| Summary: | Calibration and validation of carbon cycle models is mainly based on isotope data of the steady state of the system as well as its responses to natural and anthropogenic disturbances. The progress in the use of carbon isotopes is reviewed. The discussion covers the following phenomena: - Response of the system to the anthropogenic CO2 input: rise of atmospheric CO2, dilution of the atmospheric 14C concentration - Prenuclear 14C distribution in the ocean - 14C rise in atmosphere and ocean due to bomb produced 14C - Modulation of natural 14C production and its atmospheric response, as reflected in 14C/C ratios recorded in tree-rings - Past changes of the atmospheric CO2 concentration inferred from ice core studies. The information is used to derive the probable range of the CO2 release, resp. uptake by the biosphere, and to check the general validity of carbon cycle models.
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