Measurements of delta^<13>C of atmospheric CO_2 and its application to the carbon cycle

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The concentration of atmospheric CO_2 has been increasing steadily for the last 200 years, due to human activities such as fossil fuel combustion and deforestation. In order to elucidate the causes of such an increase of the concentration quantitatively, it is necessary to figure out the carbon cycle on the earth's surface. However, the roles of the biosphere and the oceans in the carbon cycle are not yet well understoond. To solve this difficulty, measurements are requireed for the carbon isotopic ratio delta^<13>C of atmospheric CO_2 as well as for its concentration.In this study, CO_2 samples collected in the troposphere over Japan and in the western Pacific and the Antarctic regions were analyzed for delta^<13>C with a precision of better than 0.02*. The seasonal cycle of delta^<13>C was clearly found at northern latitudes and the equator, due to the seasonal change in the biospheric activities, but it was hardly observable in the southern hemisphere. It was found that values of delta^<13>C decreased seculary at a rate of about 0.03*, due mainly to fossil fuel combustion. Interannual variations in the long-term trend of delta^<13>C were also found in association with ENSO event and other factors, which were almost opposite in phase with those of the CO_2 concentration. The analyzes of delta^<13>C and the CO_2 concentration with a box diffusion model and a 2-D atmospheric transport model showed that an imbalance of the CO_2 exchange between the atmosphere and the biosphere could be responsible for such interannual variations and that the biosphere does not act as a strong net source for atmospheric CO_2.