PDQ,
You had some interesting questions about my comments to Little Hawk, where I tried to show the potential impact of atmospheric CO2. So I said:
"...its quite easy to calculate how much energy all that CO2 could absorb - its about 6E+20J (followed by 20 zeros) joules for every absorption cycle."

and your comments are:
'The trouble I see with that is: you are telling us how much energy the CO2 can absorb/hold/release in the atmosphere, not the amount of heat it does absorb/hold/release. Not the same thing.'

'At 361 ppm, CO2 is involved with a tiny portion of the total radiated heat, the rest passes through the atmosphere and makes contact with nothing.'

I agree with your first comment. I was trying to show that 361 ppmv or 380 ppm(w/w) was indeed enough to trap a significant amount of heat from first principles. Those simple numbers illustrate that potentially that small amount of CO2 could easily absorb enough energy to shift the climate. As you say, what actually happens in real life is highly complex, and that is what the practicing scientists are trying to do. One approach is the oft-criticized global climate models, which are literally and metaphorically 'over my head' (doesn't mean they're right or wrong, just that I don't understand them).

The other approach, which bears on your second comment, is more based on radiation physics and involves looking at the total radiation budgets. From classical physics (Stefan- Boltzmann) you can calculate the infra-red emission given off by the earth bearing in mind its average temperature. And you can use satellites to measure the radiation actually going out into space from the earth's atmosphere. And you can also measure the wavelength of any infra-red radiation coming back down to earth from the atmosphere. What is found is:

1. The earth radiates about 390 Joules per second (or Watts if you prefer) for each square meter of its surface. The energy is emitted in a smooth bell curve with almost nothing at 5 micron radiation and not much beyond 50 microns. The peak is between 15 and 20 microns.

2. That peak at 15-20 microns is quite close to the CO2 absorption band at 14-15 microns and in principle you'd expect to see strong absorption there. And looking at the radiation going into space from the atmosphere, you do indeed see a strong band. Instead of a nice smooth bell curve with a peak at 15-20 microns there is a curve that looks like someone has put an axe into the top (and around the sides as well). The dent in the curve is pretty well at the strongest point of the CO2 absorption.

3. There is significant (>30W/m2) energy being radiated back to the earth's surface at 14 microns.

There is a summary paper by Kiehl and Trenberth (of the US meterological society) where they look at these heat flows. They would probably disagree with your second statement, as they comment 'that very little radiation is actually transmitted directly to space as though the atmosphere were transparent'.

My view is that there is is little doubt that CO2 is a greenhouse gas and that it contributes to our current climate.

For sceptics of man-made climate change, there are two areas of genuine scientific uncertainty as far as I can tell. One is the complexity of the earth's climate with all of the inputs from solar variation, Milankovitch cycles, aerosols, vegetation impacts on albedo, which makes the predictive climate models necessarily complex. Some sceptical scientists are meteorologists rather than climatologists, and the gist of their criticism is usually around the fact that the earth's system is too complex to be quantitatively modeled.

The other point is made by physical scientists - the atmospheric absorption by CO2 is already so strong that adding more will not make that much more difference, because most of the 14 micron radiation is already absorbed. And again, calculating the answer to that point is very complex - is the 14 micron band saturated at all parts of the atmosphere, or just in the lower troposphere? Unfortunately, one of the new satellites meant to look at that issue blew up on launch last year.

So what will happen if we add a large 'slug' of greenhouse gases to the atmosphere rather quickly in geological terms? The paleoclimate stuff is interesting but not conclusive because there haven't been many occasions where the CO2 levels have changed as fast as they are right now. Well, we're now doing the full scale experiment and it remains to be seen what kind of results we get back...

Sorry if that's all too long! These are not questions where its easy to give a quick answer.