From	Stefan Rahmstorf	Date	Fri, 01 Oct 2004 11:49:05 +0200
To	Eystein Jansen
CC	wg1-ar4-ch06@joss.ucar.edu
Subject	[Wg1-ar4-ch06] Ch6-Climate Sensitivity
Hi co-authors, here are some thoughts on what to say on climate sensitivity in our chapter - this is an attempt to focus on the main, simple messages for policy makers. (I think we should try retaining those important messages and not lose sight of them amidst all the details, complexity and caveats.) The main policy-relevant question could be phrased as follows: Does the past climate history tell us how sensitive the climate system is to CO2? I submit that the answers to this we get from different time periods are the following. Deep Time: Reconstructions are too uncertain (and boundary conditions too different, e.g. continents in different places, different ocean circulation) to draw quantitative conclusions about sensitivity to CO2, but there is clear evidence that times of high CO2 in Earth history tend to be ice free (Royer et al. 2004). A second piece of evidence is the Late Paleocene Thermal Maximum, which shows that the climate has responded by warming to a large carbon release into the atmosphere. Just how large this carbon release was is not known, since several origins of the carbon are possible, which have different isotope signature and would thus imply different amounts. But the temperature response was large (6K), and if anything this response would point to a high sensitivity. Glacial-Interglacial Changes: We have by now sufficiently good quantitative reconstructions of CO2 and other forcings as well as temperatures in order to derive useful quantitative estimates of climate sensitivity. LGM was the most recent time in history in which CO2 concentration differed greatly from pre-industrial values, by as much as it does now. It is the closest test case for response to CO2 changes that we have. There are two basic methods to derive climate sensitivity: (i) Based on data analysis - e.g. Lorius et al. 1991 (concluding sensitivity is 3-4 K). This method has the caveat that this sensitivity applies to colder climate, which may differ somewhat from that which applies in present climate as the strength of feedbacks is expected to depend on the mean climate (e.g., stronger snow-albedo feedback in colder conditions). (ii) Based on combining data and models - e.g. Schneider von Deimling et al. 2004. Does not have the above caveat, but depends on models. Lag of CO2 behind temperature does not imply a lack of CO2 effect on climate, since the lag is small (centuries, not millennia). Holocene, last millennium ?? Overall conclusions Qualitatively, climate history is at least consistent with the accepted CO2 sensitivity. There is no evidence for much lower or much higher CO2 sensitivity (note that CO2 is not the only forcing). The more recent climate history (as far back as ice core data go) does allow quantitative inferences. The results of these estimates all lie within the IPCC range and provide strong support for this. Paleodata may even allow to reduce this range, since at least one study argues that values above 4K are very likely inconsistent with the reconstructed LGM climate: for high CO2 sensitivity, tropical cooling in the glacial should have been larger. Cheers, Stefan _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06