date: Mon, 14 Sep 1998 14:38:34 -0700
from: "Jonathan T. Overpeck" <jto@ngdc.noaa.gov>
subject: climate of the last millennia...
to: p.jones@uea.ac.uk, Keith Briffa <k.briffa@uea.ac.uk>,  mann@snow.geo.umass.edu, rbradley@climate1.geo.umass.edu,  drdendro@ldgo.columbia.edu, coleje@spot.colorado.edu,  Brian Luckman <luckman@sscl.uwo.ca>

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

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Dr. Jonathan Overpeck
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