ࡱ> 9;:7 '!bjbjUU ,,7|7| l J J J 8  * $ 07i 7   B    V V@ 0+ @J  0    Direct Estimates of the Oceanic Inventory of Anthropogenic Carbon Scott C. Doney and Christopher L. Sabine As the atmospheric concentration of the greenhouse gas carbon dioxide continues to grow from human fossil fuel emissions and land-use change, increasing attention is directed toward better constraining the past, present and future oceanic uptake of anthropogenic carbon. With a total carbon inventory about 50 times larger than the atmosphere, the oceans are the largest reservoir of mobile carbon on centennial-to-millennial time scales. The ocean is also an effective sink for excess anthropogenic carbon. The net ocean carbon uptake for the 1980s was approximately 2 Pg C/yr (Pg equals 1015g), or roughly a third of the atmospheric fossil fuel emissions. Until recently, calculations of the oceanic carbon uptake came almost exclusively from box models and ocean general circulation models, but a number of data-based approaches are now emerging. The WOCE/Joint Global Ocean Flux Study (JGOFS) global CO2 survey increased by an order of magnitude the amount of high quality data on the ocean inorganic carbon system, and new empirical estimates of the anthropogenic carbon inventory from the WOCE/JGOFS survey and historical data sets (e.g., Gruber et al., 1996; Sabine et al., 1999) offer the first robust test of the model-based approaches. These data-based estimates also help provide important constraints on other components of the global carbon cycle such as the terrestrial biosphere, where integrated inventory measurements are even more difficult to obtain. As outlined by Wallace (1995), three basic observational techniques have been proposed for estimating oceanic carbon uptake: calculating the net air-sea CO2 flux integrated over the globe; measuring the temporal change in carbon inventories from repeat surveys; and computing the present excess carbon inventory relative to a preindustrial distribution. The third method is based on the premise that the anthropogenic CO2 content of a water parcel can be estimated from the measured dissolved inorganic carbon (DIC) by subtracting the contributions from the biological and solubility pumps. The effect of the decomposition of organic and inorganic matter is determined from oxygen and total alkalinity using Redfield stoichiometric ratios. The DIC of waters in equilibrium with a preindustrial atmosphere can be calculated from the thermodynamic constants. Since surface water CO2 is rarely in equilibrium with the atmosphere, an additional correction for the air-sea CO2 disequilibrium also must be made. Transient tracer ages are used to estimate the air-sea disequilibrium in the main thermocline, where essentially all of the water is influenced to some degree by the penetrating anthropogenic carbon signal. Two major innovations of Gruber et al. (1996) that revitalized this area of research are to reorient the analysis along isopycnal mixing surfaces and to break down the preformed DIC into equilibrium and disequilibrium components. The anthropogenic carbon inventory approach requires high quality measurements of DIC and associated hydrographic properties in order to remove the large background and natural variability in ocean carbon. Typical DIC concentrations range from 1800 to 2400 mmol/kg, and the surface seasonal cycle is often as large as 100 mmol/kg. By comparison, the total anthropogenic signal in surface water for the 1990s is roughly 40-50 mmol/kg (Figure 1), dropping off rapidly with depth to 5-10 mmol/kg at the base of the main thermocline. By standardizing analytical techniques and developing new standards or Certified Reference Materials for DIC, the WOCE/JGOFS Global CO2 Survey team was able to improve dramatically the shipboard accuracy (2 mmol/kg) and precision (1 mmol/kg) to levels where the detection of the anthropogenic signal (estimated error 5-10 mmol/kg) became feasible. The Global CO2 Survey along the WOCE Hydrographic Program one-time survey cruise track mapped all major ocean basins except the Arctic. As shown in Figure 1, the estimated anthropogenic CO2 values are highest in the surface waters, decreasing rapidly with depth through the main thermocline. The largest water column inventories of anthropogenic CO2 are associated with the subtropical convergence regions (Figure 2). The water column inventories and basin-scale integrals from the data-based methods are comparable to numerical global circulation model results. Substantial regional differences remain, though, due perhaps to uncertainties both in model transport and in the empirical method (Gruber et al., 1996; Sabine et al., 1999; Wanninkhof et al., 1999). The general agreement between the two widely different approaches, however, is encouraging. A comparison of results from the different techniques allows the scientific community to refine the uncertainties for the terrestrial carbon sink and the overall perturbation of global carbon cycle, a key issue for future climate change projections. References Gruber, N., J. L. Sarmiento, and T. F. Stocker. 1996. An improved method for detecting anthropogenic CO2 in the oceans. Global Biogeochem. Cycles,10: 809-837. Sabine, C. L., R. M. Key, K. M. Johnson, F. J. Millero, A. Poisson, J. L. Sarmiento, D. W. R. Wallace, and C. D. Winn. 1999. Anthropogenic CO2 inventory of the Indian Ocean. Global Biogeochem. Cycles, 13: 179-198. Wallace, D. W. R. 1995. Monitoring global ocean inventories, OOSDP Background Rep. 5, 54 pp., Ocean Observ. Syst. Dev. Panel, Texas A&M Univ., College Station TX. Wanninkhof, R., S. C. Doney, T.-H. Peng, J. L. Bullister, K. Lee, and R.A. Feely, 1999. Comparison of methods to determine the anthropogenic CO2 invasion into the Atlantic Ocean. Tellus, 51B, 511-530. Figure 1-A comparison of depth profile of anthropogenic CO2 (mmol/kg) derived from observations (Sabine et al., 1999) and from the National Center for Atmospheric Research climate system model Ocean Model (NCOM) for 1995 along the WOCE I9 line in the eastern Indian Ocean (approximately 93E). Figure 2-Observed and modeled integrated water column anthropogenic CO2 inventory (mol/m2) along the same section as Figure 1. 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