cc: tar_cla@meto.gov.uk, tar_ts@meto.gov.uk date: Tue, 17 Oct 2000 12:03:22 +0100 from: "Mitchell, John FB" subject: RE: 'balance' Issue for TS and SPM to: 'Michael Prather' , Michael_Oppenheimer@environmentaldefense.org, Joyce Penner , John Stone , griggs Dear Michael, As you were not in New York, let me comment briefly. First, the statement is not a closed statement - because of the huge uncertainty in indirect aerosl forcing- necessarily negative, we cannot give an upper bound to the greenhouse gas contribution. We only say that it is substantial- ie at least non negligible. Second, detection studies work on the largest space scales only - probably only encompassing a N-S gradient and land-sea contrast - also atmospheric and other processes smooth out regional detail which may be apparent in the forcing pattern. See eg figure 12.3 Third, detection and attribution schemes allow for scaling of the amplitude of patterns- if the pattern amplitude is wrong then the regression approach used in optimal detection can scale the signal to correct for this.. Fourthly, natural factors are ruled out largely becuase of their time dependence. Fifthly, if the aerosol patterns look like greenhouse patterns ito the detection procedure, then attribution of a sizeable greenhouse gas contribution follows from the assertion that the observed warming is too big to be explained by natural factors alone - anthropogenic factors must provide a warming and the bigger the (negative) aerosol forcing, the bigger the GHG gas warming required to balnace it. If the patterns are different, then this makes detection and attribution easier. Finally, at least two of the studies cited in the chapter include a reperesentation of increases in tropospheric ozone. For more details, I recommend you read the chapter- especially sections 4 and 6 - you should consider all the evidence.. With best wishes John jfbmitchell@meto.gov.uk Hadley Centre for Climate Prediction and Research The Met. Office, Bracknell RG12 2SZ UK Tel +44 1344 856613/6656 Fax+44 1344 856912 > -----Original Message----- > From: Michael Prather [SMTP:mprather@uci.edu] > Sent: Friday, October 06, 2000 9:21 PM > To: Michael_Oppenheimer@environmentaldefense.org; Joyce Penner; John > Stone; griggs > Cc: tar_cla@meto.gov.uk; tar_ts@meto.gov.uk > Subject: Re: 'balance' Issue for TS and SPM > > 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."