Carbon dioxide (CO₂) is a noncombustible, non-noxious gas that naturally occurs in the atmosphere and is the most prevalent of the greenhouse gases (GHG) believed to contribute to global warming.

CO₂ concentration in the atmosphere affects plant growth and water use directly. While large atmospheric concentrations are harmful to plants, conversely, in principle, CO₂ enrichment of the air around plants will stimulate photosynthesis, (the process of converting atmospheric CO₂ , water and sunlight, into energy and oxygen) resulting in greater production of total dry matter, and in greater yields of fruits and grains.

About 86% of U.S. GHG emissions are energy related, and 95% of the U.S. emissions from energy is CO₂. Its concentration is increasing, primarily as a result of large-scale utilization of fossil fuels, deforestation, and changes in land use. The Intergovernmental Panel on Climate Change projected that increasing concentration of CO₂, plus that of other active greenhouse gasses, would cause an increase in the mean global temperature and result in other changes in the climate system.

The United States emits nearly 25% of the world’s carbon dioxide, over 5.5 billion tons annually, largely via the combustion of petroleum, coal and natural gas. A fraction (less than 2 percent) comes from other sources, including landfills and the chemical processing of cement. Total estimated emissions increased by 1.6 percent annually from 1990 to 2000, so that currently about 5,806 million metric tons ofCO₂is produced each year. Compared to 1990 emissions levels, the increase is about 836 million metric tons or almost 17 percent. In 2000 alone, U.S.CO₂emissions were 83% of total greenhouse gas emissions. By 2100, the atmospheric concentration ofCO₂is expected to rise between 540 and 970 ppm, depending on the steps taken to limit emissions.