From: Jonathan Overpeck <jto@u.arizona.edu>
To: "Neil Roberts" <C.N.Roberts@plymouth.ac.uk>
Subject: Re: ipcc chapter 6 draft
Date: Thu, 18 May 2006 15:58:25 -0600
Cc: Keith Briffa <k.briffa@uea.ac.uk>, Eystein Jansen <eystein.jansen@geo.uib.no>

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Hi Neil - Thanks for your interest in providing feedback on the draft 
chap 6 Second Order Draft. Since the IPCC has very strict rules about 
all this, I'm going to ask them (the IPCC) to send you an official 
invitation to review, along with the process - formal, but highly 
efficient - to follow. If you could send your comments in that way it 
would be a great help. We've been asked to keep everything squeaky 
clean, and not to get comments informally.

Thanks! Peck


>Dear Jonathan
>
>Please excuse me for writing direct, but Keith Briffa suggested it 
>would be simplest. I have looked through the draft chapter 6 and 
>find it an impressive document. However, bullet 4 on page 6.2, 
>starting "global mean cooling and warming....." strikes me as 
>incorrect and misleading.
>
>Whereas the mean rate of temperature change over the Pleistocene may 
>have been >10 times slower than that projected for the next century, 
>there is clear evidence that for specific major climatic 
>transitions, global (or at least hemispheric) temperature changes in 
>the past have been at least as rapid as those projected by climate 
>model simulations and incorporated in the last IPCC report.  The 
>most obvious case in point is the global warming at the start of the 
>Holocene, ca. 11.5 ka BP. Russell Coope, more than 20 years ago, 
>showed from beetles that UK temperatures rose faster than could be 
>dated within the errors of 14C dating. Subsequently this was 
>confirmed by Greeland ice cores based on layer counting (full 
>glacial to interglacial in less than 100 years), and by the Cariacos 
>basin marine record. I have worked on varved lake records from both 
>the tropics (Roberts et al Nature 1993 366, 146-148) and the 
>Mediterranean (Roberts et al The Holocene, 2001, 11, 719-734) where 
>this climate transition was accomplished in substantially less than 
>a century.  In short, several independent lines of evidence show 
>that the climate system has been capable of flipping from one 
>meta-stable state to another, very different one over timescales 
>that could be experienced by a single human lifetime.  This is not 
>an unimportant conclusion in terms of the potential for non-linear 
>responses of future climate to GHG forcing.
>
>I also looked for supporting argument for bullet 4 later in chapter 
>6, but found nothing of substance.
>
>In short, this particular bullet seems in need of critical 
>reassessment before the definitive version of the next IPCC reprot 
>emerges.
>
>Thanks in anticipation and best egards
>
>Neil


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

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