Carbon dioxide in the Earth's atmosphere comes mostly from?

Like Venus and Mars, most of our CO2 was probably there from the beginning, and has been pulled out and sequestered in limestone, chalk, coal and oil deposits over hundreds of millions of years.

For the past few million years, CO2 cycles from a low in the summer (when it is absorbed by growing plants, grass and leaves) to a high in the winter, as those organisms die and release their stored up carbon. This cycle continues, though it has gradually increased due to human fossil fuel consumption.

MOST of the additional CO2 in our atmosphere over the past century has come from the the burning of coal and oil.