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 *
> **********************************************************