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Re: Human vs. natural influences on the environment



charliew@hal-pc.org (charliew) wrote:
>OK, Mr. Evens.  How much effect does methane have relative to 
>water vapor and CO2?  Since CO2 supposedly has a 
>concentration of 300-400 ppm, and water vapor has a 
>concentration of approximately 10,000 ppm, why is methane at 
>1.7 ppm such a threat?

I am neither Evans nor a radiation physicist, but I will try to give
a hand-waving explanation.

Consider the atmosphere's transparency to heat radiation as a large
window of many small sections.  Each section may be empty or filled
with "glass" of varying insulating quality.  The rate at which heat
escapes depends on the insulating value of the whole and the temperature
of the Earth; the higher the net insulating value, the hotter Earth
gets.

The insulating value depends mostly on the least-blocked sections.
If you have a double-pane section next to an open one, the insulating
value of the whole will be increased far more by putting a piece of
plastic over the open hole than by upgrading the double-pane to
triple-pane Heat Mirror krypton-filled.

Substances do not exert heat-blocking effects uniformly.  They are
all selective to some degree, and block certain wavelengths more
than others.  If the atmosphere is already mostly opaque at the
wavelengths absorbed by water, adding more water is like upgrading
the double-pane.  On the other hand, if the atmosphere is largely
transparent at the wavelengths absorbed by methane, adding more
methane to the atmosphere is like putting plastic over the open
holes.  The effect will be proportionately far greater.  The same
applies to other greenhouse gases such as nitrous oxide and CFC's.

I do not have IR absorption charts for the various greenhouse gases
so I cannot say what the relative influence of any particular change
in composition might be.  I leave that to the radiation physicists.
But I hope I have given a reasonably accurate layman's summary of
the issue.




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