Monday, January 16, 2012


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
>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
>>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
>>For OVERNIGHT (e.g., Fedex) deliveries,
>>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

For OVERNIGHT (e.g., Fedex) deliveries,

Dr. Jonathan Overpeck
NOAA National Geophysical Data Center
3100 Marine Street, RL3, Rm A136
Boulder, CO 80303
tel: 303-497-6160

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