Data are presented which define the role of atmospheric carbon dioxide in plant growth. The amount of atmospheric carbon dioxide required to support a unit amount of carbon in the biosphere is determined from this data. This permits the determination of the minimum amount of atmospheric carbon dioxide required to support the antediluvian biosphere based on the amount of coal reserves and resources. The increased atmospheric carbon dioxide may also require an additional source of atmospheric water vapor to support the antediluvian biosphere. Furthermore, increased atmospheric carbon dioxide may have implications for the respiration and diet of mankind and the animals before the Flood. Carbon dating and the preferential growth of selected plant types in the pre-Flood world would probably be affected by increased atmospheric carbon dioxide as well.