There is no doubt that CO2 levels in the atmosphere have increased dramatically in the last century and there is no doubt that human output outstrips other sources and there is no doubt that CO2 molecules absorb infrared photons. Such things are simple facts.
True, but that's only a small part of the picture. "Infra-red photons" is a bit of a broad sweeping concept. CO2 is quite transparent to a wide range of wavelengths in the IR region. There are a couple of very specific absorption bands that you can observe:
1) A very sharp (narrow) peak at around 2349/cm (sorry, those are "wavenumber" units - spectroscopy loves to use them) which is down to the asymmetric stretching of the molecule (e.g. O=C==O <-> O=C=O <-> O==C=O). Note that the symmetrical stretch (e.g. O=C=O <-> O==C==O) doesn't contribute, as there is no overall change in dipole (charge separation) when you stretch symmetrically. Expressed in more meaningful units, this peak occurs at about 4.3 micrometres. This absorption is so strongly characteristic that if you see it in any IR run of an unknown organic molecule, it's almost certainly got a carbonyl (C=O) functional group in it.
2) A slightly broader multiple collection of peaks at around 667/cm for the bending modes (can't really depict that one well with ASCII art), corresponding to around 15 micrometres.
Note that the exact wavenumber values do vary depending on the pressure / phase. The above are for gaseous CO2 at low pressure (similar to what you'd expect from atmospheric CO2, which has a low partial pressure).
Together, these form only a very narrow range of absorption in the overall window of wavelengths we consider as being IR. There are other gases in the atmosphere in bigger concentrations that have much broader absorption than CO2 does. The most obvious is water vapour. It has a broad window of absorption in that region at which CO2 itself is transparent to IR. The concentration of water vapour in the atmosphere varies much more strongly from place to place (given that it's fundamentally part of the climate system), but typically there's always more of it around in any given sample than there is CO2. What's more, most of the activities that produce CO2 (respiration and the burning of almost all fossil fuels except refined coal) also produce water vapour, usually in larger molar ratio.
The other key factor here is to observe which part of the overall IR spectrum the warm surface of the Earth emits. Again, you will find variation depending on whether you are looking at land or water, but overall, there's more emission in the regions re-absorbed by water vapour than there is CO2.
There's no doubt that the climate is changing and that human activities are a factor, but focussing only on CO2 as the
standard, rather than using it as an
indicator is as almost poor as ignoring it all together. There are also other gases in smaller concentrations than CO2 that are actually far more potent greenhouse gases. Methane and various PFC/CFC gases have much stronger absorption spectra that more than make up for their reduced concentration.
Suppose we moved to a hydrogen economy tomorrow and the vast majority of CO2 output was replaced with H2O emissions, whilst ignoring methane (from agriculture) and various other sources. I'm not convinced that the observed/predicted warming trend would be reversed.
