NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/sci/environm/11293

Text File | 1992-09-11 | 5.4 KB | 106 lines

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From: [e-mail] (David Halliwell)
Subject: Re: Gallup Poll on Global Warming
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Date: Sat, 12 Sep 1992 04:56:51 GMT
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[e-mail] (Steinn Sigurdsson) writes:


>I don't quite know what Josh Rivero (sp?) was getting at,

    Now that I've had the chance to look back and check earlier posts,
it's Josh R_O_vero, and I apologize for the earlier mispelling.

>but the two complaints about climate modelling that I hear

    Ignore climate modelling. The 4W/sq.m is based on fairly straight-
forward measurements of the radiative absorption coefficient for CO2,
combined with a postulated doubling of atmospheric CO2. The question
being dealt with to begin with is "if we double CO2, how will this
alter radiative losses to space, with NO change in other components
of the atmosphere".

>most frequently are not over the radiative transfer but
>a) what the actual CO2 load will be given known emissions
> - the GCMs take an assumed change in CO2, but the CO2 balance
>is clearly missing a term, there's an unknown sink, and presumably
>at least one missing feedback loop - I noted that IPCC92 started
>allowing for biofeedback, which I seem to remember was assumed
>negligible in IPCC90? Also there's ocean sinks and anthropogenic
>sinks ;-)

   The global carbon cycle is indeed an important aspect of how the
earth/atmosphere system response to an atmospheric input of CO2 from
combustion of fossil fuels, but has absolutely no relevance to the
question "What would the climatic response be to doubling atmospheric
CO2", which is the question the climate models attempt to answer.
The first step in answering this climate question is to determine that
doubling CO2 would decrease radiative losses to space by 4W/sq.m,
all other factors being constant.


>b) Are albedo changes adequately modelled?
>A definitely niggle is illustarted by a major glacial
>vs CO2 concentration chart that was in Nature last month,
>showing a major glacial epoch occurred while CO2 concentrations
>were O(10) time current ones. Clearly some combination of
>albedo and continental shifts managed to cool the Earth despite
>a much greater greenhouse effect.

    Return question. Given a decrease of 4W/sq.m in radiative losses
to space, what global climatic changes will occur that could lead
to albedo changes? Warming? Cooling? Albedo changes are a response to
other changes (i.e. a feedback). Do you have a physical reason for
expecting albedo to change, and how will this modify the equilibrium
result of the changing climate? The major change affecting albedo would
be the distribution of snow and ice, which is usually included in
climate models to some degree. An important factor here is whether the
model dynamically determined sea ice on the basis of the current model
climate, or whether it simply accepts a dictated sea ice cover. Ocean
dynamics are generally poorly handled in GCMs, and are a major potential
source of error. Vegetation cover can also affect albedo, and I think
that most GCMs do *not* allow for vegetation changes.

   Not having read the recent Nature article, I cannot comment directly
on the results. Did the paper suggest that the order of magnitude increase
in CO2 *caused* the cooling? Or was the increase in CO2 only one of several
climatic forcing factors which could have led to change? I could easily
imagine an ice-covered earth with 50x CO2, but half the solar output we
have now. We can only apply the logic "increased CO2 is related to
colder temperatures" if our sample has controlled (eliminated) other
factors which affect temperature.

   It is of fundamental importance that we identify factors which are
independent of earth's climate, but have a significant impact on climate,
and factors which are the earth's response to these outside factors. An
example of an ouside/independent factor would be solar output or
earth-sun orbital variations. Albedo, clouds, radiative emissions to
space are the earth's response to an input of solar energy. Atmospheric
CO2 levels are slightly dependent on earth's climate, but in general
we can assume that atmospheric composition is not greatly affected by
the current climatic state. (The argument that it is, e.g. Gaia, is
another discussion.)

    The climate models attempt to answer the question "How would the
climate change if we took today's climate and doubled the atmospheric
CO2 levels?" This approach takes atmospheric composition (of gases)
as a given quantity, and tries to determine how one change in concentration
influences climate.

    This old .sig of mine seems particularly appropriate today.


Dave Halliwell                   |   "So, once you know what the
Department of Geography          |   question is, you'll know what
University of Alberta            |   the answer means."
Edmonton, Alberta                |      -   Douglas Adams