date: Fri, 20 Feb 2004 13:59:11 -0000 from: "Emma L. Tompkins" subject: FW: Abrupt Climate Change to: FYI 4 -----Original Message----- From: John Shepherd [mailto:j.g.shepherd@soc.soton.ac.uk] Sent: 20 February 2004 12:53 To: Alex Jackson; Gareth Morgan; Claire Powell; Stephen Powell; Lindsey Stones; Ben Ward; Maxwell Gonzales; Mathieu Theron; Helen Luke; Rachel Hadfield; Adam Williams Cc: tyndall-soton@soc.soton.ac.uk Subject: Abrupt Climate Change Date: Wed, 11 Feb 2004 17:08:25 +0100 From: Stefan Rahmstorf User-Agent: Mozilla/5.0 (Windows; U; WinNT4.0; en-US; rv:1.4) Gecko/20030624 X-Accept-Language: en-us, en To: Andy Revkin CC: Mark Cane , changelings@ldeo.columbia.edu, lea@geol.ucsb.edu, Sandy.Tudhope@glg.ed.ac.uk, rich@ldeo.columbia.edu, peter@ldeo.columbia.edu, koutavas@mit.edu, jchiang@atmos.berkeley.edu Subject: Re: finally preparing to write an update on prospects for abruptness X-Filter-Version: 1.7 (lamont.ldeo.columbia.edu) X-MailScanner-SOC: Found to be clean X-MailScanner-SOC-SpamCheck: not spam, SpamAssassin (score=-4.361, required 5, BAYES_00 -4.90, HTML_MESSAGE 0.10, HTML_TITLE_EMPTY 0.12, MIME_HTML_ONLY 0.32) Hi all, I think it is too general to debate whether warm climates or cold climates are more stable or prone to abrupt change. We need to be much more specific about the physical mechanisms, and about the specific situation we are in during this century. In terms of the past, we know of rapid ocean circulation changes due to freshwater input, which for the YD and 8k events was apparently meltwater due to the deglacial warming happening at the time. Obviously, the exact same stuff cannot happen now, because we are not going through a major deglaciation, we are now starting from a much warmer world with much less ice. On the other hand, deglacial warming was 4-6 degrees C occurring over several thousand years - IPCC projections have us go through similar amount of warming much much faster, within a century. The question is: can this rapid climate change reduce ocean surface density to the point where NADW formation stops, through both warming and freshening of the surface waters of the North Atlantic? Note that it is the warming, i.e. rate of change, not the warm climate per se, which causes this. In models with CO2 rising and then held constant, the THC weakens during the transient warming phase but then recovers (unless it got across the collapse threshold), often it is stronger in the warm climate than before. There is a large uncertainty about the amount of extra freshwater that will enter the North Atlantic in global warming scenarios. Most models do not include Greenland meltwater since they do not have an ice sheet model coupled in, for example (something to consider when you find no THC collapse in model simulations in the literature), yet the estimates for Greenland runoff vary hugely. To say something about the risk of THC collapse, one needs to do a systematic risk analysis looking at those uncertainties - something I have argued for many years (see the two Nature News&Views on this topic on my web site), it is not enough to look at a few "best guess" GCM scenarios. For a first attempt at this, see [1]Rahmstorf, S. and A. Ganopolski, 1999: Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, 43, 353-367. In terms of impacts, we can probably all agree that such an event would have major repercussions - regional cooling aroung the northern Atlantic, dynamic sea level changes, shifts in precipitation patterns, some far afield (ITCZ shift). I'm heading an international project looking at such impacts, the project web site is [2]http://www.pik-potsdam.de/~stefan/Projects/integration/ There is observational evidence for big changes currently going on in runoff (Peterson et al), salinities in the North Atlantic (Dickson et al., Curry et al) and Faroe bank channel overflow (Hansen et al). These observational people tend to be far more worried than us modelers. Ron Stouffer and I have a session at the EGU in Nice on this topic in April. Cheers, Stefan -- Stefan Rahmstorf Potsdam Institute for Climate Impact Research (PIK) For contact details, reprints, movies & general infos see: [3]http://www.pik-potsdam.de/~stefan