How have humans added an excess amount of carbon dioxide to the atmosphere?

Carbon dioxide is produced as a by-product of the burning or combustion of almost all types of fuels, including gasoline, natural gas (methane) and coal. People have been burning a lot of these things, in recent centuries.

Because the carbon in fossil fuels has been out of circulation for 300 million years, releasing it now is overloading the carbon cycle, and causing the excess.