The carbon cycle, and its evolution: from the geological era to the pre-industrial era

As an introduction, the paper gives an overview of the evolution of the atmosphere before and during the geological era, the advent of the vegetable kingdom and photosynthesis, the formation of coal, petroleum and natural gas deposits, and the ocean contribution to the equilibrium of the CO2 content in the atmosphere. These processes were very slow, and their knowledge relies essentially on models. Closer to us, the analyses of air bubbles enclosed within the polar ice cover gives accurate values of the CO2 content over the last hundred thousand years. This content influenced the climate and, reciprocally, the climate influenced the equilibrium between the atmospheric and CO2 contents in the ocean and continental biomass. On the continents, the vegetation uses CO2 mainly in the spring for its growth whereas it produce CO2 when the organic matters are rotting in the fall. The same thing exists in the oceans, where a huge quantity of carbon is also under the form of carbonates and bicarbonates. Before the industrial revolution, an equilibrium was continuously adjusted between the CO2 contents in the atmosphere and the oceans: this carbon was carried to the ocean bottom in down-welling zones by various phenomena, explaining the reason for its higher CO2 content, where it was released in the up-welling zones. This natural equilibrium has been destroyed since the beginning of the industrial era, by the production of an excess of CO2 At the present time half of the CO2 industrial emissions is stored in the atmosphere, the other half is naturally split between the ocean and the continental biomass. This paper is a review of this long history and describes the present level of knowledge of the processes that control the carbon cycle. (author)