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Re: atmospheric blanket / greenhouse effect



Robert Cohen wrote:


It seems to me that the "not isothermal" argument follows from the sa=
me
argument that produces the "warmer surface". The situation, as it
appears to me, is this:

1. Let's first assume that the albedo is the same with and without an
atmosphere (we know it is not but this, at least, focuses our attenti=
on
on the CO2/IR situation).

2. Let's further assume that none of the non-reflected radiation from
the sun is absorbed by the atmosphere (actually, it is probably about
10%).
......................SNIP...............................
8. Consequently, the earth is absorbing more radiation than it would
otherwise without an atmosphere (usually called the "greenhouse
effect").

9. Consequently, the earth must be warmer than the atmosphere.

10. Assuming the atmosphere at the surface has the same temperature a=
s
the earth, this means the atmosphere is not isothermal.

I find your argument above to be superior to the _Bad Science_ one we were
referred to. A satisfying explanation needs to show why the atmosphere is
warm, not just state that it is. A further refinement would recognize the
importance of convection in the lower atmosphere. The average temperature
profile in the lower atmosphere is exactly that followed by a parcel of
air warmed by contact with the Earth's surface and then rises
adiabatically through the troposphere. That has to be more than just
coincidence.


6) Clouds complicate things considerably. A
thin layer of clouds can actually make the
climate warmer, whereas thick clouds make it
cooler (all compared to the baseline no-cloud
situation).

Yes, clouds can influence the albedo, which I've conveniently ignored=
.

The affect of water in general is too often ignored in the greenhouse
discussion. The emphasis is always on CO2. Unfortunately, CO2 is only a
player in a few narrow bands of the IR radiated upward from the Earth.
Within those narrow bands it is basically opaque and between the bands it
is almost totally transparent. Doubling the CO2 content of the atmosphere
produces no significant change in the IR radiation budget. It is water
that has the most dramatic influence because it absorbs IR over a wide
band and is a significant absorber without being totally opaque. A
variation in the water content of the atmosphere produces a significant
change in the radiation budget. (Having lived in Providence, RI and Reno,
NV for long periods, I can attest to the amazing difference between day
and night temperatures in the two regions because of humidity and cloud
cover.) It's curious how, in recent years, introductory texts have
concentrated almost exclusively on the role of CO2.

Bob at PC