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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.
Summary of the Global Carbon Cycle, showing natural CO2 emissions and human CO2 emissions from burning fossil fuels, compared to CO2 absorbtion by vegetation and land and the oceans. Numbers represent the flux of carbon dioxide in gigatonnes per year, adapted from Figure 7.3 in the IPCC AR4. Note that the IPCC numbers refer to carbon rather than carbon dioxide (GtC rather than GtCO2), and these differ by the ratio of the molecular weights, 44/12 = 3.67, as described here.