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Re: [Phys-L] some climate concepts, numbers, and references

1) Concept: The incremental amount of energy per unit time
is only /logarithmic/ in the amount of CO2 in the atmosphere.
That's because the atmosphere is more-or-less optically thick
at the relevant wavelengths already. So, adding more CO2 cuts
down on what's coming in, not just on what's going out.

I've never heard anyone promote the absorption of incoming EM waves as an important factor for CO2's impact on global temperature.

The two main factors are most commonly explained as:
1) The CO2 bands are optically thick near the centers of the band, but less so at the tails. Adding more CO2 reduced the outgoing IR at the edges of the band
2) Since CO2 is optically thick, most of the radiation comes from near the top of the troposphere. Adding a bit more CO2 means the outgoing IR comes from even higher in the troposphere where it is even colder. Less IR from the top means something else (ie the surface) has to emit more IR through the "atmospheric window" to restore balance.

There is a superb bit of software called MODTRAN that calculates IR in the atmosphere. In particular, there is an easy-to-use interface here related to thermal IR: http://forecast.uchicago.edu/modtran.html I highly recommend that anyone interested in the physics of global warming play around at this site.

You can look up or look down from various altitudes; you can add clouds; you can adjust key greenhouse gas level, you can change the surface temperature. The output includes the thermal IR spectrum (either plotted with wave number or wavelength) and the total EM heat flux. So for example, if you look down from 100 km, you get the heat (W/m^2) leaving the planet under the given conditions.

(One note: the output is calculated from a fixed initial temperature profile. So if you add more CO2, there is no attempt to calculate what new temperature the atmosphere might reach in response. It is not a climate model; it is an IR calculator.)