For the decade of the 1980s, the global carbon cycle can be summarized as follows (units are PgC. One Pg [petagram] = one billion metric tonnes = 1000 x one billion kg ): Atmospheric increase = Emissions from fossil fuels + Net emissions from changes in land use - Oceanic uptake - Missing carbon sink 3.3(±0.2) 5.5(±0.5) 1.6(±0.7) 2.0(±0.8) 1.8(±1.2) Attention on the global carbon cycle over more than 25 years has focused on the apparent imbalance in the carbon budget in the above equation - the so-called "missing sink," missing because the accumulation of carbon has not been observed. The average annual emissions of 7.1 PgC during the 1980s (5.5 ± 0.5 Pg from combustion of fossil fuels and 1.6 ± 0.7 Pg from changes in land use) are greater than the sum of the annual accumulation of carbon in the atmosphere (3.3 ± 0.2) and the annual uptake by the oceans (2.0 ± 0.8 PgC/yr). An additional sink of 1.8 PgC/yr is required for balancing the budget. The terms in the global carbon equation can be shown graphically over the period 1850-1990 [Figure 1].

		Figure 1

In the last few years several independent analyses based on geochemical data (data from the atmosphere and oceans) and a series of carbon budgets based on data from forest inventories have shown that carbon is accumulating in terrestrial ecosystems, although estimates of the magnitude and location of the accumulation vary among the analyses. Direct evidence for an uptake of carbon from measurements on the ground (forest inventories) is only available from countries of the northern mid-latitudes, and a similar but unmeasured uptake may also be occurring in the tropics, where forest inventories are rarely carried out. Two approaches have been used to calculate changes in terrestrial carbon storage with data obtained from terrestrial ecosystems, rather than with atmospheric or oceanographic data. One approach is based on the changes in carbon that result from changes in land use (conversion of forest to agricultural land, abandonment of agricultural land, harvest and regrowth). This is the approach we use at the Center. The other approach uses measurements of forest biomass obtained through forests inventories to determine change directly. These latter studies may also calculate changes in the amount of carbon stored in wood products and soil, but in this respect the two approaches are similar. If a significant fraction of the missing carbon sink is to be found in mid-latitude forests, one would expect direct measurement of biomass to show greater accumulations of carbon than analyses in which calculated accumulations result only from regrowth following previous harvests or abandonment of agricultural land. Data from Canada, the conterminous U.S., Europe, and the former U.S.S.R. show this is indeed the case. Accumulations of carbon in biomass and soil are 0.5-0.8 PgC/yr greater than expected from past management practices (land-use change). In the tropics (where forest inventories are rare), the total net flux of carbon from changes in land use (1.6 PgC/yr) is consistent with recent estimates of flux based on atmospheric data, but the geographic distribution of the flux is not the same. Globally, terrestrial ecosystems are calculated to have been a net source of 1.0 to 1.3 PgC/yr to the atmosphere during the 1980s. For the period 1850 to 1990, a geochemical summary of the global carbon cycle is as follows (the terrestrial term having been determined indirectly by difference) (units are PgC):

Fossil fuel = Atmospheric + Oceanic - Terrestrial emissions increase uptake net release 220 (±22) 145 (±10) 100 (±25) 25 (±25)

	The terrestrial net release may be the result of different processes, however: Terrestrial = Release from + Accumulation in net release land-use change undisturbed ecosystems 25 (±25) 120 (±30) 95 (±55) The last term may, again, be referred to as the missing carbon sink because it has not been observed.
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