Abstract

In this chapter, we focus on the land sink of anthropogenic CO2, because humans have a history of using the terrestrial biosphere for our purpose and because efforts to control atmospheric CQ> levels involve deliberate manipulation of the biosphere. We present atmospheric evidence for the land sink and use information about its intcrannual variations to infer its stability.

3.1 Introduction

The \launa Loa CO2 record is a clear documentation of the increasing concentration of C02 in the atmosphere as a result of anthropogenic activities. By 1999, the atmospheric COj abundance had increased by 25% since the beginning of the pre-industrial era. The cumulative increase, together with the concomitant increase in CI I4, NiO, CFCs, and other greenhouse gases, presents a total radiative forcing of —2-3 W/nr to the climate system in the 1990s, This forcing is countered to some degree by the increase in sulphate and other aerosols in the atmosphere*

The decreasing HC/12C ratio in tree rings (Suess, 1955) proves that the atmospheric Ct)2 increase is d ue to the addition of fossil (14 C-free) carbon. However, the CO* increase rate, as determined from the atmospheric record, is only 50%-60% that emitted by fossil fuel combustion (Figure 3.1). Thus, the land and oceans have absorbed the remainder of the fossil fuel CO2 as well as the CO2 released due to land use modification. It is important to figure out the partitioning of the carbon sink between the land and sea inasmuch as the residence time of carbon is shorter on land than that in the oceans. Terrestrial storage may be transient and easy to destabilize with climate warming.

3.2 Evidence for a Land Sink

The existence of a land sink for anthropogenic COz has been inferred since the early carbon budget calculations using one-dimensional ocean models (e.g., Oeschger et aL, 1975). Because of the heterogeneity of the land surface and the lack of terrestrial flux measurements of long duration, direct detection of the land sink has been difficult. The most u neon trovers ial evidence for the land sink is now found in the changing ratio of

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and its magnitude must be added to the displayed sink strength to \ ield the actual sink for anthropogenic COj.

O2/N2 in the atmosphere (Keeling and Shertz, 1992), Atmospheric O2 concentration (and hence the O2/N2 ratio) decreases with the combustion of fossil fuels and with the net uptake of carbon by the terrestrial biosphere. The O2/N2 to CO2 ratios are different for the two processes, but O2/N2 ratios are not altered by atmosphere-ocean carbon exchange. Using the high-precision measu rem ants of O2/N2 in the atmosphere, Keeling and Shertz (1992) infer, for the period 1991-1994, a net carbon uptake of 2.0 ± 0.9 PgC/y by the terrestrial biosphere. This translates into an anthropogenic carbon sink of 3.6 PgC/y for a deforestation source of 1.6 PgC/y. As a residual, the ocean sink for the period is 1.7 ± 0.9 PgC/y.

Atmospheric variations could also potentially distinguish between the terrestrial and oceanic sinks (Keeling et al., 1995; Francey et al,, 1996), Atmospheric has been decreasing from —6.4%oin the preindustrial era to — 8%uin the 1990s with the addition of fossil fuel carbon, which has an isotopic value of —25%o, that of the plant material from which fossil fuels arc derived. The observed atmospheric <5nC decrease is less than expected if all the fossil fuel CO* remained airborne, further supporting the need for terrestrial and oceanic uptake, which remove "lighter" carbon and leave an atmosphere with a "heavier" 1 ^C/^C ratio. However, constraining the ocean-land par^ titioning of the carbon sink requires information about (1) the isotopic disequilibrium associated with the gross fluxes and (2) the discrimination of the land uptake processes. These are discussed below.

Gross fluxes, assumed to cancel in CO?, leave a signature in the atmosphere. Fluxes from the atmosphere to the land or ocean carry today's "light" nC/'"C ratios, whereas the returning fluxes are "heavier," with the difference determined by aiepwan, 3amnineHHbiM aBTopcKMM npaBOM

the age of the donor pools. There are considerable uncertainties in the magnitudes and distributions of the isotopic disequilibrium associated with gross terrestrial and oceanic exchanges (Fung et al., 1997). The oceanic isotopic disequilibrium is governed principally by the oceanic circulation. The isotopic disequilibrium associated with the biosphere depends on the age of the respired carbon, to which the old, slowly decomposing soil carbon pools contribute a small percentage of the total flux but contribute a significant weighting of the <5' 'C signature. In tropical forests, the disequilibrium is large even though carbon turnover ts fast in the soil, because trees there live for 30-41) years. In the tundra, the disequilibrium is also large, because decomposition is slow in the cold climate. Fung et al. (1997) used a comprehensive biogeochemistry model to study factors contributing to isotopic disequilibrium associated with terrestrial fluxes. They obtained a value of 24%o PgC/y that is larger than those used in previous studies (Tans et al, 1993; Keeling et al, 1995; Francev et al, 1996). With their estimate, the isotopic disequilibrium associated with the land and ocean gross fluxes explains about half the discrepancy between the observed atmospheric decrease and the source contribution {Figure 3.2), A greater degree of isotopic disequilibrium would contribute to a "heavy" atmospheric <5 l iC and imply a smaller net uptake by the biosphere.

Because nC discrimination by terrestrial uptake is —10 times greater than by oceanic uptake, it is not unreasonable to anticipate a land carbon sink to contribute to the observed atmospheric &UC trend. However, if one assumes at this point that terrestrial photosynthesis has a discrimination of 18%o, one obtains a smaller oceanic uptake than obtained from the O2/N2 analysis. The dilemma stems from the uncertainty about the type of vegetation responsible for the carbon uptake. Cj and C4 vegetation have different photosynthetic pathways and hence different degrees of discrimination against ,JC during photosynthesis. ¿,jC of Cj plants varies with ambient temperature and humidity and CO2 levels but is around —25%o whereas that for Q plants is around — 1 2%o . C4 photosynthesis thus imparts an isotopic signature in the atmosphere that is similar to oceanic uptake. By combining their C isotopic disequilibria with the ocean-land sink partitioning derived from O2/N2 by Keeling and Shertz (1992), Fung et al. (1997) estimate that M5% of the terrestrial uptake is by C4 plants. Corn, legumes, and some grasses are C4 plants.

In summary, the O2/N2 and S] SC records show that the terrestrial biosphere and oceans have removed the anthropogenic CO2 from the atmosphere. Not all the terrestrial uptake is by forests (Cj vegetation); a small fraction may have occurred in grasslands and agricultural lands.

3.3 Global CO2 Variations

In this section, we explore the interannual variations of the contemporary carbon budget, and we use the variations to deduce how the sinks have responded to climate perturbations. The globally integrated budget ofatmospheric CO2 can be written as follows:

Its variation from 1980 to 1994 is shown in Figure 3.1. Fossil fuel emission of CO2 is estimated from United Nations statistics (Andres et al, 1996), and the atmospheric

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Deforestation

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