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Abstract

Climate on earth strongly depends on the radiative balance of its atmosphere, and, thus, on the abundance of the radiatively active greenhouse gases. Largely due to human activities since the Industrial Revolution, the atmospheric burden of many greenhouse gases has increased dramatically. Direct measurements during the last decades and analysis of ancient air trapped in ice from polar regions allow to quantify the change of these trace gas concentrations in the atmosphere. From a presumably "undisturbed" pre-industrial situation several hundred years ago until today, the CO2 mixing ratio increased by almost 30%. In the last decades this increase was nearly exponential, leading to a global mean CO2 mixing ratio of almost 370 ppm by the turn of the millenium. The atmospheric abundance of CO2 the main greenhouse gas containing carbon, is strongly controlled by exchange with the organic and inorganic carbon reservoirs. The world oceans are definitely the most important carbon reservoir, with a buffering capacity for atmospheric CO2 largest on time scales of centuries and longer. In contrast, the buffering capacity of the terrestrial biosphere is largest on shorter time scales from decades to centuries. Although today equally important, the role of the terrestrial biosphere as a sink of anthropogenic CO2 emissions is still poorly understood. Any prediction of future climate strongly relies on an accurate knowledge of the greenhouse gas concentrations in the present day atmosphere, and of their development in the future. This implies the need to quantitatively understand their natural geophysical and biochemical cycles including the important perturbations by man's impact. In attempting to disentangle the complexity of these cycles, Radiocarbon observations have played a crucial role as an experimental tool enlightening the spatial and temporal variability of carbon sources and sinks. Studies of the “undisturbed” natural carbon cycle profit from the radioactive decay of 14C in using it as a dating tracer, e.g. to determine the turnover time of soil organic matter or to study internal mixing rates of the global oceans. Moreover, the anthropogenic disturbance of 14C through atmospheric bomb tests has served as an invaluable tracer to get insight into the global carbon cycle on the decadal time scale.