Reply To: Questions regarding DPF.

[jamesr]

Brian H wrote:

In part, this says that any given CO2 molecule may or may not re-radiate energy it has gained from either thermal contact or IR influence. It may just bump a nearby molecule and lose its excess energy to some other gas. If it does re-radiate, it will be in some random direction, possibly back to space, possibly sideways, possibly down. This does not permit any kind of averaging computation.

Bingo. The more CO2, the more absorption. The more absorption, the more downward re-radiation. The more downward re-radiation, the less gets through to warm the stratosphere. Therefore the stratosphere cools, the surface warms. Exactly what we are seeing.
False. Only a small fraction of the absorbed radiation ends up going down; there is no conservation of “radiation”

The fraction of absorbed radiation being re-radiated down would be almost exactly a half, since the CO2 molecule will deexcite in a random direction. The half that is re-radiated upwards still has a chance of being absorbed again higher up. Of course one initially scattered down could also be re-scattered upwards. This kind of linear chance of interaction and being scattered out of a particular direction is the similar to say gamma rays being absorbed/scattered as they pass through sheets of metal. If you double the thickness (equiv to doubling the thinkness of the atmosphere) you half the amount getting through. Similarly if you double the density of atoms/molecules, and so increase the probability of an interaction, you half the amount getting through. The result is an exponential fall off of radiation of those particular energies corresponding to the CO2 absorbtion bands with height, with the constant in the exponent being proportional to the number density of CO2 molecules.

The total effect is small compared to the infuence of water vapour, but CO2’s absorption bands are at different wavelengths – that happen to lie around the black-body emmision from the earth.

How specifically significant the anthropogenic CO2 contribution to greenhouse warming is, I would happily debate on, but the effect itself I think is sound.