the latter half of the twentieth century."Ken Gregory, Centre for Business and the Environment, UK, (Exp.)

Page 5 Line 41"There is still considerable uncertainty in estimates of the sulphate aerosol forcing ...". And all the otherforcings, except for GHGs. The sentence continues "and the climate response." It is not clear whetherthis is uncertainty in actual climate response, in climate model response, in climate model response tosulfate aerosols specifically or to climate model responses to forcings generally, whether to globalmean response, or to spatial distribution of response, and whether the latter is due to uncertainty inclimate model response or to uncertainty in distribution of the forcing. Needs clarification. Rathersurprisingly the next sentence reads "This uncertainty does not have a large effect on the detection ofthe combined signal from anthropogenic greenhouse gas and sulphate aerosol forcing in the globalclimate." This statement certainly would seem to require quantitative examination later, but does notseem to have been systematically examined.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 5, line 41It is not clear why the uncertainty of S-aerosols are mentioned when the direct S-aerosol effect is aminor contributor to aerosol forcing uncertainty? Suggest mentioning that there are gaps in ourunderstanding of the indirect effects that preclude definition of a best estimate of past radiative forcing,that patterns for different aerosol forcings are different and independently uncertain (refer to figure5.2).Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 5, line 47From the body of evidence since the SAR, we conclude that there has been a discernible humaninfluence on global climate.

Considering all the uncertainties you described many times I think would better to write this:

"From the body of evidence since the SAR, we conclude that relevant human influence on globalclimate is highly probable."

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fPCC WGf TAR Gov/Expert Review (Expert Comments) Chapter 12 21111113

In conclusion, in order to wipe out any misunderstanding and any reason to argue, I think that in someplace in the chapter 12 (as well as in the "Technical Summary" and" Summary for Policy Makers") itshould be clearly stated that:

cc the causes of the past short term climatic variations and of the present warming are not wellunderstood, and its environmental consequences also are not well known. However there is a highprobability that such a warming is due to the anthropogenic activity (i.e. anthropogenic greenhousegases emissions) and, no matter the causes, a future higher warming will have heavy effects on humankind."

In chapter 12 the same concept is given many times, but in a shy, unclear and contradictory form.Walter Dragoni - Dept. Scienze Terra- Perugia University - ITALY, (Exp.)

Page 5, line 47Should be "With the addition of the body of evidence since the SAR"Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 5, line 47Recommend not using a first person conclusion to assess the probability of a result. The probability ofdetection coming from the lines of evidence stated in this page should lead to the final objectiveanswer. Instead, the authors have given an absolute subjective conclusion?Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 5, line 47The statement "we conclude that ..." seems to imply that an unambiguous detection and attribution hasin fact been accomplished. This is inconsistent with the statement on line 49 of this page, beginningwith "This work suggests ...". Could the word "conclude" be qualified by a confidence limit?Dr A.P.M. Baede, KNMI, Netherlands (Exp.)

Page 5, lines 47,48The headline detection statement makes no mention of greenhouse gases and is entirely consistent withthe possiblity that the sole anthropogenic influence on climate is from stratospheric ozone depletion. Inmy opinion the headline detection statement needs to go beyond this and state that we have a highconfidence in a greenhouse gas influence on climate.

All the quantitative detection studies I am aware of have detected an anthropogenic influence ontwentieth century near-surface temperature change made up at least in part of a contribution due togreenhouse gases. While detection of other factors such as sulphate aerosols remains much less clearcut, confidence in the detection of the influence of greenhouse gases is high.

I would suggest something of the form "The body of evidence indicates that there has been adiscernible change in global climate due to anthropogenically caused increases in greenhouse gasemissions. "Peter Stott, Hadley Centre, UK, (Exp.)

Page 6, line 17This is a clear definition of detection and attribution, and should be retained. The difficulty is that it isnot used consistently throughout the chapter. As stated, detection implies a consistency test betweenthe data and the statistics of natural variability. In the appendix and most of the chapter, detection isrelated to the attribution of an anthropogenic signal with a background of natural variability?Haroon Kheshgi, IPIECA, USA, (Exp.)

Line 47Insert 'of surface temperature' after '5-year means'.Nathan Gillett, University of Oxford, UK (Exp.)

Page 9 Line 1Insert 'in surface temperature' after '50-year JJA trends'.Nathan Gillett, University of Oxford, UK (Exp.)

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Line 2'found larger differences' Than what? The previous sentance refers to a comparison with obs rather thaninter-model comparisons, so it's not clear to what the factor six is being compared, or indeed what it is.Nathan Gillett, University of Oxford, UK (Exp.)

Page 9 Line 3,4Change to 'Gillett et al. compared simulated variability in the free atmosphere of HadCM2 with that ofdetrended radiosonde data'.Nathan Gillett, University of Oxford, UK (Exp.)

Page 9 Line 54,55Change to 'because we do not have well-resolved observations of the unforced atmosphere'. Even withonly one forcing agent, we surely couldn't estimate the natural variability in its response if we only hadobservations in which that forcing was present.Nathan Gillett, University of Oxford, UK (Exp.)

Page 10, line 9Now we have coupled models being termed OAGCMs cf. CGCMs in Chapter 9 and AOGCMs inChapter 10.Professor A. Henderson-Sellers, ANSTO, Australia, (Exp.)

Page 10 Line 51'and longer' -> 'to centennial'. Milankovich etc. at longer timescales.Nathan Gillett, University of Oxford, UK (Exp.)

