AR4 on "1998 was the warmest year"

From Climate Audit via Greenie Watch. As most CA readers know, a few years ago, I wondered how they knew that 1998 was the warmest year in a millennium – a claim that you don’t see in AR4. Nor, at first (second or even fifth) glance does the assertion, once so prominent, even seem to be addressed in AR4. The Climategate letters offer an interesting vignette. Chapter 6 authors were not unaware of the matter and worked over language on the issue like New York or London lawyers, eventually inserting a clause deep in the chapter that gave them cover, intentionally leaving the issue out of the chapter Executive Summary. On July 28, 2006, Chapter 6 Coordinating Lead Author Overpeck (1154090231.txt) wrote to Briffa (copy Jansen) passing along a question from WG1 Chairman Susan Solomon, asking the reasonable question about what happened to claimes that 1998 was the warmest year, 1990s the warmest decade. Hi Keith – in our TS/SPM discussions, Susan has raised this question: “In the TAR they spoke of 1998 being the warmest year in the millennium and the 1990s the warmest decade. I don’t see that chapter 6 addresses any of these time scales. I am not saying you should do so – but are you planning to say anything about it and why you aren’t doing so? and if you’re not planning to say anything at all, can you please tell me what you think about it, just for my own info?” Would you please give me your feedback on this, with enough thoughtful detail to hopefully make me/Susan fully informed (a para should be enough). Thanks, Peck On Aug 1, 2006, Briffa replied (728. 1154484340.txt) with a comment that would not be out of place at Climate Audit (one of the interesting things about Climategate letters is how often they express views in private that are expressed publicly at CA). Briffa: Peck, The TAR was, in my opinion, wrong to say anything about the precedence (or lack thereof) of the warmth of the individual year 1998. The reason is that all reconstructions have very wide uncertainty ranges bracketing individual-year estimates of part temperature. Given this, it is hard to dismiss the possibility that individual years in the past did exceed the measured 1998 value. These errors on the individual years are so wide as to make any comparison with the 1998 measured value very problematic, especially when you consider that most reconstructions do not include it in their calibration range (curtailed predictor network in recent times) and the usual estimates of uncertainty calculated from calibration (or verification) residual variances would not provide a good estimate of the likely error associated with it even if data did exist. Now Briffa didn’t leave it quite like that. He continued with the opinion that confidence could be attached to decadal averages that could not be attached to individual years: I suspect that many/most reconstructions of NH annual mean temperature have greater fidelity at decadal to multidecadal timescales (based on examination of the covariance spectrum of the actual and estimated data over the calibration period. This is the reason many studies implicitly (Hegerl et al.,) or explicitly (Esper et a;., Cook et al.) choose to calibrate directly against decadally-smoothed data. The exception is the Briffa et al (tree-ring density network based) reconstruction back to 1400. This has probably the best year-to-year fidelity – but for summer land only and does not go back anyway to the MWP. We are on much safer grounds focusing on decadal/multi-decadal timescales and so this is where we place the emphasis. As for the warmest decade’ – this is likely to be the 1990s or the last 10 years – but again, the proxies do not cover this period, and we do anyway state that post 1980 is the warmest period – which I think is fair enough. Overpeck (728. 1154484340.txt) acknowledged this message the next day, passing it on to Solomon and Jansen: Hi Keith – thanks. This makes sense to me. I’ll cc Susan so she understands the issue better, and also can advise on any strategy we should adopt to make sure we communicate effectively. thanks again best, peck On Sep 1, 2006 (739. 1157138720.txt), Overpeck and Jansen adopted the strategy of inserting some protective language in the chapter text, while leaving it out of the Executive Summary, and urgently requested Briffa to write some language on the matter (it had not been specifically addressed in the drafts sent to reviewers). As for the 1998/2005 warmest in last 1000 years issue, we suggest adding nothing new to the ES, in line with our chapter policy from Bergen, BUT adding something in the chapter along the lines of: ” There is currently insufficient knowledge to form a consensus on the issue of how the warmth of individual years of the last 100 years compare with individual years of the last 1000 years” Keith, would you like to make a suggestion on the wording and placement? On Sep 13 (744. 