Terrestrial biogeochemical cycles: Global estimates with remote sensing

The carbon and nitrogen cycles are crucial for understanding the changing Earth system, influencing atmospheric concentrations of greenhouse gases, primary productivity of the biosphere, and biogenic emissions of reactive trace species. The carbon budget of the terrestrial biosphere has attracted special attention because of its role in atmospheric changes in carbon dioxide. The terrestrial biosphere influences atmospheric CO2 through three main modes: First, large, nearly balanced fluxes of CO2 in photosynthesis and respiration exhibit a degree of interannual variability which can influence atmospheric CO2, at least on annual to decadal time scales. Second, land use changes release C02 to the atmosphere. Third, poorly understood processes are likely resulting in enhanced uptake of CO2 in certain ecosystems, acting as a sink in the global carbon cycle. This sink may result from forest demographics, atmospheric N deposition, or direct CO2 fertilization, or some synergistic combination of those processes. Global estimates of terrestrial carbon cycle components requires the use of remote observations; however, the appropriate remote sensing strategies are quite different for the various components.