cc: felzer@ucar.edu, sjagtap@agen.ufl.edu, franci@giss.nasa.gov, kittel@ucar.edu, nanr@ucar.edu, m.hulme@uea.ac.uk, mmaccrac@usgcrp.gov date: Tue, 18 May 1999 15:50:00 -0600 (MDT) from: Dave Schimel subject: Re: CO2 concentrations to: Tom Wigley I'm glad that Tom thinks this clarifies matters. I'm more confused than ever but often the darkest darkness precedes the dawn. Point 4) appears to argue that 1% in CO2 is about right (but 1% gives levels higher than 712 ppmv). Is it that 1% applied to the eq CO2 of ~479 gives about the right final eq CO2 and deltaQ? In any case, I would be thrilled to simply use the Joos 1s92a time series. Dave On Tue, 18 May 1999, Tom Wigley wrote: > Dear all, > > I've just read the emails of May 14 onwards regarding CO2. I must say > that I am stunned by the confusion that surrounds this issue. > Basically, I and MacCracken are *right* and Felzer, Schimel and (to a > lesser extent) Hulme are *wrong*. There is absolutely, categorically no > doubt about this. Let me explain. > > (1) The Hadley Centre run is meant to simulate the climate change > consequences of the full IS92a emissions scenario. > > (2) In this scenario, there are the following concentration and forcing > changes over 1990-2100: > Item C(2100) DQ(1990-2100) > CO2 708 4.350 > CH4 3470 0.574 > N2O 414 0.368 > Halos 0.315 > TropO3 0.151 > ----------------------------- > GHGs 5.758 > SO4 (dir) -0.284 > SO4 (indir) -0.370 > ----------------------------- > TOTAL 5.104 > > These are the numbers I used in Ch. 6 of the SAR. They do not agree > precisely with numbers in Ch. 2, because I used the models and formulae > embedded in MAGICC. The differences between Ch. 2 and Ch. 6 are > irrelevant to the present issue. > > (3) How does one simulate the combined effects of all the GHGs in a > climate model that only has CO2? The standard way is to take the GHG > radiative forcing (ending in 5.758W/m**2 in 2100 in this case) and > convert this to *equivalent* CO2 concentration changes. If one uses > the old (IPCC90) forcing formula for CO2 (which is what was used in the > SAR), viz DQ=6.3 ln(C/C0), then C(2100)/C(1990) is 2.494. Note that the > 1% compounded change would be C(2100)/C(1990)=(1.01)**110=2.988. Thus, > 1% compounded CO2 gives roughly the correct *forcing*. > > NOTE THAT THE ACTUAL CO2 CHANGES ARE *NOT* THE CO2 CHANGES USED IN THE > MODEL. THE MODEL USES ARTIFICIAL CO2 CHANGES, SCALED UP TO ACCOUNT FOR > FORCING FROM OTHER GHGs. > > NOTE THAT THE ACTUAL CO2 CHANGE IS FROM 354ppmv IN 1990 to > 708ppmv IN 2100. THIS IS *NOT* A 1% COMPOUNDED INCREASE. > > NOTE, FURTHER, THAT WHAT MIKE HULME SUGGESTS IN HIS POINT 8 IS ALSO > WRONG. IT IS WRONG TO *BACK OUT* THE CO2 FROM FORCINGS. THE CO2 WAS > SPECIFIED A PRIORI. > > NOTE FINALLY THAT MIKE *DOES* GIVE THE 708ppmv VALUE IN HIS POINT 9. > USING THIS WOULD BE OK, BUT I RECOMMEND USING THE SLIGHTLY DIFFERENT > BERN MODEL RESULTS (SEE BELOW). > > (4) Now, some minor wrinkles. In the Hadley Centre model for CO2, > DQ=5.05 ln (C/C0). Hence, to get a forcing of 5.758W/m**2, they need to > use C(2100/C1990)=3.127. Note that this is a little closer to the 1% > compounded result than my above calculation. The Hadley Centre may well > have used a slightly different total 1990-2100 GHG forcing than mine, so > they may have backed out a compounded CO2 increase rate even closer to > 1% than the above. In any event, if they decided to go with 1%, then > this was a perfectly reasonable choice in order to capture the total GHG > forcing. > > (5) The 708ppmv C(2100) value is what comes out of my carbon cycle > model. In the SAR, in Ch. 2, we considered results from three different > carbon cycle models; mine, the Bern (Joos) model, and Atul Jain's > model. For illustrations in the SAR, we used the Bern model. The > mid-2100 value with this model, for IS92a, was 711.7ppmv. A later > version of this model, used in IPCC TP4, gives 711.5ppmv. Jain's model > gave 712.3ppmv. > > (6) The bottom line here is that, for a consistent pairing of Hadley > Centre climate and CO2, one MUST use the ACTUAL CO2 numbers that went > into calculating the radiative forcing, NOT the equivalent CO2 numbers. > The climate response reflects all GHGs, whereas the plants are > responding only to CO2. > > (7) I am attaching the Joos CO2 time series. I recommend using the > actual values rather than trying to fit a compound CO2 increase to > them---which, in any event, should not be done using just the end point > values. This, however, is your choice. Differences will be negligible > in terms of plant response. > > I hope this clarifies things. It has always seemed pretty obvious and > clear cut to me. I hope it will now to all of you. > > Cheers, > Tom > > > ********************************************************** > *Tom M.L. Wigley * > *Senior Scientist * > *National Center for Atmospheric Research * > *P.O. Box 3000 * > *Boulder, CO 80307-3000 * > *USA * > *Phone: 303-497-2690 * > *Fax: 303-497-2699 * > *E-mail: wigley@ucar.edu * > **********************************************************