If you read the story, it was in response to a global warming critic that claimed there was no such thing as the greenhouse effect at all. I’ve since met and communicated with several such individuals.

With regard to positive feedback in the atmosphere, i meant things like increasing water vapor.

That said, in some ways it is a better experiment, especially as there is no water vapour present in the containers and the thermal mass of the system appears to be less, due to the absence of liquid in the vessels. I tried a similar set up (see above) using methane and nitrogen and got much larger temperature rises than with the coke-in-bottles (up to nearlly 60C), but I was unable to show that the methane vessel consistently reached a higher temperature. I am not at all a global warming sceptic and want to seek an explanation for the failure of my experiments, but my physics isn’t up to it and I don’t get much time to experiment (or sunlight: I may need to wait until next spring).

I don’t understand your second or third paragraphs. Of course, these experiments would only demonstrate that CO2 is a GHG. If it works at all, it would have to be with a large concentration of CO2. You can’t expect a positive result when comparing a few cm of CO2 at 550ppm with the CO2 in the atmosphere at the same concentration (where the IR is absorbed and re-emitted sequentially through the whole 10km path length). See http://www.aip.org/history/climate/simple.htm#L_0141

I’m not sure what “addition(al) positive feedback in temperature through other interactions with the atmosphere” means. Do you mean the absorption and re-radiation discussed in the link to the aip site (above), or do you mean things like increasing water vapour, methane and CO2 as a result of warming?

However, even if that experiment (the one I show a link to) was run to steady-state, i.e. run until the temperatures in both the CO2 and air containers stabilized, it would show that even with 100% C02 vs air, the delta T would not support the global warming predicted by the IPCC models. A most interesting experiment would be to run the experiment with the CO2 doubled from the current concentration in our atmosphere. I suspect the delta T would be hardly measurable.

So, the whole issue is not whether CO2 is a greenhouse gas, it of course is. The issue is whether CO2 causes addition positive feedback in temperature through other interactions with the atmosphere as the IPCC claims. That’s the harder claim to swallow.

What I’m trying to suggest is that the infrared energy abosrbed by 4g of methane is less than that absorbed by 100g of PET, let alone several 100g of dark water (in the coke case) and that the temperature difference is likely to be marginal. I can’t explain why your experiment works and the ones I tried didn’t.