cc: Barrie Pittock , Luis Jose Mata , Stephen H Schneider , "Kozak, LeeAnn" , tar_cla@usgcrp.gov, tar_la@usgcrp.gov, tar_reved@usgcrp.gov, wgii.bureau@usgcrp.gov, John Houghton , yhding@public.bta.net.cn, djgriggs@meto.gov.uk, Neil Leary , kwhite@usgcrp.gov, ipcc@usgcrp.gov, maureen.joseph@environmental-change.oxford.ac.uk, giorgi@ictp.trieste.it, Jerry Meehl , cubasch@dkrz.de, ckfolland@meto.gov.uk, stocker@climate.unibe.ch, hewitson@egs.uct.ac.za, Barrie Pittock date: Thu, 07 Sep 2000 18:29:53 +1200 from: Martin R Manning subject: RE: Extremes table to: tkarl@ncdc.noaa.gov Tom Thanks for your interesting comments on the uncertainty issue. Some responses... Being a physical scientist I have no problems with the frequentist approach and this would be my preference where it can be applied. I also agree that it is confusing, and possibly illogical, to associate uncertainties with probabilistic statements - I have already argued against doing that in the WG II SPM. Your experience in forecasting must provide a strong basis for considering measures of uncertainty or probability for WG I statements - but I would argue that you can only apply this approach where the basis for predictions is well established (e.g. the models being used are well validated and relatively stable). The TAR also has to deal with some highly policy relevant issues that depend on models which are still being developed. In these cases we have no guidance from experience with previous forecasts and I do not see how the frequentist approach to uncertainty can be used. Your comments about not having a large "sample" echo the same concern. But your response to this problem is.... >"Without a large sample, it seems to me that it is not possible to assess >our confidence in our probabilistic statements. Our confidence is already >built in --- for low probability events we don't expect them to occur very >often. This is why I believe in working group 1 we changed from >confidence statements to likelihood statements." ... and I am afraid I do not follow this argument at all. If you can not apply a probabilistic approach then it seems to me that you have to look for another measure of confidence in model results. To say that the confidence is "built in" sounds like giving up rather than addressing the problem. It does not recognise inherent limitations in making assessments where views on the key driving processes are still evolving, model structure still changing, and model validation still sparse, as in the case of the THC. Estimates of confidence, based on expert judgement as to how comprehensive the present models are, may not meet the philosophical rigour of the frequentist approach, but do provide important information in these cases. The Uncertainties Guidance paper gave a number of ways in which you might develop such confidence levels from expert judgement on an objective basis. Your approach seems to limit the definition of uncertainty to the range of results from current models and in doing so may be rejecting an important source of information, viz any expert consensus on how good the models really are. Thus my bottom line is that we should be able to make statements about our "confidence" in a result even when defining "uncertainty" in that result may be problematic. And I really think that WG I need to consider and state their confidence in some key statements - such as the THC issue. There are of course other problems with producing simple summary statements about THC changes. It is hard to define exactly what one is talking about - most coupled ocean atmosphere models show a weakening of ocean circulation patterns in response to GH warming - so the < 1% probability by 2100 statement being made by WG I presumably refers to some level of major collapse in the THC. How do you set that level? How non-linear is the dependence of the probability estimate on the chosen degree of THC collapse? Then there are the time-scale issues - what is the probability that GH warming by 2100 causes significant change in THC after 2100? For these reasons I would prefer to see WG II deal with singular climate change separately from the table being discussed for Ch 3. Thanks very much for your comments - I am still learning from this interesting discussion. Martin ----------------------------------------------------------- Martin R Manning National Institute of Water & Atmospheric Research 301 Evans Bay Parade, Greta Pt PO Box 14-901, Kilbirnie Phone: +64 +4 386 0535 Wellington Fax : +64 +4 386 2153 New Zealand m.manning@niwa.cri.nz -----------------------------------------------------------