Carbon dioxide (chemical formula CO2) is a naturally occurring
chemical compound composed of two oxygen atoms covalently bonded
to a single carbon atom. It is a gas at standard temperature and
pressure and exists in Earth's atmosphere in this state, as a
trace gas at a concentration of 0.039 per cent by volume. As
part of the carbon cycle, plants, algae, and cyanobacteria use
light energy to photosynthesize carbohydrate from carbon dioxide
and water, with oxygen produced as a waste product. However,
photosynthesis cannot occur in darkness and at night some carbon
dioxide is produced by plants during respiration. Carbon dioxide
is produced by combustion of coal or hydrocarbons, the
fermentation of sugars in beer and winemaking and by respiration
of all living organisms. It is exhaled in the breath of humans
and land animals. It is emitted from volcanoes, hot springs,
geysers and other places where the earth's crust is thin and is
freed from carbonate rocks by dissolution. CO2 is also found in
lakes at depth un