From: userDHAL@mts.ucs.UAlberta.CA (David Halliwell)
Subject: Re: Gallup Poll on Global Warming
Organization: University Of Alberta, Edmonton Canada
Distribution: usa
Date: Sat, 12 Sep 1992 05:40:12 GMT

rovero@oc.nps.navy.mil (Josh Rovero) writes:

>The parameterizations of sub-grid scale phenomena (like cumulus cloudiness)
>have everything to do with the feedback mechanisms and net radiative balance.
>There are a range of feedbacks (depending on method) that can be positive
>or negative.

   Yes, they definitely affect feedbacks, but they have no influence on
how doubling CO2 affects net radiative balance ALL OTHER FACTORS BEING
CONSTANT. The 4 W/sq.m will be eventually compensated by other changes,
but before these changes can take effect, doubling CO2 causes this 4W/sq.m
change.

>Example.  Increase temperature 1 degree celsius due to any cause (natural
>or anthropogenic).  Look at range of model parameterizations of cloudiness,
>sea/air heat fluxes, etc.  Some may result in a net albedo change that
>would cause a reduction in incoming shortwave radiation GREATER than the
>reduction in outgoing flux due to CO2.  Exactly the same thing we are
>seeing now with Pinatubo dust...... coolest summers on record in mamy
>northern hemisphere sites.

    And if that albedo change leads to cooling, then exactly the same
feedback mechanism will also reduce the effect of that cooling mechanism
so that the cooling effect is reduced, and it will *never* overpower
the initial warming caused by the factor that led to the initial 1C
warming. No matter how strong the feedback, the end result will still
be a (perhaps extremely tiny) warming. You can't have your feedback
alter the initial change in temperature without having it also alter
the temperature caused by the feedback!

    Pinatubo is *not* a feedback mechanism. The reduction in radiation
reaching the surface is caused by volcanic aerosols, not a change in
global temperature. If *warming* reduces surface radiation, then cooling
will increase it, and (unless the system shows hysteresis) the cooling
then eliminates the reduction caused by warming. The final equilibrium
will be a temperature cooler than the temperature expected without
feedback, but warmer than the initial temperature before the 1C warming.
 

>There is a great deal of discussion about feedback mechanisms and whether
>they are positive or negative.  To quote from Albert Arking's article
>"The Radiative Effects of Clouds and their Impact on Climate", Bulletin
>of the AMS, V72 #6, June 1991:

>  On cloud feedback in climate simulations, there are wide discrepancies
>  amongst models.  Investigators find, in general, that the sign of 
>  cloud feedback is positive when one allows the spatial distribution of
>  clouds to vary in response to climate change.  When one additionally
>  takes into account the effect of changes in cloud properties, that 
>  result is less certain.  The conclusion is: 1) clouds MAY have a strong
>  influence in climate change, but 2) we are far from knowing the
>  maginitude, and even the sign, of this influence.

    I agree entirely. We do not know enough about cloud response to
know whether it we be a positive or negative feedback. I can't follow the
logic that leads to the conclusion that this lack of knowledge will
reduce the global temperature change that could result in the absence of
cloud feedbacks.

>Arking's statements seem reasonable based on my experience with numerical
>weather prediction models.  The "we're all gonna die" certainty of
>others is not reasonable at this point -- there is too much that we don't
>know.

    Agreed. There are very few climatologists that claim "We're all
gonna die". However, a great number of them do feel that our best
estimate of the effect of doubling CO2 is "global mean surface temperature
increases of 2-4C, with effects being greater in polar areas". I think
that some of your comments illustrate important uncertainties in this
estimate, but a good number of them could *increase* the change rather
than decrease it. As you said: there is too much we don't know.

Dave Halliwell
Department of Geography
University of Alberta
Edmonton, Alberta
Newsgroups: sci.environment