date: Sat, 25 Jun 2005 16:08:23 +0200
from: Stefan Rahmstorf <regentage@gmx.de>
subject: [Wg1-ar4-ch06] Exec bullet points on abrupt change
to: Eystein Jansen <eystein.jansen@geo.uib.no>, Jonathan Overpeck <jto@u.arizona.edu>, IPCC Chapter 6 <wg1-ar4-ch06@joss.ucar.edu>

   Hi friends,
   we were left with three semi-finished bullet points on THC changes and abrupt climate
   change in Beijing. In the powerpoint file sent by Peck on 2 June, these are the slides #
   11, 18, and 19.
   Bullet 18 is really about shift in e.g. drought regime and other regional changes, which
   I'm not expert on so won't comment.
   Bullets 11 and 19 are within my expertise, they concern THC changes and DO and Heinrich
   events. I would like to suggest the following wording for these two bullets (and I propose
   to keep them together):
     * More than 20 times during the past 100,000 years, large and abrupt climate shifts have
       occurred known as Dansgaard-Oeschger events and Heinrich events. Temperatures changed
       by up to 12 C within a decade around the northern Atlantic (e.g., in Greenland). These
       events persisted for centuries and had global repercussions, such as major shifts in
       tropical rainfall patterns. They were probably not associated with large changes in
       global mean temperature, but rather with a redistribution of heat between northern and
       southern hemisphere.

     * Strong evidence, both from sediment data and from modelling, links abrupt climate
       events to changes in the Atlantic ocean circulation, although details of the mechanism
       are still under discussion. Our current understanding suggests that the ocean
       circulation can become unstable and change rapidly when critical thresholds are
       crossed. While it is unclear where these thresholds are and how much they differ
       between glacial and modern climate, it cannot be ruled out that future warming and
       meltwater inflow could again trigger major ocean circulation changes.

   The rationale here is that there are in my view two key conclusions from our
   paleo-knowledge about these abrupt events: (1) that very big and abrupt events have
   happened quite a few times in the past (this is a warning that the climate system is
   capable of such things, purely from data), and (2) that we actually know something about
   the mechanism, namely that the ocean circulation (triggered by whatever) can change in a
   fast and major way, and this could happen again. I believe these are robust conclusions
   that I hope everyone can share.
   I'm now working on the draft of 6.3.2 which will underpin these two bullets.
   Stefan
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