Actually, in the last 50 years the CO2 level in the atmosphere has increased about 30%, from ~310 ppm to a little over 400 ppm.
And if you go back further it was so low that we were in danger of all plant life dying. Further than that it was over 800 ppm. Your point?
Let me add something because I might just know what you mean. I was referring to the more recent rise from 350 ppm to 400 ppm, which is an increase of 50 ppm. We are talking very small numbers here, and a 50 ppm increase adds (roughly) 1/100th of 1% more CO2 in our atmosphere than there was before.