Re: atmospheric blanket / greenhouse effect

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

> 4) Item (2) cannot be the whole story.  An opaque
> atmosphere in thermal equilibrium with the
> surface would radiate at least as much as the
> surface would.  The radiation would be "seen"
> to come from the top of the atmosphere, but
> there's no way that could reduce the radiative
> heat loss.
>
> So I surmise that an essential part of the story
> is that the atmosphere is not isothermal.  The
> troposphere temperature profile is closer to
> being isentropic than isothermal.  Going up from
> there, the stratosphere is isothermal, but muuuch
> colder than the surface.

It seems to me that the "not isothermal" argument follows from the same
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 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 and up
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.

> 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?

> 5) The distribution of CO2 versus height must
> be important also.
>
> 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.

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301