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Re: atmospheric blanket / greenhouse effect



On 07/27/2003 10:44 PM, Larry Woolf wrote:

See also the Bad Meteorology site by Craig's buddy, Alistair Fraser:
http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadGreenhouseFAQ.html

On the Bad Science site as a whole, there is some good stuff.
The Bad Greenhouse section is, however, somewhat disappointing.

It correctly deprecates the "blanket" and "greenhouse" metaphors.
Alas, however, it goes on to suggest a substitute explanation
that is somewhere between a non-explanation and a wrong explanation.

a) If you want to say simply
-- The earth is observed to be warm.
that's a scientific fact. It isn't a scientific *explanation*,
nor does it pretend to be. That's fine as far as it goes. If
you want a fact without explanation, the whole discussion
reduces to a one-liner.

b) If you try to "explain" by saying
-- The earth is warm "because" it is observed to be warm.
that's bogus; that's circular.

c) If you try to "explain" by saying
-- The earth's surface is warm "because" the atmosphere is
observed to be warm.
that's not much better. That's just kicking the question
upstairs, so to speak. That's not a scientific explanation.

The Bad Greenhouse page says the surface is warm because of
radiation received from the atmosphere. On the associated
FAQ page it says the atmosphere radiates "because it has a
temperature". It is contemptuous of questions about where
the atmosphere got its energy. This cannot be considered a
good explanation.

If the atmosphere gets its energy from the sun or from
the earth's surface, the argument is _at best_ circular.
If I steal $100.00 from you and then give you a $100.00
gift, it doesn't make me a philanthropist.

But wait, it gets worse.

The Bad Greenhouse page argues that the atmosphere
radiates a lot because it is warm and is opaque
at thermal (IR) wavelengths. But if you think
about the physics for a moment, you realize that the
atmosphere radiates upward as well as downward. For
any given parcel of air, for every downward photon
that helps the energy budget, there is (on average)
an upward photon that hurts the energy budget.

So for an atmosphere in thermal equilibrium, this
sounds like a break-even situation that doesn't
explain anything.

In fact, an isothermal atmosphere is worse than break-
even, because in the absence of an opaque atmosphere,
you might have been able to make a "selective surface"
argument, i.e. to claim that the surface acts as a
black-body in the visible, but is a less-efficient
radiator in the IR. The atmosphere, being opaque,
is necessarily efficient in the IR.

So in this sense the page has it exactly backwards:
it tries to explain things by saying the atmosphere
radiates a lot. But in fact, any correct explanation
must show that the atmosphere's upward radiation is
_less_ than what you would expect at the observed
surface temperature.

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

As suggested in the note that started this thread,
the only way I can see to make sense of this is to
invoke the fact that the atmosphere is not isothermal.

An amusing way of condensing the argument is the
following:

A) Consider the _top_ of the atmosphere, i.e. the
layer above which the atmosphere is so non-dense
that it becomes optically thin in the IR.

B) Argue that the temperature of that layer is
what you would expect based on a balance between
incoming solar energy and outgoing sigma T^4
radiation. Add small corrections if you think it
is a somewhat selective radiator, i.e. a non-black
body.

C) Work your way down to the surface following
the highly non-isothermal profile of the troposphere.
Conclude that the surface temperature is warmer
than the radiation-balance temperature calculated
in step (B). QED.

===

I'm not an expert and I'm not sure my argument is
correct and complete. But it is more complete and
more plausible than other things I've seen.