date: Sat, 25 Jun 2005 16:08:23 +0200 from: Stefan Rahmstorf subject: [Wg1-ar4-ch06] Exec bullet points on abrupt change to: Eystein Jansen , Jonathan Overpeck , IPCC Chapter 6 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 ºC 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 _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06