cc: tar_cla@meto.gov.uk, tar_ts@meto.gov.uk date: Fri, 06 Oct 2000 13:20:58 -0700 from: Michael Prather subject: Re: 'balance' Issue for TS and SPM to: Michael_Oppenheimer@environmentaldefense.org, Joyce Penner , John Stone , griggs Dear David, John, Joyce, and Michael My apologies, I have been unable to contribute to this very important debate until I cleared my chapter. The wording in the SPM draft we were discussing (15 Apr draft given below) is far too strong a statement: it removes the fundamental issue that this finding is basically still a balance of the evidence. Admittedly what is new since the SAR is that more weight has accumulated on the "have-detected-human-influence" side of the balance (as Michael O notes). Nevertheless, there are still some large and open problems (e.g., indirect aerosol effects) that prevent this from being a closed case. Today a new SPM draft appeared (6 Oct, below) that chooses more measured words (I only wish that 'balance' could somehow be worked in). BUT the final bullet in the new section stands out in that it avoids the major new uncertainties that have been identified - merely by doing a GHGas+Sulfate vs. GHGas alone model does not address the uncertainties in "other" forcings, such as other aerosols or the history of the increase in tropospheric ozone - which cannot be explained well and is certainly not documented. I doubt that these studies considered the range of uncertainty in tropospheric ozone growth or in OC/BC aerosols and indirect effects. This last bullet cannot be supported from what I found in Chapters 4 and 5. I leave these issues for discussion in NY, Michael ------------------------------------------ SPM (15 Apr 2000) "From the body of evidence since IPCC (1996), we conclude that there has been a discernible human influence on global climate." -------------------------------------------- new SPM (6 Oct 2000) "There is now stronger evidence for a human influence on global climate than at the time of the IPCC Working Group I, Second Assessment Report, and it is likely that increasing concentrations of anthropogenic greenhouse gases have contributed substantially to the observed global warming over the last 50 years. . . . Uncertainties in other forcings do not prevent identification of the effect of anthropogenic greenhouse gases over the last 50 years. The sulphate forcing, while uncertain, is negative over this period and changes in natural forcing during most of this period are also estimated to be negative."