cc: schimel@cgd.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 15:36:19 -0600

from: Tom Wigley <wigleyatXYZxyzr.edu>

subject: Re: CO2 concentrations

to: felzeratXYZxyzr.edu

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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