date: Mon, 14 Sep 1998 14:38:34 -0700 from: "Jonathan T. Overpeck" subject: climate of the last millennia... to: p.jones@uea.ac.uk, Keith Briffa , mann@snow.geo.umass.edu, rbradley@climate1.geo.umass.edu, drdendro@ldgo.columbia.edu, coleje@spot.colorado.edu, Brian Luckman 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