Tracing the global carbon cycle with bomb radiocarbon
Bomb radiocarbon (14C) emitted to the atmosphere by nuclear explosion tests has not yet reached equilibrium within the Earth carbon system. In the present thesis this fate of radiocarbon is investigated using coarse-grid models to trace high-precision 14CO2 observations available in the atmosphere since the 1950s. The goal of the study is to progress our quantitative understanding of bomb radiocarbon following the pathways of the global carbon cycle. Inversely, I wanted also to determine new constraints on the atmospheric carbon budget buried in the long-term observations of atmospheric 14CO2. Three relevant findings came out at the different stages of my research. First, the man-made passive tracer SF6 was shown to be a powerful tool for investigating air mass transport in atmospheric transport models. Second, a serious mismatch in the global bomb radiocarbon budget has been detected, suggesting that the oceans take up 25 less anthropogenic CO2 than hitherto believed. Third, tracking both the tropospheric and the stratospheric 14C observations during the period of major bomb 14C activity excursions was found to uncover the cycle of air mass through the stratosphere. The above tools were finally used in a first assessment of the seasonal cycles of recent atmospheric 14 CO2.
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