Oxygen isotopes in atmospheric carbon dioxide can be used to obtain a better understanding of biospheric carbon fluxes, which must be understood in order to predict vegetation and climate change in response to anthropogenic greenhouse forcing. The main objective of this proposal is therefore to improve understanding of carbon flow in the biosphere by enhancing a comprehensive global 3D model of the movement of stable oxygen isotopes in plants, soil and the atmosphere. This objective will be achieved by measuring unknown fractionation factors, and by improving descriptions of plant and soil processes, including processes that had previously been omitted, and incorporating complementary tracers in the existing model. The resulting model will be the most comprehensive description of oxygen isotopes in atmospheric carbon dioxide up-to-date, where complementary tracers constrict individual aspects of the isotope cycle. The model will strictly constrain gross biospheric carbon fluxes, allowing direct comparison between measured and modelled oxygen isotope ratios in atmospheric carbon dioxide. This isotope model will then be incorporated in an Earth Systems Model (ESM) currently being developed. This will allow further detailed studies of biosphere/hydrosphere, biosphe re/atmosphere and biosphere/climate interactions, as well as studies of human influences on the Earth climate system. The ability to track oxygen isotopes in the biosphere components of the ESM will substantially reduce the uncertainty in inferred carbon sources and sinks on scales ranging from regions to continents, thus greatly improving the ESM's predictions of climate change and climate variability.
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