Page 11, Lines 5-14:You may consult the paper byP. Thejll and K.Lassen: Solar forcing of the Northern hemisphere land air temperature: New data. Thepaper is submitted for a journal publication and is available as a report of the Danish MeteorologicalInstitute at present. The paper was presented and well accepted on the EGS conference 2000 in Nice.The paper shows that the NH surface temperature was well anti-correlated with length of the Solarcycle until 1970 but the correlation is getting very weak when the more recent data are included in theanalysis. The paper contradicts the conclusions of Friis-Christensen and Lassen in 1991 (Length of theSolar cycle: An indication of Solar activity closely associated with climate. Science, 254, 698-700,1991). Note that Lassen is co-author on both papers. I got this information with the following email(shortened):

"A preprint is available as a.pdffile (and a .ps file) at the following address on the Internet:http://web.dmi.dk/f+u/publikationiSr99-9.pdf(orSr99-9.ps). You may use the figures as you like aslong as you cite the DMI report. The paper is being published elsewhere as well, but the report is ourcurrent reference for the work.Peter ThejllSolar-Terrestrial Physics DivisionDanish Meteorological InstituteLyngbyvej 100DK-2IOO Copenhagen [email protected] telephone: (+45) 39157477 fax: (+45) 39 1574Dr. Ulrich Schumann. DLR Oberpfaffenhofen, Institute of Atmospheric Physics, Germany,(Exp.)

Page 11 Line 8Over what period is the 0.6-0.7Wm-2? If it's from Little Ice Age to present, then this should be stated.Otherwise it's not clear what a figure this exact means.Nathan Gillett, University of Oxford, UK (Exp.)

Page 11, line 27The reference to Willson, 1997 is not correct here, and should be removed as that paper dealt only withsolar irradiance changes in solar cycle 21 and 22, NOT in the first half of the twentieth century.Instead, a reference could be made to Chapter 6 which shows the various irradiance reconstructionsduring the zo" century.

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Judith Lean, Naval Research Laboratory, Washington DC USA

Page 11, line 28The statement "which overwhelms the direct effect of a small increase in solar radiation (Chapter 6)"does not properly reflect the discussion in Chapter 6 that the irradiance increase claimed by Willsonmay not be a real, but rather instrumental. Thus, this sentence could be rephrased to something like:"which overwhelms the direct effect, if real, of the small increase in solar radiation that Willson (1997)claimed (Chapter 6)."Judith Lean, Naval Research Laboratory, Washington DC USA

Page 11 line 41Our 2.5 dimensional climate model with quasi-geostrophic dynamics was also applied to the simulationof the past 300 years, and we have also shown that volcanic and solar impacts were insufficient toreproduce the recent warming.Please add the following reference between the parentheses:Bertrand, C., J.P. van Ypersele, and A. Berger (1999) Volcanic and solar impacts on climate since1700. Climate Dynamics, 15,355-367.Prof. J.P. van Ypersele, Belgium, (Exp.)

Page 11 line 48 :Our 2.5 dimensional climate model with quasi-geostrophic dynamics was also applied to a detailedstudy of the impact of solar variability, using different reconstructions, and we have also shown thestronger temperature response of the low-frequency solar variability.Please add the following reference between the parentheses:Bertrand, C. and J.P. van Ypersele (1999) Potential role of solar variability as an agent for climatechange. Climatic Change, 43 (2),387-411.Prof. J.P. van Ypersele, Belgium, (Exp.)

Page 11 line 53Should read "solar radiative forcing" not "solar irradiance"Dr Joanna D Haigh, Imperial College of Science, Technology and Medicine, UK, (Exp.)

Page 11 lines 54,55Sentence starting "Others find that" The evidence for models significantly underestimating solarforcing is extremely weak in Stott et al2000b and does not support the statement in the TAR. Analysisusing a new regression scheme (total least squares regression) show that the early century warmingmay be consistent with an enhanced response to solar forcing. However there are very largeuncertainties in the weak noise contaminated solar signal and the contributions of solar forcings to theearly century warming are very poorly constrained. Therefore unless there is other evidence to supportthis statement, this sentence should be dropped.Peter Stott, Hadley Centre, UK Met. Office, UK (Exp.)

Page 11, line 55.I don't believe that Stott et al, 2000b fmd that solar forcing may be significantly underestimated.Simon Tett, Hadley Centre, UK, (Exp.)

Page 12 Line 2"like to find differences" probably inappropriate. Better: "differences in responses would make iteasier to attribute response to forcing."Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 12, line 7.Note that representing greenhouse gases by equivalent C02 overestimates the stratospheric cooling.Furthermore any stratospheric cooling due to C02 is likely to be overwealmed by stratospheric coolingcaused by ozone decline.Simon Tett, Hadley Centre, UK, (Exp.)

Page 12 line 17Remove references to Rind and Balachandran 1995 and Forster and Shine 1997 as these do not discussclimate response to solar-induced ozone changes.

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Dr Joanna D Haigh, Imperial College of Science, Technology and Medicine, UK, (Exp.)

Page 12 Line 20Could cite Shindell's planetary wave feedback mechanism to explain circulation changes in response tosolar forcing e.g. Shindell et al. 2000 (Northern Hemisphere winter climate resopnse to greenhouse gas,ozone, solar and volcanic forcing', submitted to JGR.Nathan Gillett, University of Oxford, UK (Exp.)

Page 12, line 25 and onI am still worried by the use of the Mann et al paper as a reference in the matter of whether asignificant correlation has been established between C02, the Sun, volcanoes' influences and theclimate. I am not convinced that the multivariate regression analysis performed in that paper is correct.Basically my criticism has to do with the use of renormalization of the data in each single window ofthe data - this method is unorthodox and unsubstantiated in the paper. I recommend removal of thereference to Mann et al, here. As the other papers cited in that line do an ample job of bringing thepoint across Mann et al is not needed there.Dr. Peter ThejIl, Danish Meteorological Institute, Denmark (Exp.)

