Decades ago it was determined that CO2 's ability to trap heat rising from Earth's surface declines logarithmically or very rapidly (see first figure below). This means that early on, at low concentrations, CO2 does exert a significant warming of the lower atmosphere. But as the absorption bands in which CO2 captures this rising hear begin to get saturated, CO2 can capture less and less heat with each additional unit of CO2 . Depending on how sensitive or reactive one thinks Earth is to additional CO2 , the level of influence of rising CO2 today can be very small or still of significant impact. Once again, we have chosen the path recommended long ago by Winston Churchill who once said, "The farther backward you look, the farther forward you are likely to see." As we look back at Earth's climate history, far beyond the popular 1980's and 1990's which happened to see a supposedly rapid rise in temperature coinciding with a real and admittedly rapid rise in airborne CO2 , we find many examples where rises in CO2 were accompanied by declining temperatures (see the Predictions vs. Reality figure below).
These real world observations lead us to believe that Earth is not very sensitive to CO2 and that many other factors have a stronger influence on the climate. This is one of the reasons that one of the world's most prominent scholars, Professor of Meteorology, Dr. Richard Lindzen of M.I.T., has been "going crazy" for decades at humanity's infatuation that CO2 is a major cause of global warming. ("Resisting Climate Hysteria" by Richard S. Lindzen, 7-26-09)
Observe how rapidly (logarithmically) CO2's ability to cause additional temperature rise declines. Warming from a doubling (or tripling) atmospheric co2 will be very small. This physical limitation explains how Earth could have entered an ice age when CO2 levels were several thousand parts per million. Note that CO2's warming effect is strong only at low levels of atmospheric CO2, predominantly at less than 100 ppm.