date: Tue, 6 Oct 1998 17:02:39 -0700 from: "Jonathan T. Overpeck" subject: Re: climate of the last millennia... to: Keith Briffa Hi Keith - thx for the good comments. As Mike responded, I agree you hit the nail on the head w.r.t. backlash potential. About that mtg in January, although I can't promise snow-free land surface in Boulder, we can avoid outdoor activities. Assuming you can get a cheapish ticket, we'll find the $$ to get you over if Jan 15-16th is a doable time for you (right after AMS - the dates chosen by Phil). Ed is here next week and I'll make sure he is invited. We'll keep it small - just a couple per proxy, and get more info to you soon. thx! Peck >Hi Peck et al. > A little late but I'd like to put in my twopence worth regarding >your original message and Phil's reply. I have been tied up with a load of >stuff so don't interpret my lack of speedy response as a lack of interest >in these matters. > My first comment is that I agree with all of your general remarks >and with your implied rebuke to Phil that we should be very wary of seeming >to dam certain proxies and over hype others when we all know that there are >real strengths and weaknesses associted with them all. The truth is that >all of this group are well aware of this and of the associated fact that >even within each of these sub-disciplines e.g. Dendro, coral etc. there is >a large range of value , or concern with the external usage of our data. >However, my own and Phil's concerns are motivated ,like yourself, by the >outside world's inability to appreciate these points and the danger that we >will all be seen as uncritical or niave about the real value of proxy data. >The rationale for the recent Jones et al paper, and some things that I have >written in the past is to inform would be users , particularly the >modellers, that there are critical questions to be addressed about how the >palaeo-data are best used in a 'detection' or 'model validation' context. >Many in the palaeo-community understand these issues , but perhaps there >has been some reluctance to air them in sufficient depth or in the right >situations where they will be heard/seen by those people who now seek to >use the data . I believe that many of the modellers , having been >blissfully unaware for years of the need to work with the palaeo-community, >are now expecting too much . This carries the danger of a backlash as they >undertake simple assessments of the palaeo-series and conclude that they >are all of very little use. The problem is that as we try to inform them we >may get the balance between valueable self criticism and scientific >flagellation wrong. The more so when the whip is seemingly aimed at others! >There is no doubt though, that many palaeo- types are not concerned with >the 'bigger issues' of climate change , so it is up to those who do ,such >as this group, to try to sort out some sensible approach to how we do >explore the good and bad ,fairly, in our collective data and how we present >this to the outside world. The meeting you propose is a good way >forward.If he is already not included, I also urge you to invite Ed Cook. > I hate cold feet and I don't ski so I vote for anywhere away from >snow. > >To answer the question about the degradation in tree-ring chronology >confidence back in time - yes, we ( that is several of us in tree rings , >and rising out of them, in average temperature or rainfall series, have >suggested a basis for quantifying chronology error as a function of series >replication and time-dependent chages in the correlations of the series >that go to form the mean chronology. The problem is tricky because the >error is timescale ( i.e frequency) dependent also. This is just the >chronology. Calculating confidence limits on reconstructions derived from >one or more chronologies must take account of the regression error (again >likely to be timescale dependent) while incorporating the additional >uncertainty associated with the chronology. When the reconstructions are >derived using a spatial transfer function ( such as in canonical >correlation or our similar Orthogonal Spatial Regression technique )the >reconstruction at each point in the predictand network has some ,different, >uncertainty relating to the error in each predictor series and the >magnitude of its influence in the specific regression equation relating to >that point. Finally, as regards this issue, if you have detrended or >high-pass filtered the original predictor series in some way (i.e. >tree-ring standardisation) , you have some potential long-timescale >uncertainty around the final reconstruction which can not be represented by >any analyses of the remaining prdictors or their association with a >relatively short instrumental predictand series. I have a half drafted >paper on this which I intended to submit to Tree-Ring Bulletin - perhaps >one day! > > Your question about Jasper, the sample depth, in my opinion , IS >responsible for the early high values. So don't put much faith in the early >warmth. We have devised a simple method of scaling down the variance in >average series to take account of the inflated variance that occurs when a >reduced number of series are averaged - such as at the start of this >chronology . We used this in our recent Nature paper looking at a possible >volcanic signal in the density data averaged over the northern network. Ed >has incorporated this in the latest version of his super tree-ring >standardisation/chronolgy construction program , but it was not used in the >Jasper work . > > I agree that we must be careful not to appear to be knocking other >proxies- even if this is not intended . We must also be explicit about >where problems lie and in suggesting the ways to overcome them. I for one >do not think the world revolves only around trees. The only sensible way >forward is through interpretation of multiple proxies and we need much more >work comparing and reconciling the different evidence they hold. Let's have >more balance in the literature and more constructive dialogue /debate >between ourselves. > > Keith > > > > >At 02:38 PM 9/14/98 -0700, Jonathan T. Overpeck wrote: >>Hi Phil et al. - just read the Jones et al. Holocene paper (v. 8, p. >>456-471) and had a couple