Balancing The Global Carbon Cycle With Terrestrial Ecosystems

The long-term (1850-1990) net flux of carbon between terrestrial ecosystems and the atmosphere, based on geophysical analyses (Sarmiento et al. 1992), indicates a net release of 25 PgC to the atmosphere, most of which was released prior to 1940. In contrast, changes in land use have resulted in a release that totals 120 PgC, most of which occurred after 1940. By difference, undisturbed terrestrial ecosystems seem to have accumulated about 95 PgC since 1850, presumably as a result of changing environmental conditions.If the difference between the geophysical and land-use estimates represents changes in terrestrial ecosystems in response to a changing global environment, the implications for future climatic change are ambiguous: a global warming seems likely to release carbon from soil, but either the increased mineralization of soil nitrogen or the elevated concentrations of CO2 in the atmosphere might counter this release. The net direction of the feedbacks is difficult to predict.On the other hand, if undisturbed ecosystems have been accumulating carbon over the last 50 years or so, the future of this accumulation is less ambiguous. Deforestation in the Tropics is about 15 x 10(6) ha (or almost 1%) yr(-1) currently and the rate seems to be increasing. The net effect of converting forests to cleared land will be a reduction in the capacity of ecosystems to accumulate carbon in the future even if they have in the past.This long-term accumulation of carbon in undisturbed ecosystems is calculated by difference. It has not been directly observed. Measured rates of tree growth suggest that the accumulation is not in vegetation. On the basis of accumulation rates of organic matter, it appears not to be in soils either (Schlesinger 1990). It is impossible to rule out the oceans as an additional sink or to be certain that the imbalance in the global carbon equation is not the result of errors in one or more of the other terms.