7.3.2.2 Uptake of CO₂ by Natural Reservoirs and Global Carbon Budget

7.3.2.2.1 Ocean-atmosphere flux

To assess the mean ocean sink, seven methods have been used. The methods are based on: (1) observations of the partial pressure of CO₂ at the ocean surface and gas-exchange estimates (Takahashi et al., 2002); (2) atmospheric inversions based upon diverse observations of atmospheric CO₂ and atmospheric transport modelling (see Section 7.2.3.4); (3) observations of carbon, oxygen, nutrients and chlorofluorocarbons (CFCs) in seawater, from which the concentration of anthropogenic CO₂ is estimated (Sabine et al., 2004a) combined with estimates of oceanic transport (Gloor et al., 2003; Mikaloff Fletcher et al., 2006); (4) estimates of the distribution of water age based on CFC observations combined with the atmospheric CO₂ history (McNeil et al., 2003); (5) the simultaneous observations of the increase in atmospheric CO₂ and decrease in atmospheric O₂ (Manning and Keeling, 2006); (6) various methods using observations of change in ¹³C in the atmosphere (Ciais et al., 1995) or the oceans (Gruber and Keeling, 2001; Quay et al., 2003); and (7) ocean General Circulation Models (Orr et al., 2001). The ocean uptake estimates obtained with methods (1) and (2) include in part a flux component due to the outgassing of river-supplied inorganic and organic carbon (Sarmiento and Sundquist, 1992). The magnitude of this necessary correction to obtain the oceanic uptake flux of anthropogenic CO₂ is not well known, as these estimates pertain to the open ocean, whereas a substantial fraction of the river-induced outgassing likely occurs in coastal regions. These estimates of the net oceanic sink are shown in Figure 7.3.

With these corrections, estimates from all methods are consistent, resulting in a well-constrained global oceanic sink for anthropogenic CO₂ (see Table 7.1). The uncertainty around the different estimates is more difficult to judge and varies considerably with the method. Four estimates appear better constrained than the others. The estimate for the ocean uptake of atmospheric CO₂ of –2.2 ± 0.5 GtC yr^–1 centred around 1998 based on the atmospheric O₂/N₂ ratio needs to be corrected for the oceanic O₂ changes (Manning and Keeling, 2006). The estimate of –2.0 ± 0.4 GtC yr^–1 centred around 1995 based on CFC observations provides a constraint from observed physical transport in the ocean. These estimates of the ocean sink are shown in Figure 7.6. The mean estimates of –2.2 ± 0.25 and –2.2 ± 0.2 GtC yr^–1 centred around 1995 and 1994 provide constraints based on a large number of ocean carbon observations. These well-constrained estimates all point to a decadal mean ocean CO₂ sink of –2.2 ± 0.4 GtC yr^–1 centred around 1996, where the uncertainty is the root mean square of all errors. See Section 5.4 for a discussion of changes in the ocean CO₂ sink.

Figure 7.6. Individual estimates of the ocean-atmosphere flux reported in Chapter 5 and of the related land-atmosphere flux required to close the global carbon budget. The dark thick lines are the revised budget estimates in the AR4 for the 1980s, the 1990s and the early 2000s, respectively.

