A common type of experiment to demonstrate the greenhouse effect is essentially to direct heat lamps at the bottom of two closed jars, one with regular air in it, and one with a higher concentration of $\rm CO_2$. The jar with more $\rm CO_2$ in it heats to a higher temperature, and this is attributed to its infrared-absorbing and back-radiating qualities, i.e. to be a demonstration of the atmospheric greenhouse effect.
- https://www.youtube.com/watch?v=xitg2tDZnXg (linked from this comment)
- https://edu.rsc.org/experiments/modelling-the-greenhouse-effect/1543.article but with uncovered beakers (linked from this comment)
- https://royalsocietypublishing.org/doi/10.1098/rsos.192075 with just one container, with a constant-input heat source, when CO2 is added the temperature increases
However I came across a critique of this class of experiments that the effect is actually due to the fact that CO2 has a worse thermal conductivity than air (from this source, at 300K air has 26.2 milliwatts per meter kelvin of conductivity while carbon dioxide has 16.8).
The authors claim to have performed the experiment with both $\rm CO_2$ and Argon and demonstrated that Argon provided a similar effect as the $\rm CO_2$ -- Argon having a similar conductivity to $\rm CO_2$ but, of course, being non IR-interacting.
These are their results for replicating the effect with CO2:
At minute ~60 they pump $\rm CO_2$ into the container, with a resultant heating of the air inside, while at minute ~260 they pump it back out.
This is their graph demonstrating a comparison of the effect between $\rm CO_2$ and Argon:
That is, they observe a similar effect with Argon.
Their (machine-translated) conclusion:
The noble gas argon is an IR-inactive gas that can neither absorb nor emit thermal radiation. If $\rm CO_2$ and argon show the same warming effect, one must look for the cause outside of thermal radiation. Heavy gases have a lower specific thermal conductivity λ than air (the table in Fig. 4). When these gases are introduced into the tube, they reduce the heat flow within the apparatus. The heavy gases act like an insulating layer. Thus it can be determined:
The Ditfurth experiment does not show the greenhouse effect, but is a phenomenon of heavy gases.
If they did perform the experiment correctly with Argon it seems it definitively demonstrates the point.
Note that the critique is published by EIKE which according to desmog.com is a "climate change denial organization". This doesn't automatically make the critique wrong, but, it must perhaps be taken with a grain of salt. And of course even if the critique is correct, it doesn't invalidate any of the other evidence for the greenhouse effect, it just means this type of experiment is not a valid way to demonstrate it.
- Is the critique indeed valid? On the face of it it seems sensible, in that it's at least possibly true, but I lack the expertise to critically evaluate the claim.
- If it's not valid then where does it go wrong?