From	Jonathan T. Overpeck	Date	Fri, 14 Jul 2006 16:46:20 -0600
To	Henry Pollack
CC	Eystein Jansen, Valerie Masson-Delmotte, Tim Osborn, Keith Briffa
Subject	Re: Borehole in the Southern Hemisphere
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 Mail and Fedex Address: Institute for the Study of Planet Earth 715 N. Park Ave. 2nd Floor University of Arizona Tucson, AZ 85721 direct tel: +1 520 622-9065 fax: +1 520 792-8795 http://www.geo.arizona.edu/ http://www.ispe.arizona.edu/ Attachment Converted: "c:\documents and settings\tim osborn\my documents\eudora\attach\huang1997GRLHoloceneBoreholes.pdf"