Space/Science — HabitableZone

Space/Science » in reply to re: A runaway greenhouse effect is quite unlikely.

Figure of speech, somewhat analogous to the term "reaction cross section" occuring at nuclear scales".
Baffler — (archived)

Greenhouse gas molecular dimensions and vibrational modes are in resonance at certain orientations with the frequency (waveLENGTH) of an IR photon. Dipolar molecules are "tuned" to interact with certain frequencies of EM radiation. Remember, these molecules are oscillating and colliding and rotating and distorting, changing shape and cross-section. I.e., at any one time, a certain proportion of the molecules are the right "size" to interact with an incoming photon.

The photons arrive from multiple directions, bump up electrons to different energy levels, then are reemitted later randomly as electrons return to their ground states. These interactions scatter the IR light and the quantiized emission/absorption forces conservation of energy and momentum to heat up the gas. (If quantum effects and electron transitions forces emission at a certain frequency, excess energy must go into molecular motion to satisy conservation laws).

Regardless of the mechanism, the greenhouse molecules at high altitude are primarily receiving their IR from the hot earth below, and then emitting it randomly in all directions. Those photons escaping at the top of the atmosphere are no longer available to participate in the process. The net effect is that heat energy is lost to the earth and dissipated into space. At lower levels of the atmosphere, the gas is dense, in violent thermal agitation, and the IR flux is essentially omnidirectional, which is why it is still warm enough at night to support life.

Not that any of this nit-picking changes the basic argument we're discussing here, and which you don't even disagree with: that CO2 is opaque to certain regions of the far infrared band.

I understand spectroscopy, although I confess I am more familiar with very hot rarified plasmas than I am with gaseous molecular solutions. However, if there are any real physicists out there reading this, I welcome them to correct my description of the phenomenon.

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