Page 12 Line 45Qualitative comparisons. The figure (12.5) looks like a quantitative comparison to me. But to beuseful the figure should really display the global mean forcings (as a function of time) as well assurface temperature changes.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 13, lines 14-22.Forcings are all "adjusted" I assume?Simon Tett, Hadley Centre, UK, (Exp.)

Page 13, line 32This discussion should mention the neglect of aerosols of other types. It should refer to Chapter 5 forthe various patterns for these aerosol forcings. The radiative forcing figure (ch6) should be referred toin order to show that S-aerosols are only one of the contributors to aerosol radiative forcing. And itshould mention that these forcings are neglected in all the results throughout this chapter.Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 13 line 38-40The statement regarding the ratio of sulphate to greenhouse gas forcing seems to require back up byanalysis of data or citation. For forcings linear in concentrations, the a forcing that is proportional toloading of a species that is proportional to emissions (e.g., sulfate) is proportional to a forcing that isproportional to the concentration of a species that is proportional to the integral of emissions (e.g.,C02) for exponentially increasing emissions. So the statement is certainly not a priori correct.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 13 Line 43Indirect forcing of tropospheric aerosols "contributes a negative forcing which could be negligible orcould exceed 2 W m-2." This is important, but its implications do not seem to have been followedthrough quantitatively. The last sentence of the para states: "Given its high level of uncertainty,studies using simulations including estimates of indirect sulphate forcing should be regarded aspreliminary." Of course I concur with this. But shouldn't studies that do not include these estimates,and thereby implicitly set this forcing to zero, be accorded the same preliminary status?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 13, line 43The upper limit is on the order of 4w/rn2 (including both indirect effects) making this statement wrong.Suggest "Aerosol cooling of 2wlm2 would imply ... "Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 13 Line 44Why limit to the last century? and should "change in" be struck?Stephen E. Schwartz, Brookhaven, USA (Exp.)

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Page 13, line 55.If negative forcings offset a substantial fraction of the greenhouse gas forcing why has there beensubstantial warming in the last 20 years?Simon Tett, Hadley Centre, UK, (Exp.)

Page 14, line 4Not just S-aerosols have indirect effects.Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 14 Line 7-9"When a simple representation of aerosol effects is included, the rate of warming is reduced." Thefinding depends strongly on the magnitude of the assumed aerosol forcing.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 14 line 13Add "warming" before "effects"Chris Folland, Hadley Centre, UK, (Exp.)

Page 14, lines 17-18.Would be good here to mention the new NRC report. See Chapter 8, comment 1.Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 14 Line 23The reference to Figure 9.14 sees inappropriate here. correct?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 15 Line 3,4'may be sensitive to' -> 'is sensitive to the EOF truncation and'Nathan Gillett, University of Oxford, UK (Exp.)

Page 15, lines 18-25.Note that figure 12.9 is using a century timescale analysis. Given problems in estimating century scalevariability given the limited length of control simulations I am very concerned about this result. See mylater remakrs on figure 12.9.Simon Tett, Hadley Centre, UK, (Exp.)

Page 16, line 45Should this read Figure 12.5?Melissa Free, NOAA ARL, USA, (Exp.)

Page 16 Line 45Is the reference to Figure 12.4 correct? Figure doesn't seem to support the statement. Same for thereference to that figure in line 50.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 16, Lines 45, 50.I think "Figure 12.4" here should actually be "Figure 12.5" (2 occurrences).Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA, U.S.A. (Exp.)

Page 16 Line 46Should the qualifier "greenhouse gas" be struck? Isn't it the model's sensitivity to forcing generally thatis being spoken of?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 17, lines 20-29The paragraph seems to be confusing surface-troposphere and lower troposphere-upper tropospheretemperature differences. The Gaffen et al. (2000), Santer et al. (2000) and Brown et al. papers dealwith differences between surface and lower tropospheric temperatures and with the lapse rates in the

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lower troposphere (500-700 mb) rather than with trends at 300 mb or above. A different paper by DianGaffen does show profiles of temperature trends in the tropics:Gaffen, D., M. Sargent, R. Haberman and J. Lanzante, Sensitivity of tropospheric and stratospherictemperature trends to radiosonde data quality, 1. Climate 13, 1776-1796 (2000).

This paper, the figures in chapter 9, and other work seem to show warming higher in the tropicaltroposphere than the draft text here indicates. I think the model maximum is also higher than indicatedhere. The text should probably read more like:

Observations indicate that, over the last three to four decades, the tropical atmosphere has warmed inthe layer from the surface to between 200 and 100 mb and cooled above (Tett et al. 1996; Vinnikovetal. 1996; Gaffen et a12000b; Chapter 9, Figure 9.13). Model simulations of the recent past produce awarming of the tropical atmosphere to 100 mb or so, with a strong maximum around 200-300 mb notseen in the observations. This discrepancy is less evident if volcanic forcing is taken into account butdoes not go away entirely (Bengtsson et al. 1999) ....Melissa Free, NOAA ARL, USA, (Exp.)

Page 17, line 25 and 40These two references to Brown et al, 1999 refer to two separate papers although there is only oneBrown et al, 1999 in the reference list. Additionally the Second reference (line 40) is incorrect as thatpaper is now in print and should read Brown et al, 2000.so:line 25 should read as it is (Brown et al, 1999) but additional reference added to reference list of:Brown, S. J., D. E. Parker and D. M. H. Sexton. (1999): Differential changes in observed surface andatmospheric temperature since 1979. Hadley Centre Technical Note No. 12. The Met. Office,Bracknell, UK.line 40 should read Brown et al, 2000 and the reference on page 35, line 39 should be updated to:Brown, S. 1., D. E. Parker, C. K. Folland and I. Macadam (2000): Decadal variability in the lower-tropospheric lapse rate. Geophys. Res. Lett., 27(7), 997-1000.Simon J. Brown, Meteorological Office, UK, (Exp.)

