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Re: atmospheric blanket / greenhouse effect



On 07/28/2003 10:14 AM, Robert Cohen wrote:
>
> 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
> attention 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%).
>
> 3. The earth warms us and emits IR.
>
> 4. Let's assume that all of the IR is absorbed by the atmosphere
> (again, it is probably only 80% or so).
>
> 5. The atmosphere warms up and emits that radiation down to earth
> andup to space.
>
> 6. Let's assume the radiation emitted by the atmosphere up to space
> equals the amount of solar being used to warm the earth (radiative
> equilibrium).
>
> 7. Let's assume the earth absorbs all of the radiation emitted
> downward (so we don't continue iterating out indefinitely).
>
> 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
> as the earth, this means the atmosphere is not isothermal.

That's a nice plausible scenario, and there are
physical situations where such a scenario plays
out. For instance, in a high-performance Dewar, there
are layers of "superinsulation" i.e. many many layers
of film. Each layer comes into radiative equilibrium
with its two immediate neighbors. The result is much
less radiative transport than you would get with just
a vacuum. (Of course they make the film highly reflective
to further reduce the transport.)

Having said all that, evidence suggests that that
this process is not dominant in our atmosphere.
The non-isothermal nearly-isentropic profile of
the troposphere is set by convection, swamping
the effect of radiative transport. (The height
of the tropopause corresponds to the height of
the tallest thunderstorms.) Furthermore, the
stratosphere is pretty much isothermal,
indicating that in the absence of convection,
radiation and re-radiation aren't sufficient to
set up a large temperature gradient.

>> I don't know at what altitude the density drops to the point of
>> becoming optically thin in the IR. I imagine this is an important
>> part of the story.
>
> It doesn't that I needed to assume anything about this, did I?

Are you claiming the magnitude of your effect is
independent of the thickness of the atmosphere?