From: scott@abyss.ATMOS.ColoState.Edu (Scott Denning)
Subject: Re: averting doom
Date: Wed, 06 Jan 1993 23:06:42 GMT
Distribution: na
Organization: Colorado State University, Fort Collins, CO  80523

In article <1993Jan6.195617.673@newshost.lanl.gov> u108502@beta.lanl.gov  
(Andrew Poutiatine) writes:
> >>>>from a U.P. story
> >>>>
> >>>>	     WASHINGTON (UPI) -- Life on Earth as we know it will
> >>>>     come to an end in 1,500 million years and the planet will
> >>>>     look more like its dusty, volcanic sister Venus in 2,500
> >>>>     million years, scientists said Wednesday.
> >>>
> >
> >Any star will change slowly during its main sequence lifetime, 
> >and in the case of the Sun and sunlike stars this means becoming 
> >slightly hotter and more luminous.  The prediction quoted above 
> >seems to be saying that this part of the Sun's main sequence evolution
> >will be sufficient to raise Earth's temperature to the point Earth 
> >becomes unable to support life.

This thread has become unbearable.

All main sequence stars are believed to evolve slowly, becoming more
luminous over billions of years.  The sun is thought to be about 30% 
brighter (hotter) now than it was 4.5 Ga ago when the Earth was new.
The truly remarkable aspect of this phenomenon is that there is evidence 
of liquid water on the planet for almost the entire geologic record (say,
3.5 Ga).  Earth's climate is extremely sensitive to solar luminosity,
due to the ice-albedo feedback. For example, using simple energy-balance  
calculations, Budyko (1969) estimated that a decrease of insolation of  
just 1.6% would cause the polar ice caps to grow to the equator.  What's
more, because of the very high albedo of such a "white Earth" makes
this climate extremely stable to perturbations ... it locks in with 
little hope of ever melting.  If the Earth were to suddenly lose more 
than a quarter of its energy input, such a fate would be unavoidable.

Thankfully for us, the atmosphere 4 billion years ago was mostly CO2
and water vapor (just as volcanic emissions are today) (Walker, 1977). 
The resulting enhanced greenhouse effect led to much warmer temperatures  
than the present-day atmosphere could sustain with a dim sun. As the sun  
slowly became brighter over geologic time, the evolving biosphere drew
down the CO2 more and more, sequestering the carbon in the extensive
carbonate rocks of the Earth's crust (Worsley and Nance, 1989). Thus
the greenhouse effect of the atmosphere adjusted downward as the solar
luminosity slowly increased, maintaining the surface temperature 
between the freezing and boiling points of water for 4 billion years.

We don't need technological solutions to this "problem." The system has
been adjusting to changing external forcing for a long time and isn't
likely to quit now.
____________________


Budyko, M.I., 1969:  The effect of solar radiation variations on the  
climate of the Earth.  Tellus, 22, 612-619.

Walker, J.C.G., 1977:  _Evolution of the Atmosphere_. Macmillan Publishing  
Co., NY.

Worsley, T.R., and Nance, R.D.  1989.  Carbon redox and climate control  
through Earth history:  A speculative reconstruction.  Paleogeography,  
Paleoclimatology, and Paleoecology (Global and Planetary Change Section),  
75, 259-282.

______________________

A. Scott Denning    (303)491-8346
<scott@abyss.atmos.colostate.edu>
Dept. of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
Newsgroups: sci.environment,ca.environment,ba.transportation