Page 17 e.g. line 26:Final Brown et al reference is:Brown, S., Parker, D.E., C.K Folland and I. Macadam: 2000: Comparison of MSU2R with observedtropospheric and surface temperature trends. Geophys. Res .Lett., 27,997-1000.Chris Folland, Hadley Centre, UK, (Exp.)

Page 17, line 28-29.Could the authors explain why reducing the strength of the negative lapse rate feedback would increaseestimated climate sensitivity?Simon Tett, Hadley Centre, UK, (Exp.)

Page 17 Line 31-44It seems inappropriate to defer to the NAS panel here. The authors should give their own independentexpert opinion and then state whether this concurs with or disagrees with that of others, e.g., NAS.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 17, line 36The Gaffen et al. paper should be referenced here too.Melissa Free, NOAA ARL, USA, (Exp.)

Page 17, line 36.Santer et al. (2000) has major flaws, and should not be referenced. Their result is highly conditionedby the use of Pinatubo and not the whole volcanic record, as well as ending in 1998. We have amanuscript coming out on this. History repeats itself (See Santer et al 1996 vs. Michaels andKnappenberger 1996, and later comment). Instead of using a subset of a larger available temperaturerecord, this time they used a subset of the volcanic record and a convenient endpoint in the temperaturehistory.Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 17, line 42.

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I think claiming that current simulated free-atmosphere changes are consistent with observations maybe slightly mis-leading. Allen and Tett had to truncate to a relatively small eigenvector space. I beleivethat many current models have probelms in simulating stratospheric variability.Simon Tett, Hadley Centre, UK, (Exp.)

Page 17, line 42The sentence should be changed to read:The simulated and observed troposphere-surface temperature differences and lower-tropospheric lapserates are generally not consistent for [specify time period?] although it is not clear ....Melissa Free, NOAA ARL, USA, (Exp.)

Page 18 line 13In the final version of chapter 12, please refer to a brand new section in Chapter 2 that will have new,more positive evidence about trends in Lower Stratosphere and Upper Tropospheric Water Vapour.This is likely to be available in Canada.Chris Folland, Hadley Centre, UK, (Exp.)

Page 18 Line 24I don't think Fyfe et al. 1999 show that the observed trend is inconsistent with natural variability.Osborn et al. 1999 show this for the NAO - but that is HadCM2 too.Nathan Gillett, University of Oxford, UK (Exp.)

Page 19 Line 32-44The discussion of Figure 12.6 should specify the global mean forcings for GHGs and sulfate that wereemployed. Evidently the forcings were time dependent so the time profile of the global mean forcingsshould be shown. Also the pattern of forcing at, say, the end of the model run. Without thisinformation one really has no idea what fraction of the greenhouse only forcing was being offset by thesulfate forcing. It would be even better to present figures as band d for the greenhouse gas onlyforcing. It would seem useful to show similar plots to those in Figure 12.6 for the study of Boer.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 19, Lines 43-44.Change: " ... with strong warming in the first half of the century." To: " ... with strong warming,particularly in the high latitude North Atlantic, in the first half of the century."Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA, U.S.A. (Exp.)

Page 19, line 47.A problem with Wigley, 1998a. The autocorrelation structure in half of the models used washemispheric ally inverted.Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 19 Line 55"consistent with model simulations". The forcings of those model simulations should be specified.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 20, line 13There is some sort of typographical error here.Melissa Free, NOAA ARL, USA, (Exp.)

Page 20 line 13-14Sentence needs rewriting.Chris Folland, Hadley Centre, UK, (Exp.)

Page 20, Lines 14-15.I think the sentence beginning "Walter et al. (1998)" has some missing words or information.Warming ofO.9-1.3K due to what?Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA, U.S.A. (Exp.)

Page 20 Line17not sure the term "Granger causality" is widely known. Seems to deserve explanation.

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Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 20 Line 24"consistent with trends in greenhouse gas, aerosol and solar forcing". The forcings should be specified.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 20 Line 41,42Correlations are sensitive to erros in the global mean response ifuncentered. Suggest changing to 'andare insensitive to erros in the amplitude of the spatial pattern of response and, if centred, to the globalmean response'.Nathan Gillett, University of Oxford, UK (Exp.)

Page 20, lines 46-63.Gee it's hard for the authors to admit that Legates and Davis were right!Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 21 Line 1"idealised perfect model". Do the authors really wish to characterize the cited model or study asidealized and perfect?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 21 Line 6"reasonable combinations of greenhouse gas and aerosol patterns". Specify the patterns. What makesthem reasonable? Is this entire study hypothetical, assessing the ability to discern spatial correlations,or was it applied to such a determination? If just the former, it sounds like a methodological study thatperhaps could be omitted in the report.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 21 Line 13-22Again it would seem that the forcings need to be specified. Line 20-21 refers to halving the aerosolresponse. Do the authors mean the forcing or the response; perhaps they do mean response, given theparenthetical remark" assumed linear and additive" but I am puzzled how one can halve the responsefor one forcing; perhaps this was meant to test the sensitivity to the forcing without doing another runin which the forcing was changed. Should be explained.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 21, lines 24-39.This is very weak. Michaels and Knappenberger also pointed out that Santer et al. started at anopportune time (1963), and in fact the data from 1958-62 look a lot like the data from 1988-95. Are wesaying here that the GH warming dominated the sulfate cooling 40 years ago, too? Santer et al.excluded BOTH periods. Please note this in the text. As Hansen told me, "You really got Santer on thatone!"Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 21 Line 30,31Is tropospheric response to stratospheric ozone depletion definitely a cooling? I don't know much aboutthis, but I thought this is ambiguous - maybe a reference should be included.Nathan Gillett, University of Oxford, UK (Exp.)

