Carbon Cycle Tracers

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In addition to the net carbon flux of interest, additional information can be extracted from tracers that are tightly coupled to the carbon cycle. Among these are carbon and oxygen isotope tracers and measurements of changes in the atmospheric oxygen content. Observations of these tracers in the atmosphere pose global and regional constraints on the contributions of differently "labeled" surface-air carbon fluxes. For example, radio carbon may be used to track fossil fuel CO2 (Levin and Kromer 1997), and the 13C/12C stable isotope ratio, at least on the global scale, provides constraints on terrestrial and oceanic CO2 (Ciais et al. 1995; Francey et al. 1995; Keeling et al. 1995, Piper et al. 2001). Observations of the concurrent trends in the O2/N2 ratio and CO2 permit a separation of oceanic and terrestrial carbon uptake (Keeling and Shertz 1992; Keeling et al. 1993, 1996; Battle et al. 2000).

Consistency

The quantitative reconciliation of estimates by the different methods outlined for a continental region, such as Europe (Janssens et al. 2003) or the United States (Pacala et al. 2001), or for ocean basins, such as the North Atlantic or the equatorial Pacific Ocean, is difficult. One major obstacle arises from the fact that different methods, in general, measure different fluxes or carbon flows between the major reservoirs. For example, the top-down approach estimates the net surface-air CO2 flux in a particular region, whereas an estimate for the same region based on extrapolating eddy flux measurements will not necessarily take into account disturbance factors (fire, harvest) or lateral carbon flows in or out of the target area (e.g., by wood products or carbon transport through rivers). Similarly, net ocean-atmosphere fluxes determined by the top-down approach will not directly correspond to an ocean carbon stock inventory change, since the net sea-air flux also includes a carbon flow induced by land-ocean transport of carbon through rivers (Sarmiento and Sundquist 1992). A second difficulty is induced by the varying timescales of the different approaches, necessitating averaging or interpolations in time for a consistent comparison.

The Carbon Budget and Global Partitioning between Atmosphere, Land, and Ocean

The global fossil-fuel emissions of CO2 and the net carbon balances of the atmosphere, land, and ocean over the past two decades are now rather well established through several independent observation and model-based methods (Prentice et al. 2002). The estimated accuracy of the net land and ocean uptake is on the order of 0.5—0.8 PgC y-1. Less accurate is the quantification of individual components of the terrestrial carbon balance, in particular the emissions from changes in land use and land management and the corresponding uptake by natural terrestrial processes, such as fertilization by CO2 or nutrients and changes in climate. The main issues in reconciling estimates of these different components have been assessed by House et al. (2003). An update on the global carbon budget is provided by Sabine et al. (Chapter 2, this volume).

From a scientific as well as an observational viewpoint, it is also important to determine the temporal variability underlying these global, decadal average quantities in the carbon budget. A recent compilation by Le Quéré et al. (2003) compares the various top-down and bottom-up methods for the ocean uptake. By difference from the atmospheric increase and estimates of the annual fossil-fuel emissions, the net land-atmosphere flux can also be inferred. Colorplate 7 shows the resulting interannual variability for the global ocean and terrestrial budgets (displayed as anomalies), using the different methods. In the panels on the right side, the time series have been smoothed to show only the decadal trends.

The main conclusion from this global analysis is that the terrestrial interannual and decadal variability is about five times larger than the variability of the oceanic fluxes. This result implies that the main interannual features in the atmospheric record, which are clearly correlated with large-scale climatic variations, such as events related to El Niño—Southern Oscillation and the climate anomaly after the Pinatubo volcanic eruption, are primarily of terrestrial origin.

The Anthropogenic Perturbation

The direct anthropogenic imprint on the global carbon cycle through the emissions of CO2 from fossil-fuel use and cement production is relatively well characterized based on energy production and energy use statistics (Andres et al. 2000). In addition to increasing the global concentration of CO2, the anthropogenic perturbation has a direct effect on atmospheric CO2 through the generation of a concentration gradient between the two hemispheres. One of the most compelling arguments for an anthropogenic cause of the current rise in atmospheric CO2 is provided by the observations of this gradient from the two longest direct atmospheric records available: Mauna Loa station on Hawaii and the South Pole (Keeling and Whorf2003). Both stations are reasonably representative for their respective hemispheres. Plotting this concentration difference against the difference in fossil-fuel emissions in the two hemispheres shows a remarkable linearity over the full range of the 41-year record (Figure 8.1). This behavior is exactly what would be expected if all non-fossil-fuel CO2 sources and sinks were distributed uniformly over the entire earth (Keeling et al. 1989; Fan et al. 1999). The scatter of the points around the regression line indicates the level of variability of the interhemispheric asymmetry of the non-fossil-fuel sources. If most of the interannual variability is caused by terrestrial sources, the scatter reflects the Northern Hemisphere, extra-tropical, terrestrial sources. Tropical sources, because of their approximate meridional symmetry, would not strongly affect the interhemispheric concentration difference. A quantitative analysis, however, requires the use of an inverse modeling framework (to be discussed later in this chapter).

Extrapolating the concentration difference back to zero emissions leaves a small offset, with the South Pole concentration about 0.6 ppm higher than at Mauna Loa. The origin of this offset, originally noticed by Bacastow and Keeling (1981), is not clear. It may be the result of an interhemispherically asymmetric configuration of natural sources and sinks, or it may be caused by rectification effects generated through sea-

Figure 8.1. Annual mean atmospheric CO2 concentration difference between the Mauna Loa and the South Pole stations (y axis, observations from Keeling et al. 1995, updated) shown against interhemispheric difference in annual fossil-fuel emissions (x axis, data from Andres et al. 2000, updated).

sonally varying transport patterns and seasonally varying sources (Hyson et al. 1980; Heimann et al. 1986; Keeling et al. 1989; Denning et al. 1995; Fan et al. 1999).

Top-Down Regional Estimates

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