Global Warming

I do not understand the theory behind global warming if the density of carbon dioxide is greater than the density of air. Following is a density comparison:

Density (kg/cm3) at 1 atmosphere:
-40 deg c 0 deg c 40 deg c
Co2 2.32 1.98 1.72
Air 1.51 1.29 1.12

Given the density difference should co2 stay close to the ground rather than rise?

Thank you.
Mike Moran [email protected]