date: Fri, 20 Feb 2004 13:59:11 -0000
from: "Emma L. Tompkins" <e.tompkins@uea.ac.uk>
subject: FW: Abrupt Climate Change
to: <tyn.building@uea.ac.uk>

   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 <rahmstorf@pik-potsdam.de>
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   To: Andy Revkin <anrevk@nytimes.com>
   CC: Mark Cane <mcane@ldeo.columbia.edu>, 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
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   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

