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Re: atmospheric blanket / greenhouse effect



I'm having trouble with statement 4. Radiation is like T^{4}. Thermal
equilibrium is not the same as isothermal. Pls explaint.

On Sun, 27 Jul 2003, John S. Denker wrote:

Hi --

Some recent discussions have reminded me of
something that has bugged me for a long time.
Consider the two statements:

a) Because of the atmosphere, the earth's
surface is substantially warmer than it would
be otherwise.

b) Because of the blanket of atmosphere, the
greenhouse effect makes the earth's surface
substantially warmer than it would be otherwise.

Analysis:

1) The metaphors in statement (b) are bogus.
Both blankets and greenhouses work primarily by
suppressing convection.

Surely the atmosphere doesn't work by reducing
convection. Having no atmosphere would be the
ultimate no-convection situation.

2) Instead, the atmosphere acts by playing games
with the radiation budget. The explanation
begins by saying that the air is transparent
to visible light (so energy at the peak of the
solar spectrum can get to the surface) while
being opaque to thermal radiation.

3) It is commonly asserted that the glass in a
greenhouse is similarly radiation-selective, and
it may even be technically true, but the effect
has got to be insignificant compared to the
aforementioned suppression of convection.

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.

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.

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).

==============

I've found a number of attempts to analyze the
situation, many of them quite silly and none of
them entirely satisfactory.

Can anybody out there shed some light on this?

Also, since the blanket metaphor and the greenhouse
metaphor are misleading, can anybody suggest something
better?

As a political/pedagogical point, you'll never
get people to stop using a bad metaphor unless
you can offer something better. "You can't
beat something with nothing."


--
"Don't push the river, it flows by itself"
Frederick Perls