Biogeochemical responses of the carbon cycle to natural and human perturbations; past, present, and future

In the past three centuries, human perturbations of the environ- ment have affected the biogeochemical behavior of the global carbon cycle and that of the other three nutrient elements closely coupled to carbon: nitrogen, phosphorus, and sulfur. The partitioning of anthropogenic CO2 among its various sinks in the past, for the present, and for projections into the near future is controlled by the interactions of these four elemental cycles within the major environmental domains of the land, atmosphere, coastal oceanic zone, and open ocean. We analyze the past, present, and future behavior of the global carbon cycle using the Terrestrial-Ocean-aTmosphere Ecosystem Model (TOTEM), a unique process-based model of the four global coupled biogeochemical cycles of carbon, nitrogen, phosphorus, and sulfur. We find that during the past 300 yrs, anthropogenic CO2 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) mainly through deforestation and consequently increased humus remineralization, erosion, and transport to the coastal margins by rivers and runoff. Photosynthetic uptake by the terrestrial phytomass was enhanced owing to fertilization by increasing atmospheric CO2 concentrations and supported by nutrients remineralized from organic matter. TOTEM results indicate that through most of the past 300 yrs, the loss of C from deforestation and other land-use activities was greater than the gain from the enhanced photosynthetic uptake. During the decade of the 1980s, the terrestrial organic reservoirs were in rough carbon balance. Organic and carbonate carbon accumulating in coastal marine sediments is a small but significant sink for anthropogenic CO2. Increasing inputs of terrestrial organic matter and its subse- quent oxidation in the coastal margin (increasing heterotrophy) were significant sources of CO2 in coastal waters in the 20th century. However, the coastal ocean did not evolve into a greater net source of CO2 to the atmosphere during this period because of the opposing pressure from rising atmospheric CO2. Since pre-industrial time (since 1700), the net flux of CO2 from the coastal waters has decreased by 40 percent, from 0.20 Gt C/yr to 0.12 Gt C/yr. TOTEM analyses of atmospheric CO2 concentrations for the 21st century were based on the fossil-fuel emission projections of IPCC (''business as usual'' scenario) and of the more restrictive UN 1997 Kyoto Protocol. By the mid-21st century, the projected atmospheric CO2 concentrations range from about 550 ppmv (TOTEM, based on IPCC projected emissions) to 510 ppmv (IPCC projection) and to 460 ppmv (TOTEM, based on the Kyoto Protocol reduced emissions). The difference of about 40 ppmv between the IPCC and TOTEM estimates by the year 2050 reflects the different mechanisms within the C-N-P-S cycles on land that are built into our model. The effects of the reduced emissions prescribed by the Kyoto Protocol begin to show in the atmospheric CO2 concentrations by the mid-21st century, when our model projects a rise to 460 (year 2050) and 490 ppmv (2075), relative to about 360 ppmv in 1995. However, these projected increases assume no major changes in the present biogeochemical feedback mechanisms within the system of the coupled C-N-P-S cycles, no global changes in the kind and distribution of ecosystems in response to the rising CO2 and possibly temperature, and no changes in the mechanisms of CO2 exchange between the atmosphere and the ocean, such as could be induced by changes in the intensity of oceanic thermoha- line circulation.