cc: Tim Kittel , Nan Rosenbloom , Mike Hulme , Mike MacCracken , "Tom M.L. Wigley" date: Wed, 19 May 1999 15:10:46 -0600 (MDT) from: Tom Wigley subject: Re: CO2 concentrations (fwd) to: Dave Schimel , Shrikant Jagtap , Ben Felzer , franci Dear all, If you use the IS92a CO2 data I provided, or some analytic approximation to them, could you please cite the source for the Bern model in any publications. It is: Joos, F., Bruno, M., Fink, R. Siegenthaler, U., Stocker, T., Le Quere and Sarmiento, J.L., 1996: An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake. Tellus 48B, 397-417. (Note: "Quere" has acute accents over the 'e's.) Thanx, Tom ---------- Forwarded message ---------- Date: Tue, 18 May 1999 13:08:51 -0600 (MDT) From: Tom Wigley To: Benjamin Felzer Cc: Dave Schimel , Shrikant Jagtap , franci , wigley@ncar.ucar.edu, Tim Kittel , Nan Rosenbloom , Mike Hulme , Mike MacCracken Subject: Re: CO2 concentrations Dear all, I've just read the emails of May 14 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 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 3467 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 (5.758W/m**2) and convert this to an *equivalent* CO2 concentration change. 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, HOWEVER, 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 of my IPCC 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. (5) The 708ppmv C(2100) 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, we used the Bern model. The mid-2100 value with this model, for IS92a, was 711.11ppmv. 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, since 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 * **********************************************************