CO2 is essential for life and is as important as oxygen, water and sunlight. Coal, oil and gas ("fossil fuels") were formed by removing carbon (i.e. CO2) from ancient atmospheres and oceans. Unfortunately, there is insufficient fossil-fuel carbon to recreate the fertile, ancient atmospheres. Most of that carbon has been used to form carbonate rocks such as limestone and coral. Carbonate carbon is no longer readily available for plant growth.