1158180188.txt), Briffa reverted with some language that was carefully crafted to say the least. Eystein and Peck I have thought about this and spent some time discussing it with Tim. I have come up with the following Greater uncertainty associated with proxy-based temperature estimates for individual years means that it is more difficult to gauge the significance, or precedence, of the extreme warm years observed in the recent instrumental record. However, there is no new evidence to challenge the statement made in the TAR that 1998 (or the subsequent near-equivalent 2005) was likely the warmest in the last 1000 years. This should best go after the paragraph that concludes section 6.6.1.1. I believe we might best omit the second sentence of the suggested new paragraph – but you might consider this too subtle (or negative) then. I think the second sentence is very subtle also though – because it does not exclude the possibility that the same old evidence that challenges the veracity of the TAR statement exists now , as then! I think this could go in the text where suggested , but I think it best NOT to have a bullet about this point. We need to check exactly what was said in the TAR . Perhaps a reference to the Academy Report could also be inserted here? Anyway, you asked for a straw-man statement for all to argue about so I suggest we send this to Stefan, David , Betty and whoever else you think. cheers Keith Overpeck wrote back the same day ( 744. 1158180188.txt): Keith – thanks for this and the earlier updates. Stefan is not around this week, but hopefully the others on this email can weight in. My thoughts… 1) We MUST say something about individual years (and by extension the 1998 TAR statement) – do we support it, or not, and why. 2) a paragraph would be nice, but I doubt we can do that, so.. 3) I suggest putting the first sentence that Keith provides below as the last sentence, in the last (summary) para of 6.6.1.1. To make a stand alone para seems like a bad way to end the very meaty section. 4) I think the second sentence could be more controversial – I don’t think our team feels it is valid to say, as they did in TAR, that “It is also likely that, in the Northern Hemisphere,… 1998 was the warmest year” in the last 1000 years. But, it you think about it for a while, Keith has come up with a clever 2nd sentence (when you insert “Northern Hemisphere” language as I suggest below). At first, my reaction was leave it out, but it grows on you, especially if you acknowledge that many readers will want more explicit prose on the 1998 (2005) issue. Greater uncertainty associated with proxy-based temperature estimates for individual years means that it is more difficult to gauge the significance, or precedence, of the extreme warm years observed in the recent instrumental record. However, there is no new evidence to challenge the statement made in the TAR that 1998 (or the subsequent near-equivalent 2005) was likely the warmest of Northern Hemisphere year over the last 1000 years. 5) I strongly agree we can’t add anything to the Exec Summary. 6) so, if no one disagrees or edits, I suggest we insert the above 2 sentences to end the last (summary) para of 6.6.1.1. Or should we make it a separate, last para – see point #3 above why I don’t favor that idea as much. But, it’s not a clear cut issue. Thoughts? Thanks all, Peck David Rind weighed in (744. 1158180188.txt) as follows (copies to other Lead Authors):: Leaving aside for the moment the resolution issue, the statement should at least be consistent with our figures. Fig. 6-10 looks like there were years around 1000 AD that could have been just as warm – if one wants to make this statement, one needs to expand the vertical scale in Fig. 6-10 to show that the current warm period is ‘warmer’. Now getting back to the resolution issue: given what we know about the ability to reconstruct global or NH temperatures in the past – could we really in good conscience say we have the precision from tree rings and the very sparse other data to make any definitive statement of this nature (let alone accuracy)? While I appreciate the cleverness of the second sentence, the problem is everybody will recognize that we are ‘being clever’ – at what point does one come out looking aggressively defensive? I agree that leaving the first sentence as the only sentence suggests that one is somehow doubting the significance of the recent warm years, which is probably not something we want to do. What I would suggest is to forget about making ‘one year’ assessments; what Fig. 6-10 shows is that the recent warm period is highly anomalous with respect to the record of the last 1000 years. That would be what I think we can safely conclude the last 1000 years really tells us. David Jansen (745. 