The Recent State of Carbon Cycling through the Atmosphere

A summary of our knowledge about the recent carbon cycle is presented from the perspective of atmospheric observations. Particular emphasis is given to carbon isotopes as a tool to determine global carbon sources and sinks as well as their recent anthropogenic changes. In the case of atmospheric carbon dioxide, the yearly anthropogenic emissions are small if compared to the gross exchange between the most important reservoirs: biosphere and ocean. In order to derive net fluxes caused by climatic or anthropogenic perturbations, the gross fluxes have to be known as accurately as possible. One isotopic tool is to trace the penetration of bomb 14C into the ocean surface water so as to derive the atmosphere/ocean gas-exchange rate. Moreover, the distribution of δ13C in atmospheric CO₂ can help to constrain the amount of excess CO₂ taken up (or released) by the global biosphere. As for atmospheric methane, anthropogenic emissions today exceed natural release rates by nearly a factor of two. Although the total yearly methane emissions to the atmosphere are believed to be known to within 10–20%, large uncertainties still remain in the estimates for individual sources, in particular for natural ecosystems like wetlands and the oceans. Isotope observations provide strong constraints also for the global atmospheric methane budget because different source types (biogenic vs. thermogenic) produce their own characteristic isotopic fractionations during methane production. These isotopic signatures can be used further to predict the future development of atmospheric methane concentrations and to understand better the drastic changes observed in the past under different conditions.