Radiocarbon – a Unique Tracer of Global Carbon Cycle Dynamics

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 the quantification of the change in these trace gas concentrations in the atmosphere. From a presumably “undisturbed” preindustrial situation several hundred years ago until today, the CO2 mixing ratio increased by almost 30% (Figure 1a) (Neftel et al. 1985; Conway et al. 1994; Etheridge et al. 1996). In the last decades this increase has been nearly exponential, leading to a global mean CO2 mixing ratio of almost 370 ppm at the turn of the millennium (Keeling and Whorf 1999). 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 that is 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 equally important today, 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 human impact.