cc: trenbert@ucar.edu, p.jones@uea.ac.uk, tom.delworth@noaa.gov, broccoli@envsci.rutgers.edu, cwunsch@ocean.mit.edu, rcurry@whoi.edu, jseveringhaus@ucsd.edu, shs@stanford.edu date: Tue, 29 Apr 2008 12:09:19 +0200 from: Stefan Rahmstorf subject: Re: nature paper / ocean model as short-term regional climate to: Andrew Revkin Dear Andy, thanks for asking. I think this an interesting paper and this kind of decadal forecasting will become increasingly important. On the other hand, it is still early days, this is pioneering work and many aspects of this are not yet properly understood, as Richard Wood rightly cautions in his N&V. Not least, nobody knows what the MOC really has done over the past decades. So what does this mean for the forecasts? The prediction of European cooling: I'd take that in a Bayesian sense as some evidence for possibly cooler temperatures, but not enough to make cooling more likely than warming for me. So if I had to bet money at equal odds on warming or cooling, I'd still bet on warming, although with less confidence than previously. I'm not sure their error bars give the full error - note for example that their forecast error bar for 2015 in Fig. 4 is almost zero, so even without having had time to properly look at what this error bar encompasses, I suspect that it is not the full uncertainty. And of course they predicted the 1994-2004 period to be quite a bit cooler than before, and it turned out to be warmer (Fig. 3c). Prediction of global temperature: The authors claim that this "may not increase over the next decade". However, their standard A1B run has a higher correlation with global temperature than their hindcast run, and their hindcast run predicted cooling over the past 5 years (the period 1994-2004 was hindcast as being colder than the decade centered five years earlier) when in fact warming happened, and it predicted flat global temperatures in the past 10 years (in this sense) when warming occurred. This is very interesting when you compare it to the time evolution of the MOC. Almost the entire verification period, where their method shows some skill, the MOC has been increasing. Since about 1990 it has stopped to increase and started to decrease (in their analysis, nobody knows for real), and consequently their method predicts cooling over Europe and globally since then - but up until now, for the past two points in their series, this prediction turned out to be wrong. That's why I'm not sure whether I believe it for their next two points. If the method works only while the MOC is increasing, but not while it is decreasing - maybe the past forecast skill has nothing to do with the MOC? Certainly it has not been well-validated for times of decreasing MOC. They predicted a slight cooling for Europe for 1965 and 1970, but the observed trends here are closer to the scenario run without the MOC forecast. The early phase of weak diagnosed MOC, before 1970, may also not be weak MOC at all (according to some work by Mike Mann), it may be at least partly an effect of aerosol cooling over the North Atlantic region, which has the effect of cooling regional SST and thereby looking like weak MOC - it would be hard to disentangle the two effects. So I think this is good work, I do not want to denigrate this in any way, but as I said it is very early days with this type of forecasting and many questions are still not properly understood, so I would not start to draw strong conclusions at this point. Of course the basic message is always correct: some natural variability is always superimposed on the greenhouse warming trend, so ten years of less warming or more warming don't tell us anything about this greenhouse trend - see the discussion we had on realclimate on this point: http://www.realclimate.org/index.php/archives/2008/01/uncertainty-noise-and-the-art-of-model-data-comparison/ You could test how serious the authors are: tell them that a prominent climatologist is offering them a bet of $10,000 at equal odds that global mean temperature will be warming over the next decade. Are they prepared to bet against this? Cheers, Stefan -- Stefan Rahmstorf www.ozean-klima.de www.realclimate.org