cc: Martin Manning <mmanning@al.noaa.gov>
date: Wed, 6 Sep 2006 09:32:56 -0400
from: David Rind <drind@giss.nasa.gov>
subject: Re: [Wg1-ar4-ch06] urgent IPCC need
to: Fortunat Joos <joos@climate.unibe.ch>, Jonathan Overpeck <jto@u.arizona.edu>, WGI-chap6-ar4 <wg1-ar4-ch06@joss.ucar.edu>

Hi,

I hope this isn't nitpicking, but using the words 'very likely' in 
combinaion with the reset of the sentence gives a statistical 
meaning, from the IPCC perspective, that says very strongly that ALL 
of those things contributed to the positive amplification. Some of 
them one would think very likely did: changes in greenhouse gas 
concentration, ice sheet and sea ice changes, and most likely 
biophysical feedbacks - but we really have no idea whether ocean 
circulation changes were a positive or negative feedback (and 
different modeling groups have gotten opposite results in this 
regard). The circulation changes implied by the CLIMAP SST 
reconstruction  (reduced poleward heat transport) actually was a 
negative feedback in our modeling results by keeping the tropics warm 
and minimizing the water vapor reduction. In coupled models, one set 
of studies showed that decreased deep water formation was necessary 
for the positive feedback, while another set showed increased 
circulation was the trigger. Even dust is an issue - over a vegetated 
surface, or the ocean, it is clearly a positive amplification, but as 
Jonathan knows well, modeling studies suggest it is a negative 
feedback over snow and ice surfaces. Some studies have it helping to 
force the peak of an ice age, while in another it is a prime 
component in ending it. So I would suggest the following, somewhat 
less definitive version:

*	The widely accepted orbital theory suggests that 
glacial-interglacial cycles occurred in response to orbital forcing. 
The large response of the climate system implies the necessity for a 
strong positive amplification of this forcing. Changes in greenhouse 
gas concentrations, ice sheet growth and decay, ocean circulation and 
sea ice changes, biophysical feedbacks, and aerosol (dust) loading 
are among the responses that influenced climate sensitivity.

David



At 12:57 PM +0200 9/6/06, Fortunat Joos wrote:
>*	The widely accepted orbital theory suggests that 
>glacial-interglacial cycles occurred in response to orbital forcing. 
>The large response of the climate system implies a strong positive 
>amplification of this forcing. Changes in greenhouse gas 
>concentrations, ice sheet growth and decay, ocean circulation and 
>sea ice changes, biophysical feedbacks, and aerosol (dust) loading 
>have very like contributed to this amplification.
>
>Not sure David is happy with it?
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