Page 21, line 35.Michaels and Knappenburger do not calculate the effect of this regional temperature change on theglobal detection variable used by Santer et al.Also Santer et al. (1999, already listed in references for this chapter) found that the radiosonde datachosen by Michaels and Knappenburger disagree with other datasets for the same area (such asMSU), and are not representative of global trends.Paul D. Farrar, USA(Exp.)

Page 21 Line 43The reference to Figure 9.14 sees inappropriate here. correct?Stephen E. Schwartz, Brookhaven, USA (Exp.)

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Page 21 Line 44The authors should simply state the current interpretation of the study instead of first stating the resultbefore the error was discovered, then informing us of the error, and then informing us of theconsequences of the discovery of the error. Again the forcings should be specified.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 21 Line 44The reference to Figure 9.14 sees inappropriate here. correct?Stephen E. Schwartz, Brookhaven, USA (exp)

Page 23, line 10.Allen et al, 2000a doesn't, I believe, examine any more than 1.5 m temperature. Tett et al, 2000examine 1.5m temperature and free-atmosphere changes.Simon Tett, Hadley Centre, UK, (Exp.)

Page 24, line 17.Note that finding a single simulation is inconsistent with observations is "harder" than finding anensemble average is inconsistent. Uncertainty ranges are larger in the first case than in the second.There is, however, an issue of multiple tests. i.e. one will falsly reject (at the 5 % level) 5 % of thetime. I don't think this is the case in the anlysis reported here -- one would expect roughly one case tobe falsly rejected. Further note that optimisation reduces the uncertainty ellipse -- it is a more powerfulmethodology for detection (uncertainty ellipse or ellipsoid or range doesn't include zero) and forconsistency testing.Simon Tett, Hadley Centre, UK, (Exp.)

Page 24 Line 33Change to 'G and GSO signals'Nathan Gillett, University of Oxford, UK (Exp.)

Page 24 Line 33,34,35The best-fit combination has amplitudes beta, so 'was used to derive' is overstating things. Couldreplace with 'gave'.Nathan Gillett, University of Oxford, UK (Exp.)

Page 24 Line 39To what does the statement about GHG only runs and circulation changes refer? - this should be mademore clear.Nathan Gillett, University of Oxford, UK (Exp.)

Page 25 Line 21'with one solar timeseries' - say which and explain i.e. 'with the .... reconstrunction of solar irradiance'.

Nathan Gillett, University of Oxford, UK (Exp.)

Page 25, line 26-27.Note that consistency for one simulation is weaker test than for ensemble. Do they find the ensembleconsistent?Simon Tett, Hadley Centre, UK, (Exp.)

Page 25, Lines 26-27.Change: " ... Note that Delworth and Knutson (2000) find one out of five simulations with onlyanthropogenic forcing can explain the early century global mean warming." To: "".Note thatDelworth and Knutson (2000) fmd that a combination of anthropogenic forcing and an unusally largerealization of internal multi-decadal climate variability can reproduce an early century global meanwarming, including an enhanced warming in northern hemisphere high latitudes, similar to thatobserved."Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA, U.S.A. (Exp.)

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(PCC WG( TAR Gov/Expert Review (Expert Comments) Chapter 12 21111113

Page 25 Line 40'is still of unclear origin' -> 'are not well-simulated by HadCM2' (I think the origin of the changes is

becoming clearer).Nathan Gillett, University of Oxford, UK (Exp.)

Page 25 Line 51 to page 26 Line 34The discussion of scaling factors would be greatly enhanced if the corresponding values of lambda(climate sensitivity, KlW m-2) were presented.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 26 Line 4-12This is a very important paragraph and an important conclusion. However this conclusion does notseem to have been brought forward. It would seem appropriate on page 5, line 44.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 26, lines 4-12 also Fig 12.9.This is very important material in that it is relevant to the possible values of the indirect radiativeforcing due to aerosols. In Fig 12.9 scaling factors for GS and GSI are included which are barelydifferent implying that the combined direct and indirect aerosol forcing is little different in the twocases? This is rather surprising in the light of the numbers quoted for indirect forcing in chapter 6.What values were used for indirect aerosol radiative forcing for the GSI case? You quote from Tett etal a range for indirect forcing constrained by the observed record. This is not the same as the range inchapter 6. Can you comment further on the range in chapter 6?John Houghton, IPCC WGI Co-Chair, UK,(Exp.)

Page 26, line 29.Note that each models predicted warming lies within its own estimated warming. This means thefigure is welcome reassurance on the likely range of future warming rather than dramatically chagingour understanding.Simon Tett, Hadley Centre, UK, (Exp.)

Page 26 lines 30-32Two statements here need explanation; they are not obvious: (1) 'the errors in the individualcomponents cancel approximately' and (2) 'but also a greater level of uncertainty'John Houghton, IPCC WGI Co-Chair, UK,(Exp.)

Page 27, line 11.I amextremly concerned about figure 12.9. See my comments on the actual figure.Simon Tett, Hadley Centre, UK, (Exp.)

Page 27, line 30.These are "fixed pattern" analysis so may be missing some variability in the free atmosphere such assuggested by Brown et al, 2000.Simon Tett, Hadley Centre, UK, (Exp.)

Page 27 Line 35 ff.Section 12.5 Remaining UncertaintiesThis entire section is entirely qualitative. What is required is a quantitative assessment of uncertaintiesand their implications.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 27 Line 35"The exact magnitude of natural internal climate variability remains uncertain." I don't thinkknowledge of the "exact" magnitude is required or will be attained. What is needed is an estimate ofthe range of uncertainty of this magnitude (say from the Mann and Bradley reconstructions).Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 27, line 44.By how much is the variability enhanced?

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Simon Tett, Hadley Centre, UK, (Exp.)

