cc: Tim Kittel , Nan Rosenbloom , Mike Hulme , Mike MacCracken , "Tom M.L. Wigley"
date: Wed, 19 May 1999 15:10:46 -0600 (MDT)
from: Tom Wigley
subject: Re: CO2 concentrations (fwd)
to: Dave Schimel , Shrikant Jagtap , Ben Felzer , franci
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
To: Benjamin Felzer
Cc: Dave Schimel , Shrikant Jagtap ,
franci , wigley@ncar.ucar.edu,
Tim Kittel , Nan Rosenbloom ,
Mike Hulme , Mike MacCracken
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: wigley@ucar.edu *
**********************************************************