Saturday, April 28, 2012


date: Fri, 06 Oct 2000 13:20:58 -0700
from: Michael Prather <>
subject: Re: 'balance' Issue for TS and SPM
to:, Joyce Penner <>, John Stone <John.StoneatXYZxyzGC.CA>, griggs <>

Dear David, John, Joyce, and Michael

My apologies, I have been unable to contribute to this very important debate
until I cleared my chapter.

The wording in the SPM draft we were discussing (15 Apr draft given below) is
far too strong a statement: it removes the fundamental issue that this finding
is basically still a balance of the evidence. Admittedly what is new since the
SAR is that more weight has accumulated on the "have-detected-human-influence"
side of the balance (as Michael O notes). Nevertheless, there are still some
large and open problems (e.g., indirect aerosol effects) that prevent this from
being a closed case.

Today a new SPM draft appeared (6 Oct, below) that chooses more measured words
(I only wish that 'balance' could somehow be worked in).

BUT the final bullet in the new section stands out in that it avoids the major
new uncertainties that have been identified - merely by doing a GHGas+Sulfate
vs. GHGas alone model does not address the uncertainties in "other" forcings,
such as other aerosols or the history of the increase in tropospheric ozone -
which cannot be explained well and is certainly not documented. I doubt that
these studies considered the range of uncertainty in tropospheric ozone growth
or in OC/BC aerosols and indirect effects. This last bullet cannot be supported
from what I found in Chapters 4 and 5.

I leave these issues for discussion in NY,


SPM (15 Apr 2000)

"From the body of evidence since IPCC (1996), we conclude that there has been a
discernible human influence on global climate."

new SPM (6 Oct 2000)

"There is now stronger evidence for a human influence on global climate than at
the time of the IPCC Working Group I, Second Assessment Report, and it is likely
that increasing concentrations of anthropogenic greenhouse gases have
contributed substantially to the observed global warming over the last 50 years.
. . .
Uncertainties in other forcings do not prevent identification of the effect of
anthropogenic greenhouse gases over the last 50 years. The sulphate forcing,
while uncertain, is negative over this period and changes in natural forcing
during most of this period are also estimated to be negative."

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