Page 27, line 49I don't think the ground-based radiation measurements used for the Sato and Lamb volcanicreconstructions for 1883 and after are really "proxy data". Estimates of solar forcing before the satelliteera are based on proxy evidence; volcanic forcing estimates are based on instrumental data with limitedgeographic coverage and accuracy.Melissa Free, NOAA ARL, USA, (Exp.)

Page 28, line 2Smaller forcings? The biomass burning organic carbon direct effects, e.g., are estimated to be equal tothat of S-aerosols in Chapter 6?Haroon Kheshgi, IPIECA, USA, (Exp.)

Page 28 Line 3The reference to Fig 6.12 is in error; no Figure 6.12 in review version.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 28 Line 7Change to 'has a component which can be represented as ..'.Nathan Gillett, University of Oxford, UK (Exp.)

Page 28, lines 11-16.Thank you.Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

Page 28 Line 12"The large uncertainty in climate sensitivity has not been reduced." This is an example of the worstaspect of progress reporting. Much better to state the present estimate of this uncertainty; indicate thatit is large; indicate the consequences of its being large; and only then indicate that there hasn't beenmuch decrease in that uncertainty since SAR.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 28, line 20.I think we understand more!Simon Tett, Hadley Centre, UK, (Exp.)

Page 28, line 23Why is this section identical to that at the beginning. Is this IPCC policy? And if so what is the pointof duplicating text?Professor A. Henderson-Sellers, ANSTO, Australia, (Exp.)

Page 28, line 37Add extra words at the end of the sentence "although the possibility remains that an unknown factorthat could have been responsible for at least part of the warming seen in the first part of the twentiethcentury might also be influencing climate in the latter half of the twentieth century."Ken Gregory, Centre for Business and the Environment, UK, (Exp.)

Page 30, line 20.It is possible that one may not detect a signal but that its simulated amplitude was inconsistent with theobserved amplitude. I think that would tell us something!Simon Tett, Hadley Centre, UK, (Exp.)

Page 30, line 22.I take it precision means with no substantial error due to "noise" or other uncertainty.Simon Tett, Hadley Centre, UK, (Exp.)

Pages 30-33.What are you going to do, have a final exam now? The proof is left to the student. ..Prof Patrick J Michaels, University of Virginia, USA, (Exp.)

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IPCC WGI TAR Gov/Expert Review (Expert Comments) Chapter 12 21/ll/13

Page 31, line 23.Given the same amount of data to estimate covariance matrices the space-time approach will sacrificespatial resolution for temporal-resolution.Simon Tett, Hadley Centre, UK, (Exp.)

Page 32 Line 13,17Why is the normalisation different in the centred and uncentred statisitics? C is a covariance, and R is acorrelation.Nathan Gillett, University of Oxford, UK (Exp.)

Page 32, line 21.Note one could do optimal correlations -- see Stephenson, 1997, Tellus 49A, 513--527.Simon Tett, Hadley Centre, UK, (Exp.)

Page 33 Line 9n has not been defined.Nathan Gillett, University of Oxford, UK (Exp.)

Page 34 Line 10Change to 'observed and simulated projections on signal patterns are normally distributed'Nathan Gillett, University of Oxford, UK (Exp.)

Page 34 Line 15I don't understand what is meant by 'determine the p-value by comparing values of the detectionstatistic' - make clearer.Nathan Gillett, University of Oxford, UK (Exp.)

Page 36 Line 22'in press' -> '27, 993-996'

Nathan Gillett, University of Oxford, UK (Exp.)

Page 39, lines 21-22Reference should read:Sexton, D.M.H., Rowell, D.P., Folland, C.K., and Karoly, D.J. (2000) Detection of anthropogenicclimate change using an atmospheric GCM. Climate Dynamics (submitted).David Sexton, Hadley Centre, UK (Exp).

Page 40, lines 8-9This Vinnikov et al. paper submitted to Science has now been published, with a revised title. Therevised title and publication information are: Global warming and Northern Hemisphere sea ice extent,Science, 286, 1934-1937. (The authors and date of publication are correct on lines 7-8.)Dr C Parkinson, NASA Goddard Space Flight Center, USA (Exp.)

Page 41, table.Allen and Tett detected GSO (in an analysis with just GSO) and G (in a G&GSO analysis) Note thatTett et al, 2000 also looked at changes in the free-atmosphere in a similar manner to Allen and Tett.Tett et aI, 2000 detected G, SITO, GSIOTO and Nat or in their notation (WM, OA, OTA and Natural).[SI = Sulphates Indirect and Direct, TO = Tropospheric ozone] Note that Tett et al, 2000 also includedchanges in tropospheric ozone in their simulations. The table needs to define Nat.Simon Tett, Hadley Centre, UK, (Exp.)

Page 43, Figure 12.2.:More detail is required in the caption. Are these equilibrium responses or transient? Was the forcing astep function or a ramp, and if a ramp, with what slope? Some measure should be given of globalmean forcing and response. I am surprised that the magnitude of the NH aerosol forcing was aminimum in august; most models have it greatest in summer (mainly due to length of daytime).Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 44

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IPCC WGI TAR Gov/Expert Review (Expert Comments) Chapter 12 21/11/13

Presumably model had greenhouse gas changes in too - if so mention these in caption - i.e. 'geenhousegas changes, Mount Piatubo volcanic aerosol.. ..'Nathan Gillett, University of Oxford, UK (Exp.)

Page 45Caption line 2: Should "changes in" be struck?Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 45Why is one pattern a covariance, and the other an EOF of temperature change? Wouldn't it be better tocompare two covariances?Nathan Gillett, University of Oxford, UK (Exp.)

Page 46, Figure 12.5 c and d.Figure here is not quite the same as the technical summary. Were the simulations masked by theobservational mask? I (and co-authors) now have a version of the paper in which the simulations weremasked by the observed mask and use gray shaiding to show the range from the different ensembles.We also have carried out an ensemble of four simulations with both natural and anthropogenic forcingsfrom which a similar diagram could be produced.Simon Tett, Hadley Centre, UK, (Exp.)

