From: steinly@topaz.ucsc.edu (Steinn Sigurdsson)
Newsgroups: sci.environment
Subject: Re: Gallup Poll on Global Warming
Date: 12 Sep 92 14:43:23
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In-reply-to: userDHAL@mts.ucs.UAlberta.CA's message of Sat, 12 Sep 1992 04:56:51 GMT

In article <dhalliwe.716273811@muskwa.ucs.ualberta.ca> userDHAL@mts.ucs.UAlberta.CA (David Halliwell) writes:


   steinly@topaz.ucsc.edu (Steinn Sigurdsson) writes:



   >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".

I absolutely agree that with CO2 doubling _and_ _NO_ other changes
there will be warming; that follows directly from the energy balance
and frequency dependent opacity in radiative transfer. 
Unfortunately this does not answer the question actually asked
by the public: namely how much CO2 increase will various emission
levels lead to, what climate change will result from this, _and_ how
bad will it be.

   >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.

But determining emission levels permitted depends critically on
how much given emissions will increase the atmospheric CO2 (and the
rather subtler question, would decreasing the rate of emissions
increase the atmospheric content or are absorption mechanisms ramping
up and would they bring the levels back down is emissions stabilised
at a lower level - clearly they would at low enough levels...)

   >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.

Hmm, I hadn't thought of vegetation: the obvious ones are snow/ice
and cloud cover; and as you know there some controversy over whether
tropical warming would lead to increase cloud cover and/or greater
precipitation at the poles.

      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.

The graph was CO2 (estimated) vs time with major galcial epochs
marked in. Two of them were now and a couple of hundred million years
ago (don't have article with me) at times of CO2 minima, but one
about 400 million years ago was smack in the middle of a CO2 peak,
at approx ten times current: solar constant will not have changed 
significantly on that time scale. I don't think the authors discussed 
causes, two obvious ones are a unique continental geometry
favouring large ice caps, and a asteroid/volcanic catastrophe causing
a prolonged winter and the resultant albedo forcing an ice age until
some other effect flipped the climate to a warmer state?

      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.

Yup, the problem is that is not the question public policy asks...

|  Steinn Sigurdsson	|I'm not the one to tell            		|
|  Lick Observatory	|This world how to get along                   	|
| steinly@lick.ucsc.edu |I only know that peace will come       	|
| "standard disclaimer"	|When all our hate is gone         - B.B. 1991	|



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