7.3.2.2.2 Land-atmosphere flux

The land-atmosphere CO₂ flux is the sum of the land use change CO₂ flux (see Section 7.3.2.1) plus sources and sinks due for instance to legacies of prior land use, climate, rising CO₂ or N deposition (see Section 7.3.3 for a review of processes). For assessing the global land-atmosphere flux, more than just direct terrestrial observations must be used, because observations of land ecosystem carbon fluxes are too sparse and the ecosystems are too heterogeneous to allow global assessment of the net land flux with sufficient accuracy. For instance, large-scale biomass inventories (Goodale et al., 2002; UN-ECE/FAO, 2000) are limited to forests with commercial value, and they do not adequately survey tropical forests. Direct flux observations by the eddy covariance technique are only available at point locations, most do not yet have long-term coverage and they require considerable upscaling to obtain global estimates (Baldocchi et al., 2001). As a result, two methods can be used to quantify the net global land-atmosphere flux: (1) deducing that quantity as a residual between the fossil fuel and cement emissions and the sum of ocean uptake and atmospheric increase (Table 7.1), or (2) inferring the land-atmosphere flux simultaneously with the ocean sink by inverse analysis or mass balance computations using atmospheric CO₂ data, with terrestrial and marine processes distinguished using O₂/N₂ and/or ¹³C observations. Individual estimates of the land-atmosphere flux deduced using either method 1 or method 2 are shown in Figure 7.6. Method 2 was used in the TAR, based upon O₂/N₂ data (Langenfelds et al., 1999; Battle et al., 2000). Corrections have been made to the results of method 2 to account for the effects of thermal O₂ fluxes by the ocean (Le Quéré et al., 2003). This chapter includes these corrections to update the 1980s budget, resulting in a land net flux of –0.3 ± 0.9 GtC yr^–1 during the 1980s. For the 1990s and after, method 1 was adopted for assessing the ocean sink and the land-atmosphere flux. Unlike in the TAR, method 1 is preferred for the 1990s and thereafter (i.e., estimating first the ocean uptake, and then deducing the land net flux) because the ocean uptake is now more robustly determined by various oceanographic approaches (see 7.3.2.2.1) than by the atmospheric O₂ trends. The numbers are reported in Table 7.1. The land-atmosphere flux evolved from a small sink in the 1980s of –0.3 ± 0.9 GtC yr^–1 to a large sink during the 1990s of –1.0 ± 0.6 GtC yr^–1, and returned to an intermediate value of –0.9 ± 0.6 GtC yr^–1 over the past five years. A recent weakening of the land-atmosphere uptake has also been suggested by other independent studies of the flux variability over the past decades (Jones and Cox, 2005). The global CO₂ budget is summarised in Table 7.1.

7.3.2.2.3 Residual land sink

In the context of land use change, deforestation dominates over forest regrowth (see Section 7.3.2.1), and the observed net uptake of CO₂ by the land biosphere implies that there must be an uptake by terrestrial ecosystems elsewhere, called the ‘residual land sink’ (formerly the ‘missing sink’). Estimates of the residual land sink necessarily depend on the land use change flux, and its uncertainty reflects predominantly the (large) errors associated with the land use change term. With the high land use source of Houghton (2003a), the residual land sink equals –2.3 (–4.0 to –0.3) and –3.2 (–4.5 to –1.9) GtC yr^–1 respectively for the 1980s and the 1990s. With the smaller land use source of DeFries et al. (2002), the residual land sink is –0.9 (–2.0 to –0.3) and –1.9 (–2.9 to –1.0) GtC yr^–1 for the 1980s and the 1990s. Using the mean value of the land use source from Houghton (2003a) and DeFries et al. (2002) as reported in Table 7.2, a mean residual land sink of –1.7 (–3.4 to 0.2) and –2.6 (–4.3 to –0.9) GtC yr^–1 for the 1980s and 1990s respectively is obtained. Houghton (2003a) and DeFries et al. (2002) give different estimates of the land use source, but they robustly indicate that deforestation emissions were 0.2 to 0.3 GtC yr^–1 higher in the 1990s than in the 1980s (see Table 7.2). To compensate for that increase and to match the larger land-atmosphere uptake during the 1990s, the inferred residual land sink must have increased by 1 GtC yr^–1 between the 1980s and the 1990s. This finding is insensitive to the method used to determine the land use flux, and shows considerable decadal variability in the residual land sink.

7.3.2.2.4 Undisturbed tropical forests: are they a carbon dioxide sink?

Despite expanding areas of deforestation and degradation, there are still large areas of tropical forests that are among the world’s great wilderness areas, with fairly light human impact, especially in Amazonia. A major uncertainty in the carbon budget relates to possible net change in the carbon stocks in these forests. Old growth tropical forests contain huge stores of organic matter, and are very dynamic, accounting for a major fraction of global net primary productivity (and about 46% of global biomass; Brown and Lugo, 1982). Changes in the carbon balance of these regions could have significant effects on global CO₂.