Page 46Caption: DVI should be spelled out. Magnitude of forcing corresponding to doubling C02 should bespecified. Separate panel(s) should show forcing as a function of time.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 49Figure caption: Magnitudes of forcings should be specified.Stephen E. Schwartz, Brookhaven, USA (Exp.)

Page 50 Figure 12.8Caption to (a) and (c) should be swapped with (b) and (d)Dr Joanna D Haigh, Imperial College of Science, Technology and Medicine, UK, (Exp.)

Page 51, Figure 12.9.HadCM3 should be GSIO (Greenhouse gases, sulphate aerosols, indirect and Ozone) with S changedappropriately. What does the asterisk in (a) above ECHAM/LSG mean?

I am however extremely concerned about the analysis. The analysis is based on century timescales andI do not think that the available length of controls used for the optimisation is sufficent to reliablyestimate the covariance matrix. For example there will only be roughly 10 to 15 independant 90 yearsegments in the 850-years of data from the HadCM2 control. For the Canadian climate model (CGMIand CGM2) the control runs are very short ~ 800 years or so. There will be 4-6 independant 90 yearsegments in 400-years. It seems to me that there could be substantial uncertainty introduced into theanalysis as a consequence of this. This possibly substantial uncertainty has not been considered. I alsonote that each analysis is done in a different eigenvector space and all at truncation 10. I am alsoconcerned that this figure is based on an algorithm that has not been used before in climate changedetection and attribution. To my knowledge no paper has yet been submitted on the use of thisalgorithm in detection and attribution.

There are two ways that this could be fixed:1) Drop the figure all together and back out the results from 11(a) of Barnett et al, 1999 (fig 12.7). Thishas advantages in that you don't end up showing two sets of similar results and that the analysis isprobably more robust.2) Take all the control data used for optimisation and generate one common covariance matrix. Use thiscovariance matrix in the optimisation and the separate controls for the uncertainty testing. Then swapthe populations about and repeat the analysis to see how robust the results are. This still leaves use ofthe new algorithm as a problem. This could be checked by using the "standard" mutli-variate detectionalgorithm to see how sensitive the results are. IFF there is little sensitivity then I think the figure wouldbe OK.

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Simon Tett, Hadley Centre, UK, (Exp.)

Page 51, Figure 12.9 caption.I have difficulty reconciling some of the statements in the caption with the information in the figure.Perhaps some additional explanation would clear things up. Areas of confusion: "All are consistentwith unity, so there is little evidence ... " Is CGCMl consistent with unity? "The uncertainties increasebut the greenhouse signal remains unambiguously detectable and both factors are consistent with one."The ECHAM3/LSG greenhouse factor does not appear consistent with one.Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA, U.S.A. (Exp.)

Page 51Caption, a): 'All are consistent' -> 'All but one are consistent''when both are estimated simultaneously' -> 'when all are ... .'Delete 'and both factors are consistent with one' (Not true)'The influence of natural forcing is not detectable in this indicator and period.' It is for solar inECHAM3. Delete or change.Nathan Gillett, University of Oxford, UK (Exp.)

Page 52Figure caption: Magnitudes of forcings and sensitivities should be specified.Stephen E. Schwartz, Brookhaven, USA (Exp.)

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LATE COMMENTS

Added 03-07-00:

Page 5, lines 49-50.I understand that other reviewers have taken issue with the claim that the evidence of the observedrecord suggests that greenhouse gases have made a substantial contribution to the recent observedwarming. While I believe this claim is correct and supported by the vast majority of relevant model-data comparison studies, it is inevitable that some multi-pattern detection and attribution exercises(which are particularly sensitive to the details of model-simulated response and internal variability)may assign a greater uncertainty than others to the magnitude of the greenhouse signal. A lesscontroversial statement would be as follows:

"This work suggests that most of the observed interdecadal variability in global mean temperature overthe past 50 years has been externally driven rather than internally generated, with the strongest singleexternal factor being the influence of anthropogenic greenhouse gases."

The evidence that observed interdecadal fluctuations in global mean temperature over the latter half ofthe 20th century are predominantly controlled by external forcing is less controversial than anyestimate of the magnitude of the greenhouse signal. Specifically, the estimated variance of theobserved externally forced response exceeds that attributable to internal variability by at least a factorof3, possibly considerably more in the case of some models. Moreover, it is argued elsewhere in theTAR that greenhouse gases are the largest single source of radiative forcing over the past 50 years, sothe second clause in this statement is not controversial.

The evidence supporting this specific statement is primarily contained in figures 12.5, 12.8 and 12.9. Ifthe natural and anthropogenic ensemble mean temperature series are simply added together in figures12.5 (c) and (d), it is immediately apparent that they account for the vast bulk of observed interdecadaltemperature variability. On a more quantitative level, if! compute the variance in decadal global meantemperature anomalies about the 1906-96 mean attributable to the combined response to anthropogenicand natural forcing as simulated by the ECHAM3, HadCM2 and HadCM3 models (following theanalysis shown in figure 12.9 and Allen et aI, 2000a), I find it exceeds that attributable to internalvariability by a factor of 3 to 8 depending on the model and forcing combination used (interestingly,

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(PCC WG( TAR Gov/Expert Review (Expert Comments) Chapter 12 21111/13

the implausible-sounding factor of 8 arises from the most comprehensive study, HadCM3). Thesenumbers are based on a single combined-response pattern (GS+Solar, GS+Solar+Volcanic andGS+Natural respectively) and so are not sensitive to the details of the estimation procedure.

