How would Earth's atmosphere change over time?

As we've seen, it started off as mostly nitrogen with a lot of CO2; the emergence of plants converted most the CO2 to molecular oxygen. The introduction of modern Man's Industrial Revolution was beginning to reverse the O2 back to CO2 and CO - raising the energy absorption rate of the planet, and giving us warmer temperatures.
If we're not careful, the runaway greenhouse effect that Venus suffers could act in a similar (although much less drastic) manner here.

If what?
Now if the question is "How WILL the Earths Atmosphere change over time" then you have to state the parameters, more fossil fuel use? less use? solar growth?

Burning fossil fuels is only a small part of the "Climate Change" issue. Natural events such as earthquakes and volcanoes add more gasses globally than do the activities of Man. It is a nice convinient way to tax the butts of us all.