Carbon dioxide is not the major greenhouse gas, water vapor is.

Carbon dioxide accounts for less than ten percent of the greenhouse effect, as carbon dioxide's ability to absorb heat is quite limited.

Only about 0.03 percent of the Earth's atmosphere consists of carbon dioxide (nitrogen, oxygen, and argon constitute about 78 percent, 20 percent, and 0.93 percent of the atmosphere, respectively).

Most of the carbon dioxide in the atmosphere does not come from the burning of fossil fuels. Only about 14 percent of it does.

Most of 20th Century global warming occurred in the first few decades of that century, before the widespread burning of fossil fuels (and before 82 percent of the increase in atmospheric CO2 observed in the 20th Century).

Source.