cogitator, what percent of the total atmosphere is that increase in CO2 the last 100 years? I'm bad enough at math that I'm reluctant to post the result I get, but it seems astonishingly small.
I doubt your math is wrong; CO2 is a small constituent of the total atmosphere.
But CO2 is the constituent in the atmosphere with an important property; it is the most prevalent atmospheric gas for which a change in its concentration will affect Earth's radiative balance. So despite the fact that there is a lot less CO2 than N2 or O2, that concentration is a very important variable in the climate system. Always has been and always will be.