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 ocean s and their sediments. Fossil-fuel combustion, land use and other huma n activities are, increasingly, disrupting these natural biogeochemica l cycles and processes, with the potential for far-reaching consequenc es; for example, changes in atmospheric composition affecting the glob al heat balance. The carbon, nitrogen and sulfur cycles are of particu lar importance to the functioning of the biosphere, and are also close ly linked to the physical climate system. Within IGBP, the global-scal e modelling effort is initially focussed on the carbon cycle: this is poorly understood-yet is critical to estimating future levels Of CO2 a nd other greenhouse gases, and their direct and indirect interactions with the biosphere. To assist in determining the factors that influenc e the atmospheric lifetime of CO2, the concept of a single half-life ( T1/2) 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 a ssumptions made about the possible terrestrial fertilization response: estimates of T1/2 vary between 92 years (no allowance for terrestrial effects) to 27 years (with both terrestrial regrowth plus fertilizati on). The range of these values raises further scientific questions, an d has implications for policy development.