From: Jonathan Overpeck <jto@u.arizona.edu>
To: Henry Pollack <hpollack@umich.edu>
Subject: Re: Borehole in the Southern Hemisphere
Date: Fri, 14 Jul 2006 16:46:20 -0600
Cc: Eystein Jansen <eystein.jansen@geo.uib.no>, Valerie Masson-Delmotte <Valerie.Masson@cea.fr>, t.osborn@uea.ac.uk, Keith Briffa <k.briffa@uea.ac.uk>

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Hi again Henry - I've attached an 1997 paper of your's and wonder if 
you could shed some up-to-date insights on how to best interpret. In 
particular:

1) it has been pointed out to us that the result in this paper argue 
for a globally warm period during the middle Holocene that was warmer 
than today. Our assessment (i.e., Figure 6.9) indicates that there 
was likely no period during the Holocene that was warmer around the 
global than the late 20th century. Especially outside of the tropics, 
there were periods warmer than today during the Holocene, but these 
regionally warm periods were not synchronous - at least at the 
centennial scale we can examine with proxy data. Thus, although Huang 
et al. 1997, indicates greater mean annual global warmth, it was 
unlike the synchronous global warming of the late 20th century.

Plus, we believe the warmth of the Holocene was driven by orbital 
forcing, and that what we see makes sense in that regard. Huang et 
al, 1997 can be explained perhaps (this is a question) by the heavy 
borehole coverage in the Northern mid- to high-latitudes? We also 
know that proxy data shown in Fig 6.9 also indicate more warming 
(again, not synchronous) in Southern Hem mid-latitudes - where there 
are also many boreholes.

Obviously, another issue is that the boreholes don't give the same 
temporal resolution as the other proxy records we 
synthesized/assessed, and at least in your paper, there isn't 
regional information either.

So - the point is not (unless you suggest otherwise) that Huang et al 
97 is wrong, but rather than within the limits of the data, it is 
compatible with what the higher-resolution, regionally-specific, 
multi-proxy data are showing in Fig 6.9, and that there was likely no 
period during the Holocene that was warmer synchronously around the 
global than the during the late 20th century. Do you agree with this, 
and is our reasoning accurate and complete?

2) Huang et al 1997 also shows evidence for warmth within the last 
500-1000 years that was greater than during the 20th century AND a 
cool minima 200 years ago. Both of these are highlighted in your 
abstract, and both seem incompatible with other evidence. For 
example, your own more recent work has shown the coolest temperatures 
to be about 500 years ago.

We didn't think it was within our focus to comment on these issues, 
but we are being asked to by reviewers, and it would be good to have 
your help in addressing these issues - hopefully in our responses to 
review comments rather than in our main text (which has to be 
shortened).

Many thanks for your help with this paper and the issues it raises.

Best, Peck



-- 
Jonathan T. Overpeck
Director, Institute for the Study of Planet Earth
Professor, Department of Geosciences
Professor, Department of Atmospheric Sciences

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Attachment Converted: "c:\documents and settings\tim osborn\my documents\eudora\attach\huang1997GRLHoloceneBoreholes.pdf"

