Thursday, May 24, 2012

4672.txt

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 *
**********************************************************
Attachment Converted: "c:\eudora\attach\Is95a.dat"

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