The Current Global Carbon Cycle and the Terrestrial Carbon Sink

Terrestrial biomass represents a dynamic and therefore important carbon pool, turning over relatively quickly compared with other carbon pools with an average residence time of 10 years (Gruber et al., 2004). Estimates of the amount of carbon stored globally in terrestrial vegetation and in specific biomes vary significantly (Table 2.1). Two recent estimates are 466 Pg C (Watson et al., 2000) and 652 Pg C (Saugier et al., 2001), amounts broadly similar to that found in the atmosphere, but only 25% of that stored in soils. Regardless of the estimate, ~80% of the carbon is stored in the world's forests, with 45-50% in tropical forests alone. Set against these carbon stocks the biosphere and the atmosphere exchange ~120 Pg C annually. Sabine et al. (2004) estimated annual global NPP of 57 Pg C averaged throughout the 1990s (~40% of GPP). This figure was almost balanced by 55.5 Pg C/year lost from terrestrial ecosystems through respiration, fire and land-use change. The small difference between these figures is the net land-atmosphere flux, an annual estimate of NEP which was estimated at 1.4 Pg C/year by Prentice et al. (2001) throughout the 1990s (Table 2.2). Carbon sequestration of this magnitude represented ~20% of the carbon being emitted from fossil fuel burning and cement production annually through the 1990s (Table 2.2), and is a significant increase on the NEP estimate of 0.2 Pg C/year during the 1980s (Table 2.2). While informative, this global estimate of NEP is continually being revised with other recent studies providing estimates of annual global NEP between 1.2 ± 0.8 (Le Quere et al., 2003) and 3.4, ranging from 1.8 to 5.0 Pg C/year (Houghton, 2003).

Table 2.1. Net primary productivity (Pg C/year) and carbon stored in plant biomass (Pg C) for the world's major biomes.

NPPa

Plant Cb

Plant Cc

Tropical forests

20.1

340

212

Temperate forests

7.4

139

59

Boreal forests

2.4

57

88

Arctic tundra

0.5

2

6

Mediterranean shrub

1.3

17

-

Crops

3.8

4

3

Tropical savannahs

and grasslands

13.7

79

66

Temperate grasslands

5.1

6

9

Deserts

3.2

10

8

Wetlands

-

-

15

Total

57

652

466

aFrom Sabine et al. (2004). bFrom Saugier et al. (2001). cFrom Watson et al. (2000).

aFrom Sabine et al. (2004). bFrom Saugier et al. (2001). cFrom Watson et al. (2000).

Estimates of the terrestrial carbon sink are made by subtracting the measured increase in Ca from total anthropogenic emissions. The difference is the combined uptake of the oceans and terrestrial biosphere. Partitioning carbon sequestration between the ocean and land is subsequently achieved through simultaneous high-precision measurements of atmospheric CO2, 13CO2, O2 and 18O2 (recently reviewed in Ciais et al., 2005). Once determined, the terrestrial sink can be partitioned between that associated with changes in land-use and that associated with increasing carbon sequestration in existing ecosystems. Throughout the 1980s and 1990s land-use change resulted in significant fluxes of carbon to the atmosphere; consequently NEP of terrestrial ecosystems not undergoing change was significantly higher, estimated at 1.9 Pg C throughout the 1980s (Prentice et al. 2001; Table 2.2). Of these key steps to determining global productivity there are two notable areas of uncertainty. The first is the need for improved understanding of the processes controlling the changing ratios of 12C/13C and 16O/18O in the atmosphere (Ciais et al., 2005; Randerson, 2005). The second is the need for improved estimates of land-use change and the type of land-use change. Houghton (2003) recently suggested that the proportion of the carbon sink in North America that is due to regrowth following land-use change could be as high as 98% (Casperson et al., 2000) or as low as 40% (Pacala et al., 2001) depending on land use figures used for the calculation.

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