A colorless, odorless, non-combustible gas, present in low concentrations in
the air we breathe (about three hundredths of one percent by volume). Carbon
dioxide is produced when any substance containing carbon is burned. It is also a
product of breathing and fermentation. Plants absorb carbon dioxide through
photosynthesis.
Although it is mainly released by human activities through the combustion of
fossil fuels, it is also formed by certain natural processes (see carbon cycle).
Carbon dioxide is the principal greenhouse gas that contributes to global
warming.
