Global kinetic models of the coupled C, N, P, and S biogeochemical cycles: Implications for global environmental change

The issues of the behavior of the coupled cycles of carbon, nitrogen, phosphorus, and sulfur during recent Earth history, the partitioning of anthropogenic atmospheric CO0₂ among its various sinks, the response of the global coastal margin to forcing of global environmental change, as well as projections for the near future, are considered using the Terrestrial-Ocean-aTmosphere Ecosystem Model (TOTEM), an unique process-based model of the global coupled biochemical cycles of the major bioessential elements. During the past 300 years, anthropogenic C0₂ was mainly stored in the atmosphere and in the open ocean. Human activities on land caused an enhanced loss of mass from the terrestrial organic matter reservoirs (phytomass and humus) through massive deforestation, increased humus remineralization, erosion, and runoff. Although nutrients mobilized during these processes partially supported the CO₂-induced fertilization of the terrestrial phytomass, the net response of the terrestrial organic matter reservoirs was a loss of carbon through most of the 300-year period. Organic and carbonate carbon accumulating in coastal marine environments was confirmed as a small but significant sink for anthropogenic CO₂. The coastal ocean waters have remained heterotrophic owing to the production of CO₂ in excess of consumption. The potential for the global coastal margin to act as a sink for atmospheric CO₂ through invasion of the gas, a complete reversal of its pre-industrial role as a net source Of CO₂, is likely because of the increased pressure from rising atmospheric CO₂ from anthropogenic sources. However, primarily because of slightly enhanced respiration from increasing inputs of organic matter from land and its subsequent oxidation, the CO₂ sink strength of the coastal waters is weaker than expected. Finally, results from two numerical forecasting experiments using TOTEM indicate that (1) implementation of the 1999 “Kyoto protocol” reducing CO₂ emissions to the atmosphere from industrialized countries could significantly slow the rate of rise of atmospheric CO₂ concentrations in the twenty-first century, and (2) if the ongoing increase in atmospheric CO₂ were to trigger a reduction, cessation, or reorganization of the thermhialine circulation, the Supply Of CO₂ from the deep ocean (as well as the nutrients N and P) to the coastal zone will be reduced, making the latter a potentially stronger sink for atmospheric CO₂.