Recent studies of the carbon balance of study plots in mature, undisturbed tropical forests (Phillips et al., 1998; Baker et al., 2004) report accumulation of carbon at a mean rate of 0.7 ± 0.2 MgC ha^–1 yr^–1, implying net carbon uptake into global Neotropical biomass of 0.6 ± 0.3 GtC yr^–1. An intriguing possibility is that rising CO₂ levels could stimulate this uptake by accelerating photosynthesis, with ecosystem respiration lagging behind. Atmospheric CO₂ concentration has increased by about 1.5 ppm (0.4%) yr^–1, suggesting incremental stimulation of photosynthesis of about 0.25% (e.g., next year’s photosynthesis should be 1.0025 times this year’s) (Lin et al., 1999; Farquhar et al., 2001). For a mean turnover rate of about 10 years for organic matter in tropical forests, the present imbalance between uptake of CO₂ and respiration might be 2.5% (1.0025¹⁰), consistent with the reported rates of live biomass increase (~3%).

But the recent pan-tropical warming, about 0.26°C per decade (Malhi and Wright, 2004), could increase water stress and respiration, and stimulation by CO₂ might be limited by nutrients (Chambers and Silver, 2004; Koerner, 2004; Lewis et al., 2005; see below), architectural constraints on how much biomass a forest can hold, light competition, or ecological shifts favouring short lived trees or agents of disturbance (insects, lianas) (Koerner, 2004). Indeed, Baker et al. (2004) note higher mortality rates and increased prevalence of lianas, and, since dead organic pools were not measured, effects of increased disturbance may give the opposite sign of the imbalance inferred from live biomass only (see, e.g., Rice et al., 2004). Methodological bias associated with small plots, which under-sample natural disturbance and recovery, might also lead to erroneous inference of net growth (Koerner, 2004). Indeed, studies involving large-area plots (9–50 ha) have indicated either no net long-term change or a long-term net decline in above ground live biomass (Chave et al., 2003; Baker et al., 2004; Clark, 2004; Laurance et al., 2004), and a five-year study of a 20 ha plot in Tapajos, Brazil show increasing live biomass offset by decaying necromass (Fearnside, 2000; Saleska et al., 2003).

Koerner (2004) argues that accurate assessment of trends in forest carbon balance requires long-term monitoring of many replicate plots or very large plots; lacking these studies, the net carbon balance of undisturbed tropical forests cannot be authoritatively assessed based on in situ studies. If the results from the plots are extrapolated for illustration, the mean above ground carbon sink would be 0.89 ± 0.32 MgC ha^–1 yr^–1 (Baker et al., 2004), or 0.54 ± 0.19 GtC yr^–1 (Malhi and Phillips 2004) extrapolated to all Neotropical moist forest area (6.0 × 10⁶ km²). If the uncompiled data from the African and Asian tropics (50% of global moist tropical forest area) were to show a similar trend, the associated tropical live biomass sink would be about 1.2 ± 0.4 GtC yr^–1, close to balancing the net source due to deforestation inferred by DeFries et al. (2002) and Achard et al. (2004) (Table 7.2).

7.3.2.2.5 New findings on the carbon budget

The revised carbon budget in Table 7.1 shows new estimates of two key numbers. First, the flux of CO₂ released to the atmosphere from land use change is estimated to be 1.6 (0.5 to 2.7) GtC yr^–1 for the 1990s. A revision of the TAR estimate for the 1980s (see TAR, Chapter 3) downwards to 1.4 (0.4 to 2.3) GtC yr^–1 suggests little change between the 1980s and 1990s, but there continues to be considerable uncertainty in these estimates. Second, the net residual terrestrial sink seems to have been larger in the 1990s than in the periods before and after. Thus, a transient increase in terrestrial uptake during the 1990s explains the lower airborne fraction observed during that period. The ocean uptake has increased by 22% between the 1980s and the 1990s, but the fraction of emissions (fossil plus land use) taken up by the ocean has remained constant.