Sources and Sinks of Atmospheric CO₂

Abstract

The biosphere plays an important role in determining the sources, sinks, levels and rates of change of atmospheric CO₂ concentrations. Significant uncertainties remain in estimates of the fluxes of CO₂ from biomass burning and deforestation, and uptake and storage of CO₂ by the biosphere arising from increased atmospheric CO₂ concentrations. Calculation of probable rates of carbon sequestration for the major ecosystem complexes and global 3-D tracer transport model runs indicate the possibility that a significant net CO₂ uptake (> 1 Pg C yr^-1), a CO₂ 'fertilisation effect', may be occurring in tropical rainforests, effectively accounting for much of the 'missing sink'. This sink may currently balance much of the CO₂ added to the atmosphere from deforestation and biomass burning. Interestingly, CO₂ released from biomass burning may itself be playing an important role in enhanced carbon storage by tropical rainforests. This has important implications for predicting future CO₂ concentrations. If tropical rainforest destruction continues then much of the CO₂ stored as a result of the CO₂ 'fertilisation effect' will be rereleased to the atmosphere and much of the 'missing sink' will disappear. These effects have not been considered in the IPCC (Intergovernmental Panel on Climate Change) projections of future atmospheric CO₂ concentrations. Predictions which take account of the combined effects of deforestation, the return of carbon previously stored through the CO₂ 'fertilisation effect' and the loss of a large proportion of the 'missing sink' as a result of deforestation, would result in much higher predicted concentrations and rates of increase of atmospheric CO₂ and, as a consequence, accelerated rates of climate change.