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Constraining carbon gross fluxes with oxygen isotopes

Final Activity Report Summary - COCO (Constraining carbon gross fluxes with oxygen isotopes)

Carbon dioxide (CO2) in the atmosphere is a mixture of all different kind of CO2 sources, e.g. fossil fuel burning or decomposition of old plant material, and CO2 sinks, e.g. absorption by plants or by the oceans.

We wanted to study the individual processes that admixed to the final CO2 signal. We therefore needed extra information on the CO2 in order to know from which process it came from. One bit of this extra information of a substance was the isotopic composition of the substance, i.e. its weight. The oxygen isotopic composition of CO2, for example, principally allowed for the separation of two opposing processes, namely the uptake of CO2 by plants and the release of CO2 by soils. Nevertheless, we had to understand the isotopic composition of CO2 in great detail in order to use the information with some confidence. This project advanced the understanding of the oxygen isotopic composition of CO2.

We tried to improve our understanding of the oxygen isotope cycle on the global scale with the help of a global climate model, as well as locally, in specific ecosystems. We firstly tried to understand energy, water and CO2 fluxes in a eucalypt forest in Australia with the help of a detailed model of the ecosystem. This was an extreme ecosystem where model deficiencies were easily exposed. This work led to a substantial amount of improvements in the descriptions of the ecosystem processes that were a priori not directly related to the isotope signal. Australia was an exceptional place, but we evaluated that the improvements should also be significant in other, more moderate, ecosystems. With all the improvements and added complexity, we always supplied possible simplifications that captured most of the variations and were suitable for global models.

We then included the descriptions of the ecosystem studies in a global model of oxygen isotopes in atmospheric CO2. They substantially improved the spatial pattern of the oxygen isotopes in CO2 in the model. However, they did not change the temporal behaviour of the model. This implied that some riddles about the oxygen isotopic composition of CO2 were solved during this project, nevertheless some puzzling characteristics remained.