CO2 in our atmosphere has been increasing steadily for the last 18,000 years-- long before humans invented
smokestacks (
Figure 1).
Unless you count campfires and intestinal gas, man
played no role in the pre-industrial increases. 
As illustrated in this chart of Ice Core data from the Soviet Station Vostok in Antarctica, CO2 concentrations in earth's atmosphere move with temperature. Both temperatures and CO2 have been on the increase
for 18,000 years. Interestingly, CO2 lags an average of about 800 years behind the temperature changes-- confirming
that CO2 is not the primary driver of the temperature changes (9).
Incidentally, earth's temperature and CO2
levels today have reached levels similar to a previous interglacial cycle of 120,000 - 140,000 years ago. From beginning
to end this cycle lasted about 20,000 years. This is known as the Eemian Interglacial Period and the earth
returned to a full-fledged ice age immediately afterward.
