Planetary metabolism: Understanding the carbon-cycle

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 modelling effort is initially focussed on the carbon cycle: this is poorly understood-yet is critical to estimating future levels of C02 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 C02, the concept of a single half-life (T112) is applied to three simple ocean carbon- cycle models and a model of global terrestrial carbon cycling. We find significant differences due to the inclusion of the terrestrial model and the nature of the assumptions made about the possible terrestrial fertilization response: estimates of T112 vary between 92 years (no allowance for terrestrial effects) to 27 years (with both terrestrial regrowth plus fertilization). The range of these values raises further scientific questions, and has implications for policy development