Regional

Estimating carbon balances of large regions is no trivial matter and is increasingly done by trying to reconcile results gained at two levels of enquiry: 'top-down' atmospheric

Table 2.2. The global carbon budget (Pg C/year).

1980s 1990s

Table 2.2. The global carbon budget (Pg C/year).

1980s 1990s

Atmospheric increase

+3.3 ± 0.1

+3.2 ± 0.1

Emissions (fossil fuels, cement)

+5.4 ± 0.3

+6.3 ± 0.4

Ocean-atmosphere flux

-1.9 ± 0.6

-1.7 ± 0.5

Land

Carbon uptake (NPP)

57

Carbon loss (TER + fire)

55.5

Net land-atmosphere flux (NEP)

-0.2 ± 0.7

-1.4 ± 0.7

Land-use change

+1.7 ± (+0.6 to + 2.5)

-

Residual terrestrial sink

-1.9 ± (-3.8 to + 0.3)

-

All values are taken from Prentice et al. (2001), except those for carbon uptake and carbon loss from the land, which are from Sabine et al. (2004). Positive numbers represent an atmospheric increase in carbon (source), and negative numbers an atmospheric decrease in carbon (sink).

All values are taken from Prentice et al. (2001), except those for carbon uptake and carbon loss from the land, which are from Sabine et al. (2004). Positive numbers represent an atmospheric increase in carbon (source), and negative numbers an atmospheric decrease in carbon (sink).

inversions, and 'bottom-up' scaling up of direct NEE measurements or inventory measurements that track changes in biomass (typically wood) through time (Pacala et al., 2001; Janssens et al., 2003).

Of the world's major biomes, tropical forest is thought to be the most productive area of the earth, followed by tropical grasslands and savannah (Saugier et al., 2001; Table 2.1). However, NPP does not necessarily provide a good measure for the amount of carbon that is sequestered within a biome over a given period of time, as it excludes all losses of CO2 from the ecosystem via processes other than leaf respiration. Although tropical forests unequivocally have the highest NPP of the earth's biomes, estimates of NEP are more equivocal. Inverse models suggest that the tropics appear to be somewhere between a small net source of carbon to the atmosphere and carbon neutral (Gurney et al., 2002). However, there are still challenges to reconcile this finding with ground-based data. For example, lowland humid tropical forests appear to be accumulating woody biomass at a mean rate of 3.1 t C/ha/year (Malhi et al., 2004). Although direct measurements of NEP at some study sites suggest a strong sink (Grace et al., 1995; Malhi et al., 1998), the same measurements at others do not (Saleska et al., 2003). Saleska et al. (2003) highlighted significant losses of carbon associated with disturbance, specifically from fallen trees during the wet season, as an important factor that reduced NEP. Seemingly there are clear limitations to the extent to which tropical forest can be thought of as a homogeneous biome for the purposes of scaling up. Uncertainties associated with estimates of deforestation within the tropics (Houghton, 2003; Grace, 2004), losses of CO2 from study areas, e.g. in solution in rivers, and different temporal durations of field studies, some of which may be made during unusual climatic conditions or different times after disturbance, all complicate reconciling ground-based measurements with estimates from atmospheric inversions. In light of these uncertainties, Houghton (2003) concluded that the small sink for carbon in the tropics was the result of a large release of carbon from land-use change that is being partially offset by a large sink for carbon in undisturbed forests. A second, mutually exclusive, possibility is that the source of CO2 from deforestation is smaller and the net accumulation of carbon in undisturbed forests is essentially zero.

In contrast to the tropics, there is compelling evidence that the northern temperate regions of the earth are sequestering carbon. Houghton (2003) describes how a northern mid-latitude carbon sink of 2 Pg C/year appears robust. For the northern USA alone Pacala et al. (2001) reconciled top-down estimates of the carbon sink of the continental USA with bottom-up estimates that included land-use change data and forest inventories. The study concluded that the continental USA was a substantial sink for carbon of 0.15-0.35 Pg C/year, of which accumulation of wood and other biomass was the largest component of the sink. Similar studies have been conducted for Europe. Janssens et al. (2003) calculated a European carbon sink of 0.11 Pg C/year from inventory studies, and a mean atmospheric inversion estimate of 0.29 Pg C/year, thereby offsetting 7-12% of Europe's CO2 emissions. Intercontinental trade in organic products was thought to largely account for the differences in the two estimates. Other estimates are available. Papale and Valentini (2003) used an artificial neural network to estimate that European forests were sequestering 0.47 Pg C/year.

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