comments/questions.... >> >>1) nice paper >> >>2) would you like to archive the reconstructions at the WDC-A for Paleo?? >>It would be great to add them to existing recent ones (Cook et al. - >>drought; Mann et al. NH temp; Briffa et al. NH temp, Overpeck et al. Arctic >>temp). It would be ideal to get each of the 17 proxy records PLUS the >>hemispheric recons. >> >>3) regarding proxies, I wonder how much of the "quality" issue regarding >>ice cores and some other remote proxy records is due to there not being any >>instrumental stations near them (and at the same altitude)? Also, with >>respect to coral records, I get the feeling most in the coral community now >>think there is something "funny" about long Galapagos record (age model, >>maybe more - I think a new record is being generated). Also, many coral 18O >>records (e.g., New Caledonia) are influenced by both temp and salinity >>variations. This is a solid reason why the fit of such a record to temp >>won't be as good as you'd like (or as good as a buffo dendro record). I >>think Terry Quinn is generating the trace metal data to sort temp out. >>Lastly, I've now seen a number of coral records (most not published, but >>Tarawa is an example I think) where the proxy does as well as local >>instrumental data (in this case ppt) in getting the regional signal, AND >>the local instrumental record only go back to the war. I'm guessing, just >>between us, that ENSO recons based on proxies will soon be better than >>instrumental ones before 1950 - not just before 1850! In fact, I'd bet on >>it (using some of the money Ray still owes Julie!). Thus, I worry that it >>might not be wise to dismiss reconstructions on a proxy basis, particularly >>since trees lack one important trait - they don't work for all parts of the >>globe. >> >>4) About trees.... (Keith are you still reading?? - I sent this to Ed and >>Brian too, since they might have insights). Has anyone examined how a >>tree-ring recon degrades as a function of sample size back in time. I >>always see the quality of dendro recons cast as GREAT vs.other proxies (and >>they are) based on comparison with instrumental records. But, the dendro >>records usually have the best sample replication in this same instrumental >>period, and then tail off back in time. For example, Brian's Jasper recon >>has a sample depth of ca 28 trees in the last century, but drops off to ca. >>5 in the 12th century and 1 (?) in the 11th century. The "quality" of the >>recon must degrade too?? In contrast, some non-dendro reconstructions may >>not verify as well as dendro vs the instrumental record, but they might not >>degrade with time either since the sample density doesn't change with time. >>Thus, could it be that at some point back in time, the dendro records >>degrade to the same quality (or worse) than other proxies??? >> >>5) Talking specifically about Jasper, it is interesting that the 20th >>century is as warm or warmer than everything in the last 1000 years EXCEPT >>before ca. 1110 AD. Since the sample depth before this time is 5 or less, >>how much faith should we put in those warmer than modern temps?? >> >>6) I went to the trouble of all this mainly to A) get some feedback (and >>data into the WDC) and also B) to highlight that we need to extra careful >>in judging the quality of one proxy over or under another. If a well known >>group of paleo scientists suggest that, for example, corals are not that >>useful, then it might mean more years before we have a mutli-century >>record of tropical climate variability. I think it is clear that each proxy >>has limitations (and I like the table 2 idea of Jones et al), but the real >>need is to understand that each record (not just each proxy) has pros and >>cons, and that wise use requires knowing these pros/cons. Some coral, ice >>core and sediment records are no doubt better than some dendro records >>(also, for example, with respect to reconstructing low frequency variations >>in climate). I'm NOT trying to dis tree-rings, but rather to suggest more >>balance in what we all say in the literature. >> >>7) Lastly, I think there is a need to have a small workshop to put together >>an expanded version of Jones' et al. table 2, and, more importantly, to set >>some guidelines for data generators in terms of the kinds of data and meta >>data that need to be archived to ensure best use of the data (for example, >>information of the nature of the climate signal and what might bias it - >>like the salinity effect on a coral record or method of standardization on >>a dendro record). Also, we need guidelines on what info should be archived >>with a climate reconstruction (for example, are error bars available; if >>not, why not - there are often good reasons, but the interdisicplinary user >>might not get it). It might be best if the database could be upgreaded, so >>that users would know, for example, that a proxy record or recon they want >>to use has some recently discovered problem or verification. >> >>I've asked Mike Mann if he'd like to help put together such a workshop with >>me, and I think I have some US funding for it - it would be small, with >>just a couple folks from each proxy plus some folks like Phil and Mike who >>are well-know users of paleo data. Like the idea?? >> >>Thx for reading this far. Cheers, Peck >> >>Dr. Jonathan T. Overpeck >>Head, NOAA Paleoclimatology Program >>National Geophysical Data Center >>325 Broadway E/GC >>Boulder, CO 80303 >> >>tel: 303-497-6172 >>fax: 303-497-6513 >>jto@ngdc.noaa.gov >> >>For OVERNIGHT (e.g., Fedex) deliveries, >>PLEASE USE: >> >>Dr. Jonathan Overpeck >>NOAA National Geophysical Data Center >>3100 Marine Street, RL3, Rm A136 >>Boulder, CO 80303 >>tel: 303-497-6160 >> >> >> >> >> >-- >Dr. Keith Briffa, Climatic Research Unit, University of East Anglia, >Norwich, NR4 7TJ, United Kingdom >Phone: +44-1603-592090 Fax: +44-1603-507784 Dr. Jonathan T. Overpeck Head, NOAA Paleoclimatology Program National Geophysical Data Center 325 Broadway E/GC Boulder, CO 80303 tel: 303-497-6172 fax: 303-497-6513 jto@ngdc.noaa.gov For OVERNIGHT (e.g., Fedex) deliveries, PLEASE USE: Dr. Jonathan Overpeck NOAA National Geophysical Data Center 3100 Marine Street, RL3, Rm A136 Boulder, CO 80303 tel: 303-497-6160