Question

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What experiments prove the greenhouse effect?

I am seeking for a proof that CO2 is a greenhouse gas. I posted this on Skeptic.SE recently but found no help in seeking for proof:

I assisted to a physicist conference in my university a few years ago against the case that carbon dioxide was a cause of global warming. The main point was that CO2 is not a greenhouse gas. I did a research to find evidence for either side and found absolutely nothing.

So, is carbon dioxide a greenhouse gas? If yes, has it been demonstrated in a scientific paper?

Here are some discussion articles describing arguments against CO2 being a greenhouse gas:

Home experiments

Ph.D. Tim Ball

Evidence from temperature rise through history

I understand that the sources do not prove that CO2 isn't a greenhouse gas...but I can't find evidence proving the contrary. Since a real physicist with many years of experience a demontrated to me that it is not possible by definition, I need to find proof to accept the other premise.

I found this post on physics.SE, but the asker do not seek proofs and simply accept what the others are saying.

Baically, to have a greenhouse effect, a gas needs to absorb only a portion of a wavelength and let pass another portion. Since CO2 absorbs some IR, a part of it is remitted to earth. The spectrum is absorbed by the earth (everything) and it is remitted on another wavelength which allows a different proportion to pass through the CO2 layer. The question is, why the heat would be remitted toward earth (it should go in the colder direction)? And also, if CO2 is all around the earth and blocking a certain wavelength, why would increasing it warm the earth? Am I missing something?

Answer 1

First of all the very name is misleading, as real green houses get heated by obstructing convection and not by the gases in the green house.

The question can be reformed as : does the existence of CO2 in the atmosphere contribute to the heat retention capacity of the atmosphere which is what the "greenhouse effect" is about.

The answer is yes, it has been measured and it is small . In this image one can see the gases which can absorb the incoming solar radiation, and the main contribution is from H2O, water vapor. The CO2 contribution, though small,is not negligible.

These are old measurements as you can see from the reference on the figure.

So the answer is yes to CO2 being a "greenhouse gas" among others. Now the importance given by the community of climate change advocates to CO2 is completely dependent on computer modelling of the earth's climate. The importance comes from an assumed in the programs feedback process, where a small increase in atmospheric CO2 induces a large increase in atmospheric H2O in a run-a-way mode. This is another story though.