1158204073.txt) suggested a version without Briffa’s casuistic second sentence: Hi all, My take on this is similar to what Peck wrote. My suggestion is to write: Greater uncertainty associated with proxy-based temperature estimates for individual years means that it is more difficult to gauge the significance, or precedence, of the extreme warm individual years observed in the recent instrumental record, such as 1998 and 2005, in the context of the last millennium. think this is scientifically correct, and in essence means that we, as did the NAS panel say, feel the TAR statement was not what we would have said. I sympatise with those who say that it is not likely that any individual years were warmer, as Stefan has stated, but I don´t think we have enough data to qualify this on the hemispheric mean. Best wishes, Eystein On Sep 15, 2006, Fortunat Joos wrote that if there isn’t enough evidence to say whether 1998 was the warmest year or not, they should say so. Hi, I support Eystein’s suggestion and agree with David. If there is not sufficient evidence to support or dismis claims whether 1998 or 2005 was the warmest year of the millennium than we should indeed say so. It is the nature and the strenght of the IPCC process that points from the TAR and earlier reports get reconsidered and reassessed. It is normal that earlier statements get revised. Often statements can be strenghtened, but sometimes statements can not be supported anymore. Our job is to present the current understanding of science as balanced as possible. With best wishes, Fortunat A little later, Briffa signed off on the revision, noting his own reservations about the original “too clever” language, expressing a slight worry that they had “inserted this late with no refereeing and no justification in the text” – (a scruple that he and Jones didn’t worry about when it came to matters MM): I do not disagree either – in fact I preferred not to make the “too clever” second statement in my “straw man” as I said at the time. If this is the consensus (and I believe it is the scientifically correct one) then I would be happy with Eystein’s sentence. The worry is that we have inserted this late with no refereeing and no justification in the text. I would also suggest dropping the second “!individual” in the sentence. On Sep 15 (746. 1158324958.txt), Overpeck decided to go with Jansen’t language on the “all important 1998 sentence”. Thanks Keith, Tim and Fortunat for your input. We’ll go with what we have then – Eystein’s suggestion minus the second “individual”. Eystein and Oyvind – just want to double check that you’ve deleted that 2nd “individual” in the all important 1998 sentence?? Thanks, Peck In the AR4 Final Version, section 6.1.1.1 ended as follows; Greater uncertainty associated with proxy-based temperature estimates for individual years means that it is more difficult to gauge the significance, or precedence, of the extreme warm years observed in the recent instrumental record, such as 1998 and 2005, in the context of the last millennium. AR4 agreed with MM on the “warmest year in 100 years”. Who would have known? Which leads to another question. What caused the WG1 authors to have a more guarded opinion in AR4 about “1998 is the warmest year” than in AR3? What was their justification for modifying the opinion of AR3 (relying on the statistical analysis of MB98-99) that they knew with statistical confidence that 1998 was the “warmest year”? In the penultimate comment above, Briffa observed that there was “no justification in the text” for introducing this more guarded opinion in the conclusion to the section. The obvious location in the text for justifying this more guarded opinion was in the discussion of the MM papers, which had, after all, raised this issue. After the MM papers observed the abject failure of MBH verification r2 statistics in the early segments, even MBH supporters abandoned any pretence that the reconstruction had any “inter-annual skill”. This point is conceded in a couple of Climategate letters though not publicly. But Briffa, as the author of the relevant section, did not concede even this point in the text on MBH vs MM – a point would have provided a small bit of credit to MM. Worse, between the Second Draft (submitted to reviewers) and the Final Draft, during surreptitious correspondence with Briffa, Eugene Wahl, neither an IPCC author or reviewer, inserted a statement that our analysis had a negligible impact – a statement that was contrary to the corresponding Second Draft statement and a statement that was never submitted to reviewers. Ironically, Chapter 6 Lead Authors adopted a key position of the MM papers in respect to individual years (though not yet individual decades) – a position that clearly contradicted MBH98-99 and AR4 – but failed, as Briffa observed, to document the changed view in the running text. SOURCE

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