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

Abstract

ABSTRACT. In the past three centuries, human perturbations of the environment 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 CO 2 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 CO 2 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 CO 2 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 CO 2 . Increasing inputs of terrestrial organic matter and its subsequent oxidation in the coastal margin (increasing heterotrophy) were significant sources of CO 2 in coastal waters in the 20th century. However, the coastal ocean did not evolve into a greater net source of CO 2 to the atmosphere during this period because of the opposing pressure from rising atmospheric CO 2 . Since pre-industrial time (since 1700), the net flux of CO 2 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 CO 2 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 CO 2 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 sho...

Anthropogenic perturbation of the carbon fluxes from land to ocean

“…The flux is evaluated at 1.9 Pg C yr -1 , by subtracting, from a total median estimate of 2.8 Pg C yr -1 , the smaller contributions from the other three fluxes: chemical weathering (F 2 ), sewage (F 4 ) and net C fixation (F 5 ). The soil-derived C flux is part of the terrestrial ecosystem C cycle (Box 1) and represents about 5% of soil heterotrophic respiration (FT 7 ). Current soil respiration estimates neglect the C released to inland waters.…”

“…Reconstructions of the historical evolution (preindustrial, around the year 1750, to present) of the global aquatic C cycle and its fluxes have so far relied primarily on globally averaged box models 7,74 . Increasing concentrations of atmospheric CO 2 , land-use changes, nitrogen and phosphorus fertilizer application, C, nitrogen and phosphorus sewage discharge and global surface temperature change drive these highly parameterized models.…”

Past and present of sediment and carbon biogeochemical cycling models

“…Total CO 2 addition to the atmosphere by the year 2000 has been estimated at 461 ±19 Gton C (3.84×10 16 mol C), of which 134±6 Gton (1.12×10 16 mol) were taken up by land ecosystems, leaving 327±13 Gton (2.72×10 16 mol) to be distributed between the atmosphere and the ocean. Of these, an estimated 122± 2 Gton C (1.02×10 16 mol) were transferred to the ocean, leaving 205±13 Gton C (1.71×10 16 mol) in the atmosphere (Ver et al, 1999;Mackenzie et al, 2001; see also Sarmiento et al, 1992;Hudson et al, 1994;Bruno and Joos, 1997). We address three questions based on modeling calculations of the carbon cycle related to the increase in atmospheric CO 2 during industrial time of the past 300 years:…”

“…2b) that, although not addressing the industrial rise of atmospheric CO 2 , indicate the inorganic carbon input as being less than needed to account for the CaCO 3 precipitation. Similarly, the delivery of organic carbon by rivers at the rate of 26 to 34×10 12 mol/yr (Ver et al, 1999;Fig. 2a) produces in 300 years an input of 0.8 to 1×10 16 mol C. Its storage as organic matter in sediments at the rate of 8 to 16×10 12 mol/yr (Ver et al, 1999;Fig.…”

“…The oceanic coastal zone, of an areal extent of 7 to 10% of the ocean surface and a mean depth of about 130 m (e.g. Ver et al, 1999;Rabouille et al, 2001;Lerman et al, 2004) receives a large input of organic matter from land that is added to the organic matter produced in situ. For this budgetary reason, NEP of the oceanic coastal zone exerts a relatively greater control of the carbon cycle in this domain.…”