The fact that most of the variance of large-scale temperature anomalies on these timescales appears tobe externally forced has important implications: given that greenhouse gases are accepted to be thedominant factor in recent changes in radiative forcing, it provides support for the claim that they havemade a substantial contribution to the recent observed warming. It also has very important practicalimplications for the development of strategies for decadal climate prediction.

I recognise and fully understand that the lead authors may be reluctant to introduce what appears to bea new element into the discussion at this stage, but it should be stressed that this observation followsvery simply from results presented in this chapter. Ibring it up now because the "substantialcontributor" phrase appears to have generated some (in my view misplaced) controversy, and suggestthis as an alternative line of argument to get across the very important point that these externally-forcedsignals are not just discernible, but large in magnitude relative to internal variability.Myles Allen, RAL, UK (Exp.)

The remainder of my comments on chapter 12 are of a specific, technical nature.

Page 16, line 44Should this not read "When both anthropogenic and natural factors are included, models provide aplausible explanation of the changes in global mean temperature over the last hundred years (figure12.5)" with a caveat to say that the need for natural factors is model-dependent (Delworth and Knutson,2000)? Both the studies on which that figure is based (Free and Robock, 1999, and Tett et al, 2000)concluded, following Wigley et al (1997), that both natural and anthropogenic factors are required toaccount for interdecadal temperature changes over the 20th century. This point is visually apparentfrom the obvious discrepancy between observed temperatures and the simulated anthropognic responsein the middle decades of the century.

The Tett et al (2000) study specifically concluded that observed changes in temperatures wereinconsistent with a response to either anthropogenic or natural forcing on their own, but that the model-simulated combined response to both forcings (which, since there is no evidence for non-linearity inthe response to forcing of this magnitude, may be obtained simply by adding up the individualcomponents) represents a remarkably accurate simulation of 20th century surface temperature change.The figure would be much clearer to read if the ensemble mean model-simulated response was shown,together with a shaded band indicating intra-ensemble spread, rather than the individual simulations (asis the case in the latest version of the supporting paper). It would also be clearer if the sum of theathropogenic and natural signals could be shown, either as an additional panel or replacing panel (d), tosave the reader from adding them up (and to guard against readers not realising that they should addthem up to obtain this model's best estimate of the externally-forced response). At present, the figuredoes not really support the accompanying discussion, because while the naturally-forced simulation isclearly inconsistent with the observed record over the past 20 years, the anthropogenically-forcedsimulation is also inconsistent wit h the observations over the preceding 20 years.Myles Allen, RAL, UK (Exp.)

Page 23, lines 25-26Allen and Tett showed that both G and Gsa patterns could be detected separately, with the amplitudeof the GSa fingerprint estimated from observations found to be consistent with that simulated by themodel. They were not able to demonstrate formal evidence of ozone influence, primarily becausedeficiencies in model-simulated stratospheric variability obliged them to down-weight precisely theregions where this influence might be expected to be strong. Systematic quantitative estimates of themagnitude of ozone influence is likely to require the next generation of stratosphere-resolving coupledclimate models.Myles Allen, RAL, UK (Exp.)

Page 25, line 15The figure reference is presumably to 12.9, which shows the 4-signal detection result with HadCM2.There should be a reference to Stott et al (2000a) and (optionally) to Allen et al (2000a) who extendedthe Tett et al (1999) analysis to 3- and 4-signal cases.

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Myles Allen, RAL, UK (Exp.)

Page 25, line 53The reference for this interpretation of pattern-amplitudes as scaling factors should probably be eitherNorth and Stevens (1998) or, more specifically, Allen and Tett (1999). Of course, Tett et al (2000) is agood example of an application of this idea.Myles Allen, RAL, UK (Exp.)

Page 26, line 6Are these figures based on the ordinary least squares regression algorithm? If so, Stott et al (2000b)have shown they are likely to be biased towards zero due to the small size of the ensemble simulationused. This bias is noted in the Tett et al study, so it simply needs to be flagged.Myles Allen, RAL, UK (Exp.)

Page 26, lines 10&12It would be more accurate to say that this partial compensation or degeneracy between the greenhouseand sulphate signals, combined with a reduction in the relative amplitude of the sulphate forcing in thefuture, increases the upper bound on future warming rates consistent with the signal observed to date.Allen et al (2000b) discuss precisely this point in some detail, and it is restated more accurately in lines25-34.Myles Allen, RAL, UK (Exp.)

Page 26, lines 14&15...found that, for most anthropogenic climate change simulations, the amplitude of the modelled

response to the combination of greenhouse and sulphate influence is consistent with that observed, withscaling factors ranging from 0.8 to l.2.

As a point of information, these scaling factors are, and should be, sensitive to model error: if a modelis over-estimating the response, the analysis suggests it should be scaled down: Allen et al (2000b) givethe example of the HadCM2 greenhouse-only simulation as an example of a demonstrable model error.Myles Allen, RAL, UK (Exp.)

Page 26, line 27-28...providing objective uncertainty limits on the transient climate response that are based on observationsand independent, to first order, of possible errors in the individual models' climate sensitivities andtimescales of oceanic adjustment.Myles Allen, RAL, UK (Exp.)

Page 26, lines 29-30...based on a number of model simulations. Allen et al (2000b) show that a similar range of uncertaintyis obtained if the greenhouse and sulphate components are estimated separately because, under theIS92a scenario, these factors are projected to continue to oppose each other in the future much as theyhave done in the past. If, as more recently-produced ...

For information, figure 12.10 only shows single-signal GS estimates because separate greenhouse-onlyensembles were not available for several of the models considered.Myles Allen, RAL, UK (Exp.)

Page 26, line 33Forest et al (2000) use the optimal detection approach of Allen and Tett (1999) -- their actual resultswere derived from a different model.Myles Allen, RAL, UK (Exp.)

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