date: Fri, 03 Dec 2004 16:20:34 +0100 from: Stefan Rahmstorf subject: [Wg1-ar4-ch06] response to "threshold" question of Ron Stouffer to: Ronald Stouffer Dear Ron on the risk of abrupt climate change and thresholds I have three things to contribute which I hope will be useful. (1) Results of THC expert elicitation. Attached is a ppt file with first results on what a dozen experts think the risk of THC changes due to global warming is. We hope to publish this soon so it can become a citeable source in time for the AR4. (2) Review editorial in press with Climatic Change Attached is a paper which argues what I think is an important point sometimes lost: given the large uncertainties but high potential impacts, dealing with abrupt climate change is an issue of risk assessment, not one of prediction, and it should be discussed as such. This has a number of important implications - e.g., doing a few "best guess" scenarios with models is *not* a risk assessment. Think of the risk of a nuclear power accident - looking at a few "best guess" scenarios would only tell you that the most likely thing is that the power station will work just fine, without accident. It is also senseless to try and predict an accident in the sense of running a model that will forecast that Chernobyl will blow up in may 1986. What can and should be done, however, is that we try to work out what could go wrong, and how likely this is. Another implication is that relatively low probabilities do not imply "nothing to worry" - or would you board a plane with 1% chance of crashing? I say this since one still sometimes finds simplistic statements about abrupt climate change risk in the style: "didn't happen in my GCM, so let's not worry about it". (3) I want to comment on one of David Rind's points. 4) The model response was in all senses linear - the less freshwater added, in Sv-Yrs, the smaller the percentage reduction in NADW. Thus if only 25 Sv-Yrs of freshwater were added, NADW reduction was only 50%. This implies that the model did not see a "threshold" - and that there were really no 'surprises', since the reduction developed over time, and would have been observable had anyone been looking for it. I respectfully disagree with the conclusion here. Reason: it is not clear that you would find a threshold in this type of rapid transient experiment - in fact, I would have expected that you don't. This view was confirmed by Andrey Ganopolski, who has repeated the same experiments as David in the CLIMBER-2 model. Everything is linear, just as in David's model. However, to conclude that there was no threshold is clearly wrong, since we know in our model a threshold and hysteresis behaviour exist from doing the equilibrium experiments, which is what you need to do to clearly identify thresholds. In other words, the type of experiment done by David is not the right type to show the absence of presence of thresholds, and nothing about the presence of thresholds can be concluded from these experiments. There are simple physical reasons for thresholds. (1) There is a threshold in the physical properties of water, i.e. the freezing point. (2) There is a threshold for surface density; if it drops below that, no convection will be possible any more since the water column is stable - i.e., convection is a threshold process. (3) There is the Stommel bifurcation, resulting from the large-scale positive salt advection feedback (Stommel 1961). This is also found in ocean GCMs and in all 11 EMICs participating in our intercomparison (including those with 3d GCM ocean component). Whether it is found also in fully coupled GCMs or not is not proven yet since no experiment I know of has yet been done which would be able to demonstrate or refute it. Until then, I think there is no reason to expect this bifurcation is not found in coupled GCMs, since the physics is simple and comes from the ocean component, and is found in ocean GCMs. Hard to see how atmospheric feedbacks, not found in the range of EMICS we tried, would wipe out this behaviour. Another (and important) question then is: do thresholds matter in a transient global warming situation, or are they only relevant as a theoretical concept that helps in understanding equilibrium, but not transient, behaviour? I would argue that this is still very much an open question that has hardly been investigated systematically. The first-order idea of a threshold implies that when crossing it, something changes. The "something" could be the qualitative difference between DWF recovering (e.g. Manabe&Stouffer for 2xCO2), versus it stopping (for 4xCO2 - it doesn't matter here whether it stops for good, or just for some centuries). Such a threshold is also seen in Rahmstorf & Ganopolski 99 for differing amounts of freshwater input in the context of a transient global warming run. Thus my summary would be: some transient global warming scenarios do support the idea that thresholds matter, not only for equilibria. Some model runs do not show signs of thresholds, but in my view this is no evidence for their absence, it is rather absence of evidence, related to the experimental design not suitable to show thresholds. All the best, Stefan _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06