Molecule made up of one carbon and two oxygen atoms.
Carbon dioxide is a gas under normal temperature and pressure conditions. It is part of the carbon cycle and it is currently found in a concentration of 385 parts per million in the atmosphere by volume. Carbon dioxide is the most important greenhouse gas after water vapor.
The exact concentration of carbon dioxide in the atmosphere varies seasonally during the annual cycle. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas through photosynthesis, and rise during the northern autumn and winter as plants go dormant, die and decay.
The oceans form the other major reservoir of CO2 in the carbon cycle. There is about 50 times as much CO2, mainly as carbonic acid, in the ocean as in the atmosphere. How this fact affects climate change is still under debate, but it is known that water can contain less CO2 when it warms, so there is concern that the oceans will be able to store less CO2 in the future. This will lead to more warming and oceans with even lower capacities as carbon sinks in a positive feedback mechanism.
Both animal and plants release carbon dioxide into the atmosphere and the oceans because of respiration. Some microorganisms produce carbon dioxide through fermentation. In both cases carbon from organic material is oxidized to make CO2.
Through the greenhouse effect carbon dioxide helps make the Earth inhabitable to humans and most other animals and many plants.
In the period since industrialization the amount of carbon dioxide in the atmosphere has grown significantly. This growth is due to human activity. The resulting enhanced greenhouse effect is believed to be one of the primary drivers in global warming.
The anthropogenic release of CO2 to the atmosphere happens through the use of fossil fuel. When fuel is burned CO2 and water is released. Plants remove the same amount of CO2 from the atmopsphere during growth as they release when used for fuel. Fossil fuel, however, releases a reservoir of CO2 that has otherwise been out of the carbon cycle for many hundred thousand years. The use of biofuels is carbon neutral whereas the use of fossil fuels is not.
Cutting the emissions of carbon dioxide is, therefore, one of the primary efforts for mitigating global warming. The Kyoto protocol is an international effort to reduce the emission of carbon dioxide through quotas and emission trading, primarily through less use of fossil fuel.
Volcanic activity also releases significant amounts of CO2.