Why is the standard classroom experiment demonstrating the GHG effect of CO2 wrong ?

Two identical, open top, plastic containers are lit by identical heat lamps. Probes shielded from direct radiation are placed close to the absorbing surfaces at the bottom of the conainers and monitor the temperature. When steady state is reached (27.5 C in the experiment reported) CO2 is added into one of the... show more Two identical, open top, plastic containers are lit by identical heat lamps. Probes shielded from direct radiation are placed close to the absorbing surfaces at the bottom of the conainers and monitor the temperature. When steady state is reached (27.5 C in the experiment reported) CO2 is added into one of the containers. Temperature immediately starts to rise and reaches 34.5 C in about 700 seconds and then progressively drops. This has been used as classroom demonstration of the GHG effect of CO2, however, it has no connection with it. Why ?
Update: The experiment which is criticized is described here: S.B. Lueddecke et al, "Greenhouse effect in the classroom: A project and laboratory-based curriculum" J. Geosci. Educ. 49, 274-279 (2001) They don't seem to find much difference between closed and open containers. Ben is clearly right to... show more The experiment which is criticized is described here: S.B. Lueddecke et al, "Greenhouse effect in the
classroom: A project and laboratory-based curriculum"
J. Geosci. Educ. 49, 274-279 (2001)

They don't seem to find much difference between closed and open containers. Ben is clearly right to stress the effect of convection/absence thereof. And this must the main effect (as it is for real greenhouses) in the experiment described as shown by another experiment using a heavy gas (Ar) for control without IR absorption bands.

However:
a/ simple calculation show that the increase in temp due to CO2 IR absorption can't account for more than a 5% increase w.r.t. the normal temperature difference (with air) between the container and the environment.
b/ even with sealed containers, convective effects/absence thereof at the interface between the CO2 layer and air can't be ruled out. The important point appears to be the existence of an interface here and not the thickness of the air layer.
Update 2: @antarcticice: this is not a post about denial or faith or betrayal or religion but about physics experiments and their interpretation.
Update 3: @antarticice my (over?)reacting was not meant at your posting links to other experiments but to your comment about what deniers recognize or not. I never intended to say that CO2 is not a GHG. It is, and that's happy for us otherwise we would freeze. But I hate false proofs pretending to rely on 'solid... show more @antarticice my (over?)reacting was not meant at your posting links to other experiments but to your comment about what deniers recognize or not. I never intended to say that CO2 is not a GHG. It is, and that's happy for us otherwise we would freeze. But I hate false proofs pretending to rely on 'solid science' as in this experiment and in the article to which you can find a link in FGR's comment.

@FGR isn't there some contradiction between your points 2 and 5 ? Either CO2 does not settle at the bottom, or there is some stratification but not both.
Actually, this is really what happens. CO2 settles at the bottom and prevents convective cooling. The important point is the existence of a CO2-air interface where convection stops so that the differential warming has nothing to do with radiative GH effect but with the suppression of convection as pointed out by Ben.
This can be shown by replacing CO2 by Argon which has about the same molar mass but has no infrared absorption bands.
Update 4: In a (cleaner) remake of the experiment with argon, it is found that Ar and CO2 yield the same temperature curves and clearly there can't be any GH effect with Ar. Monoatomic gases do not have the vibrational ddf which are excited by IR radiation in CO2. A simple model shows that large temperature rise found... show more In a (cleaner) remake of the experiment with argon, it is found that Ar and CO2 yield the same temperature curves and clearly there can't be any GH effect with Ar.
Monoatomic gases do not have the vibrational ddf which are excited by IR radiation in CO2. A simple model shows that large temperature rise found in the CO2 container cannot be explained by the IR CO2 absorption bands but can very well be explained by the suppression of convection.
Interestingly, if you keep monitoring the temperature long enough (without introducing more heavy gas) you find that after rising and reaching an almost steady state value, it suddenly drops with a discontinuous slope, corresponding to the moment when the level of the heavy gas falls below the temperature sensor, showing that it is really the existence of an interface which is at the root of the absence of convection. There is a real greenhouse effect here, but it has nothing to do with the radiative GH effect supposed to cause global warming.
Update 5: This is precisely what I deeply dislike in this article. The authors rightfully write that the difference of temperature is due to CO2 but without asking any question, they attribute it to the absorption features of this gas, and therefore to what is the cause of global warming..! Afterwards they boast of basing... show more This is precisely what I deeply dislike in this article. The authors rightfully write that the difference of temperature is due to CO2 but without asking any question, they attribute it to the absorption features of this gas, and therefore to what is the cause of global warming..! Afterwards they boast of basing their claims on solid science, contrasting it with the 'half truths' and 'approximations' found elsewhere. It is a real shame and it is the duty of real scientists to say so loud and clear. There are, actually, many points in their experimental protocol which show it is unreliable, to start with them not lighting the lamps and letting a steady state be reached before introducing carbon dioxide. This could allow to check that both containers reach the same T when they are in same conditions. Also to avoid the problem posed by the endothermic CO2 producing reaction, which is used to justify not heating before, why don't they use small pipes to introduce the gas from outside ?
Update 6: @antarticice sorry but I will not by any experiment 'documented' by a YouTube clip with which you can prove almost anything and its exact opposite. That's the paradise of candidate Uri Geller imitators. If you have some reference to some detailed written protocol, I would be interested.
Update 7: Sorry, I should have given a reference to the remake experiment and the model calculations. This is described in Am.J.Phys. 78,5, 536-540 (May 2010)
Update 8: @FGR I agree with most of what you say but you first answer gave to somewhat incompatible possibilities without deciding - but maybe you were forming your ideas while writing, as I often do here. Also, I don't see why you said or implied that CO2 would not stay at the bottom because it is warmer. It would... show more @FGR I agree with most of what you say but you first answer gave to somewhat incompatible possibilities without deciding - but maybe you were forming your ideas while writing, as I often do here.
Also, I don't see why you said or implied that CO2 would not stay at the bottom because it is warmer. It would have to be a lot warmer to compensate for the density difference (44/29)=(273.15+t)/(273.14+27)==>t=182 °C !
What is difficult is really to understand this convection mechanism and why it is inhibited by the interface and how in the end it overtakes. We should try to estimate the diffusion rate. I'll do that if I have time.

More basically my question was: given that this experiment does not appear credible, do you know a table-top experiment demonstrating the GHG properties of CO2 (apart from measuring its absorption lines directly ?)
Update 9: I tried to extend the question duration but for some reason I couldn't. So I shall re post it in a different form. For BA, I give it to Ben in spite of the numerous interesting points made by FGR, because he was clearly the first to spell the fundamental reason for the described experiment to be unrelated to... show more I tried to extend the question duration but for some reason I couldn't. So I shall re post it in a different form.
For BA, I give it to Ben in spite of the numerous interesting points made by FGR, because he was clearly the first to spell the fundamental reason for the described experiment to be unrelated to radiative GHG effect, namely that the effect here is entirely due to (lack of) convective heat transport.
9 answers 9