Carbon dioxide is emitted into the atmosphere when fossil fuels are burnt.
About half of the CO 2 emissions are retained in the atmosphere, the rest enters the terrestrial biosphere or is absorbed by the oceans and carried down into the deep water flow where it is ultimately deposited as sediments. Other chemicals that affect our environment are methane, sulphur and nitrogen as well as man made chemicals like CFC and other chlorine compounds. In order to model these cycles the relevant, often very complex chemical interaction need to be represented as well as the transports by the atmosphere and ocean circulation. In the investigations of the greenhouse warming of our climate it became evident that the CO 2 -cycle must be properly modelled in order to compute the atmospheric CO 2 concentration, since a large fraction of the emitted carbon dioxide is absorbed by the ocean and terrestrial biosphere and thus cannot contribute to global warming. Also important is the sulphur-cycle, which is mainly controlled by volcano eruptions and pollution due to industrial production.