Land use and energy scenarios affecting the global carbon cycle

Past increase of atmospheric CO₂ involves significant contributions from both fossil and biospheric sources. The latter are controversial, partly because these CO₂ releases may be balanced by accelerated regrowth following clearing of some forests, while others were being converted to agricultural or other nonforest land. A simulation model was used to reconstruct changes since 1860 and project four hypothetical future scenarios of CO₂ injection to 2460. Nineteen compartments and their exchanges of carbon were considered. Areal extents of tropical forests, other wooded ecosystems, and nonforests were incorporated into the model. Rapidly and slowly exchanging pools of carbon per unit area were projected by integrating income-loss differential equations numerically. Estimated cumulative releases of CO₂ from fossil fuels (plus cement) near 120 Pg of carbon (1 Pg = 10¹⁵ g) from 1860 to 1970 were assumed to equal the prompt plus delayed releases due to forest clearing. Limits of exploitable forest area and biomass were evaluated and found to contribute much less future CO₂ than usable fossil fuels. Ultimate release from the latter (7500±2500 Pg C) could increase atmospheric CO₂ manyfold: doubling the assumed 1860 levels as early as (1) the year 2032 for assumed early fossil-use scenarios and (2) the year 2045 for late fossil-use scenarios. Depending on the poorly known parametes that were programmed to constrain the organic production rates, cumulative storage, and the response of plants and soils to enhanced atmospheric CO₂, biospheric storage might reach higher levels for all scenarios than the estimates given here. However, maximizing such storage in real life would require much closer understanding and wiser management of ecosystems than history has shown.