The current levels of carbon dioxide in the earth’s atmosphere, approaching 400 parts per million, are low by the standards of geological and plant evolutionary history. Levels were 3,000 ppm, or more, until the Paleogene period (beginning about 65 million years ago). For most plants, and for the animals and humans that use them, more carbon dioxide, far from being a “pollutant” in need of reduction, would be a benefit. This is already widely recognized by operators of commercial greenhouses, who artificially increase the carbon dioxide levels to 1,000 ppm or more to improve the growth and quality of their plants.

Using energy from sunlight—together with the catalytic action of an ancient enzyme called rubisco, the most abundant protein on earth—plants convert carbon dioxide from the air into carbohydrates and other useful molecules. Rubisco catalyzes the attachment of a carbon-dioxide molecule to another five-carbon molecule to make two three-carbon molecules, which are subsequently converted into carbohydrates. (Since the useful product from the carbon dioxide capture consists of three-carbon molecules, plants that use this simple process are called C3 plants.) C3 plants, such as wheat, rice, soybeans, cotton and many forage crops, evolved when there was much more carbon dioxide in the atmosphere than today. So these agricultural staples are actually undernourished in carbon dioxide relative to their original design.