This argument should have been put to rest long ago, but I still see it here, so let's try a little though experiment:
Say there is a planet A, with an atmosphere composed a single isothermal layer that--similar to Earth--is transparent to incoming SW radiation and opaque to outgoing LW. The flux density incident at the top of the atmosphere is 1 Wm^-2. What is the flux density incident on the surface of planet A? What happens when we add a second isothermal layer to the atmosphere?
You can assume that the surface and all layers of the atmosphere are in radiative equilibrium.
2010-03-18T00:02:46Z
though experiment = thought experiment
2010-03-18T00:28:15Z
I need to proofread--I meant 100 Wm^-2 at the TOA.
MTRstudent2010-03-18T01:54:44Z
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Using Stefan's Law, assuming emissivity=1, albedo=0, initial temperature is 65K. With 1 layer it is 77K, 2 layers it is 85K.
I believe ModTRAN etc fully include this effect and have done for years.
The heat flow at the surface is related to the number of layers, n, via n+1.
The temperature is related to the heat flow by F = constant * T^4
Therefore T_2 = (n+1)^(1/4)T_1.
This allows calculation for an arbitrary number of isothermal layers.
The "CO2 is saturated" argument is simply bad physics, and was explicitly rebutted in the Scientific American article on global warming. However, denialist arguments don't vanish merely because they are shown to be wrong.
John Houghton in "Global Warming" discusses what he calls "grey body radiation". We can consider absorption and readmission, as you indicate, from successive layers of the atmosphere. Final emission into space will take place, on average, from a layer at a characteristic depth below the top of the atmosphere, and that depth is inversely proportional to concentration of the gas. The higher, the colder. So more of the gas means less energy reemitted at the frequencies where it absorbs, creating a disequilibrium that is ultimately removed only by raising the surface temperature.
So there you have it, in language that I hope even Conservative Agenda can understand.
edit: Expel says BOTH that marginal increases in CO2 don't count for much, AND that measures to reduce that increase will be more effective than generally realised. But he's shifted from saying there's no AGW effect sto saying the effects are easily dealt with, which I suppose is progress of a kind.
This did always seem like a particularly dopey argument because it has the implicit assumption that absorbers are not emitters, which is clearly wrong. As for how much effect that additional CO2 has, it needs to be mentioned that since the vapor pressure of water rises much faster than linearly with temperature, that can offset any proportionate decrease in the effect of CO2 alone, since the temperature of water will rise right along with the temperature of everything else and water vapor is, of course, the most abundant greenhouse gas.
These nonlinear effects are one of the reasons that models are so important for evaluating the consequences of changing the atmospheric composition.
I would say that the saturated argument is probably wrong, but certainly the argument that each additional molecule of CO2 added to the atmosphere is going to be less effective at capturing energy as the one before because of competition for available energy is certainly valid. But even the saturated argument is more valid than your second isothermal layer argument as clearly the amount of CO2 placed into the atmosphere does not remotely have the ability to act as a whole other layer.
That being said, it seems clear that any additional CO2 would increase temps in a logarithmic manner, not a linear, nor exponential. Leading to the question of how much change should we really see. Now if the increases in temps caused by CO2 follow a logarithmic pattern, but CO2 is increasing in an exponential way, then we could see anything from a logarithmic increase to an exponential increase, but I would tend to believe a linear increase is the most likely. Hence that idea of the past 15 years only showing a 0.12 degree increase in temps per decade seems to go along with a fluctuating linear increase as opposed to an exponential increase. Projecting this out places a 1.2 to 2.0 degree increase in the next 100 years and also means that any methods to curb CO2 emissions would be more effective than we have been lead to believe. This also makes no assumption about the strength of the feedbacks except that they are constant and that the strength of the feedbacks will not change as temps increase. Thus reducing our CO2 output would be advisable, but should be accomplished in a way that does not cause unneeded human suffering.
Paul B, I think you have been misinterpretting what I have been saying. I am upset with the scientists because they frequently declare certainty and facts, when my field of science do not, even though my science field has the ability and indeed is much more controlled. I also do not like the fact that the scientists have not felt the need to correct the exaggerrations made by warmers, but do feel the need for skeptics.
But overall, I agree that man is causing warming (not nearly as much as you all believe, I would guess 1.5 - 2.0 degrees in the next 100 years if nothing is done) and I have made quite a few suggestions of how we may go about reducing our CO2 emissions without resorting to a Cap and Trade. In general, it makes me nervous giving the gov't another method to tax. I do not understand why this does not make warmers nervous.
Pegminer, The assumption of relative humidity staying constant is a fine assumption initially, but since it has shown false, it would be silly to continue to think that H2O will rise faster than temp rises. Thus your argument for a greater than logarithmic effect due to the feedback of H2O has not shown to be the case. Therein lies the fallacy of your thought. The models do not help in determining the effect of additional CO2, they are simply calculated estimates determined from guesses as to strengths of the potential feedbacks until they can be confirmed by observational data. As of now, the models have not shown enough accuracy or precision to be confirmed by observational data, thus the degree of uncertainty is higher than what the AGWers traditionally state.