Biological systems are intimately involved in the transfers of energy and materials around our planet, affecting the geochemistry and other physical properties of the atmosphere, the land surface, and the oceans and their sediments. Fossil-fuel combustion, land use, and other human activities are, increasingly, disrupting these natural biogeochemical cycles and processes, with the potential for far-reaching consequences; for example, changes in atmospheric composition affecting the global heat balance. The carbon, nitrogen and sulfur cycles are of particular importance to the functioning of the biosphere, and are also closely linked to the physical climate system. Within IGBP, the global-scale modeling effort is initially focused on the carbon cycle: this is poorly understood--yet is critical to estimating future levels of carbon dioxide and other greenhouse gases, and their direct and indirect interactions with the biosphere. To assist in determining the factors that influence the atmospheric lifetime of carbon dioxide, the concept of a single half-life is applied to three simple ocean carbon-cycle models and a model of global terrestrial carbon cycling.