cc: Tim Kittel <kittelatXYZxyzr.edu>, Nan Rosenbloom <nanr@ucar.edu>, Mike Hulme <m.hulmeatXYZxyz.ac.uk>, Mike MacCracken <mmaccracatXYZxyzcrp.gov>, "Tom M.L. Wigley" <wigleyatXYZxyzr.edu>

date: Wed, 19 May 1999 15:10:46 -0600 (MDT)

from: Tom Wigley <wigleyatXYZxyzker.ucar.edu>

subject: Re: CO2 concentrations (fwd)

to: Dave Schimel <schimelatXYZxyz.ucar.edu>, Shrikant Jagtap <sjagtapatXYZxyzn.ufl.edu>, Ben Felzer <felzeratXYZxyzr.edu>, franci <franciatXYZxyzs.nasa.gov>

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 <wigleyatXYZxyzker.ucar.edu>

To: Benjamin Felzer <felzeratXYZxyzr.edu>

Cc: Dave Schimel <schimelatXYZxyz.ucar.edu>, Shrikant Jagtap <sjagtapatXYZxyzn.ufl.edu>,

franci <franciatXYZxyzs.nasa.gov>, wigley@ncar.ucar.edu,

Tim Kittel <kittelatXYZxyzr.edu>, Nan Rosenbloom <nanr@ucar.edu>,

Mike Hulme <m.hulmeatXYZxyz.ac.uk>, Mike MacCracken <mmaccracatXYZxyzcrp.gov>

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: wigleyatXYZxyzr.edu *

**********************************************************

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