date: Tue, 19 Jul 2005 15:48:56 +0200 from: Stefan Rahmstorf subject: [Wg1-ar4-ch06] Comments on exec bullets to: Eystein Jansen , wg1-ar4-ch06@joss.ucar.edu, Jonathan Overpeck Hi all, here is some comments on the summary bullet points. I number the bullets in sequence of appearance. 1. Need to improve on how we phrase "time-average climates" versus "events" (the PETM) - the PETM data are still averages over thousands of years, to any policy maker this would be a "time average climate". Maybe we can say something like "climate states stable over many millions of years (e.g., the mid-Pliocene), or a warm event lasting a few hundred thousand years (the PETM). (And why do we speak of warm events in plural - is there another case like the PETM?) 2. I suggest: · Post-industrial levels of atmospheric CO[2] and CH[4] have risen far above the levels found in the longest (up to 800,000 years) ice-core records, highlighting the fact that the recent unprecedented rise in these trace gases does not stem from natural mechanisms. Over these multi-millennia time scales, Antarctic temperature and CO[2] co-vary with each other. 3. I suggest: · There is no evidence that the current warming will be mitigated by a natural cooling trend towards glacial conditions. Understanding of orbital forcing indicates that the earth would not naturally enter another ice age for at least 30,000 years. Rising atmospheric CO[2] may delay the earth from entering the next scheduled ice age. 5. This bullet says: Paleoclimate models simulate a change in global mean surface air temperature change between the Last Glacial Maximum and the current interglacial of 3.1 to 5.1°C. That is incorrect. E.g., the first coupled LGM simulation (Ganopolski et al., Nature 1998) obtained 6.2 ºC, and more recently Kim et al. (2003) obtain 10 ºC. Our new large ensemble study (Schneider et al., submitted) gets a glacial cooling in the range of 6.0-7.5 ºC for model versions with mid-range climate sensitivity (2.5-3.5 ºC), and finds that the regional results (e.g., tropical SST cooling, Greenland and Antarctic cooling) of these runs agree well with paleo data - indicating that the data constraints suggest an actual global cooling in this range. The reason why these numbers are larger than many previous modeling studies is that many have only used part of the forcing changes, i.e. mostly not including either vegetation changes or dust forcing. The latter account for ~2 ºC additional cooling in our model. I think if we do want to cite numbers, we need to distinguish which numbers apply to models with the full set of glacial forcings, and which numbers apply to a subset only, which essentially makes those sensitivity studies rather than attempts to simulate the full glacial conditions. They should not be lumped together, else the range of model responses looks far more uncertain than it actually is. 9. This bullet makes the radical claim that Arctic warming caused Antarctic ice sheet decay - I look forward to the submitted Science papers that supposedly show this... But jokes aside - is it true that Eemian Arctic warming is comparable to Arctic warming at the end of this century (like, 6-8 ºC)? I got some sceptical looks when I ran that past a few people here. 14. I suggest: · The current retreat of mountain glaciers is unusual in the context of the Holocene, particularly considering that the known natural forcing mechanisms, such as the orbital decrease in summer insolation in the Northern Hemisphere, should be more favourable to glacier growth, as it was in the past. -- To reach me directly please use: [1]rahmstorf@ozean-klima.de (My former addresses @pik-potsdam.de are read by my assistant Brigitta.) Stefan Rahmstorf [2]www.ozean-klima.de [3]www.realclimate.org _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06