cc: felzer@ucar.edu, sjagtap@agen.ufl.edu, franci@giss.nasa.gov, kittel@ucar.edu, nanr@ucar.edu, m.hulme@uea.ac.uk, mmaccracatXYZxyzcrp.gov

date: Tue, 18 May 1999 17:11:02 -0600 (MDT)

from: Tom Wigley <wigleyatXYZxyzker.ucar.edu>

subject: Re: CO2 concentrations

to: Dave Schimel <schimelatXYZxyz.ucar.edu>

Dear All,

Re Dave Schimel's question: yes, 1% is about right FOR THE CLIMATE MODEL

FORCING. It is NOT right for vegetation modeling.

By the way, note that I did not specify a C(1990) value. The calculation

and results I gave are independent of what is used for C(1990) vis a vis

the climate model forcing. This is deliberate --- there is no need to

introduce additional numbers that are irrelevant to the present

discussion. I thought this was obvious, but perhaps not.

Finally, note that IS95a and IS92a are the same thing. We introduced 95a

to distinguish it (them) from 92a. The 95a scenarios employ constant CO,

NOx and VOC emissions. I hope this didn't confuse anyone.

Cheers,

Tom

On Tue, 18 May 1999, Dave Schimel wrote:

> 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: wigleyatXYZxyzr.edu *

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

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