Is there an article related to that logarithmic graph (on diminishing effect of CO2)? I'd to learn more about that.
It's an effect called the beer-lambert law. We use it a lot in spectroscopy.
In simple terms,
absorption and concentration are linear and directly proportional. The trick is that absorption is logarithmic. It's based on a value called transmittance. Transmittance is the ratio of how much light passed through divided by how much light I started with. Suppose I shine a light at a detector. With nothing stopping the light, I would say my transmittance is 1/1 or 100%. To make that into an absorption value, I take the negative log base 10 of transmittance. -Log(1) = 0, so I have 0 absorption.
If I put some chemical infront of it that so that only half the light passes through, I would say my transmittance is 0.5/1 or 50%. To convert that to a log scale of absorption, take the negative log base 10. -log(0.5) = 0.301 absorption units.
If I blocked it so only 10% of the light was passing through, my absorption would be -log(0.1/1) = 1 absorption unit. Notice how 90% of the light blocked happens between absorption 0 to 1. At an absorption of 2, 99% of light has been blocked. At absorption 3, 99.9% of light has been blocked. Going from 2 to 3 absorption, you need to increase the concentration by 50% just to block an extra ~1% of light.
Basically what this means is that the majority of light being stopped is stopped by the first particles there and the rest is diminishing returns. This is why it was such a huge deal to make sure absolutely no acetone was in the sample cuvette. 1 drop of acetone would absorb so much IR that it would ruin any sample no matter what. 2 drops of acetone wasn't any worse because 1 was already enough to completely mask everything.
The significance of adding more gas depends on where you are starting in the absorption numbers. If CO2 is currently absorbing at 0.1 absorption units, then this is serious business and adding more CO2 can have a very large effect. If the absorption is already at 3 then you can add CO2 until the cows come home and virtually nothing will change. Since CO2 is a trace gas, adding more of it can theoretically have a large effect. On a planet like Venus, adding more CO2 would do nothing because the air is already dense CO2 that absorbs everything.
http://en.wikipedia.org/wiki/Beer's_law