cc: "Pittock,Barrie" , "Whetton, Peter" , "Jones, Roger" , "Mearns, Linda (E-mail)" , "Hulme Mike (E-mail)" , "Wratt, David (E-mail)" , "Dokken Dave (IPCC) (E-mail)" date: Fri, 30 Jul 1999 13:06:04 +1000 from: "Pittock,Barrie" subject: Chapter 3. to: "Carter Tim (E-mail)" Dear Tim, I feel bad about not getting to Chapter 3, but I am really desperate to get Chapter 12 in presentable order, which it is not right now. There are some major gaps and getting contributions is hard (as you are no doubt experiencing also). I do not even recall exactly what is covered in the climate scenarios section, but thought it might be useful to flag several things I think must be covered eventually: * Possible, even probable, increases in the frequency and magnitude of El Nino events. A paper by Wenju Cai and Peter Whetton sent to Nature last week provides very solid support for a move towards more El Nino like mean state since the 1960s, continuing until 2100 or beyond. In Chapter 12 we will have to discuss the possible consequences in terms of more frequent droughts in ANZ, but opposite consequences are likely elsewhere and could be major. Peter intended to send you a copy, but the local email system failed last Friday/Monday and it may not have gone. He is back next Monday after a week off and can send it again. * Other possibly low-probability but high impact scenarios, notably decrease or cessation of both N. Atlantic and Antarctic Bottom Water formation. Key references are several including Hirst in Environmental Modelling and Software 14, 227-241 (1999); Wood et al., Nature 399, 572-575 (10 June 1999); Sarmiento and Hughes, Tellus 51B, 560-561 (1999). Right now we have Richard Matear at CSIRO Marine Labs. running an ocean chemistry model to see what it does to the deep ocean - possibly making it anoxic, with H2S emissions, and changes in biology and nutrients. Possible consequences in terms of changes in ocean circulation and upwelling, nutrient supplies and fisheries are largely conjecture as yet, but could be big indeed. Must be discussed! * The caveats that you have put in the Interim Characterizations document will have to be most fully explained in the chapter, including a strong statement re the assumption of linearity in the scaling, which is clearly invalidated by the Cai and Whetton paper which shows a qualitative change in patterns of change setting in in the 1960s, with a reversal from a more La Nina-like mean state to a more El Nino-like mean state, long after the start of the GH gas increase. * As I have discussed with Mike Hulme, a need to depart from comparing estimated mean changes to 2 standard deviations of the hisorical/natural inter-decadal variability to something more like a 1 standard deviation, since the object is not to prove that climate change is happening (against a null hypothesis of no change), but to come up with a best estimate of what the change is (which requires an estimate of the most probable, with variability or uncertainty around that). The proper null hypothesis for studying potential impacts is not no change, but that there will be some change - all we are looking for is the best estimates. * I still think the discussion of the post-stabilisation scenarios needs to be fuller than in the Interim Characterizations document, with a strong suggestion that changes in some regions may reverse sign, and that sea level may continue to rise for centuries after stabilisation. >From a policy viewpoint this implies a need for not only stabilisation at minimum GH concentrations, but also reductions in concentrations post-stabilisation in order to try and minimise the post-stabilisation sea level rise. We cannot address Article 2 of the FCCC unless we deal with this up front. Again, apologies for not doing some real drafting, but just impossible at present. Best regards, Barrie. Dr A. Barrie Pittock Post-Retirement Fellow*, Climate Impact Group CSIRO Atmospheric Research, PMB 1, Aspendale 3195, Australia Tel: +61 3 9239 4527, Fax: +61 3 9239 4688, email: WWW: http://www.dar.csiro.au/res/cm/impact.htm * As from 1 March 1999 I have become a CSIRO Post-Retirement Fellow. This means I do not have administrative responsibilities, and am working part-time, primarily on writing for the Intergovernmental Panel on Climate Change. Please refer any administrative matters or contract negotiations for the CIG to Dr. Peter Whetton, the new Group Leader, at , tel. +61 3 9239 4535. "Far better an approximate answer to the right question which is often vague, than an exact answer to the wrong question which can always be made precise." J.W. Tukey as cited by R. Lewin, Science 221,636-639.