CO2 level increase and decrease are a RESULT of global temperature, which of course is moderated by the Earth's only energy source the SUN.
The alarmists like to ignore the Vostok/Greenland scientific ice core research which shows that CO2 level LAG, global temperature increases up to 800 years.
The Medieval Warm period, which occurred from 950 to 1350 and was warmer than the modern solar grand minimum cycle (1980 - 2003) is responsible for the recent rise in global CO2 levels.
The Sun, Galactic Cosmic Ray flux, solar and planetary orbital variances and Nature are the true drivers of the Earth's climate. The AGW is a myth and CO2 is plant food.
Update: Darwin's answer in which he references Henry's Law is a typical alarmist response, where he forget to include pertinent information. Henry's Law by his own reference indicates that "atmospheric pressure" is the determining factor, which he forgets to mention.