The Missing Carbon Sink

For the decade of the 1990s, 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
3.2 (±0.2) = 6.3 (±0.4) + 2.2 (±0.8) - 2.4 (±0.7) - 2.9 (±1.1)

Attention on the global carbon cycle over more than 30 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 8.5 PgC during the 1990s (6.3± 0.4 Pg from combustion of fossil fuels and 2.2± 0.8 Pg from changes in land use) are greater than the sum of the annual accumulation of carbon in the atmosphere (3.2 ± 0.2) and the annual uptake by the oceans (2.4 ± 0.7 PgC/yr). An additional sink of 2.9 PgC/yr is required for balancing the budget. The terms in the global carbon equation can be shown graphically over the period 1850-2000 [Figure 1].

Flux of Carbon

Figure 1 - select image for larger version (opens in separate window)

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 northern mid-latitude terrestrial ecosystems, although estimates of the magnitude and location of the accumulation vary among the analyses.

In the tropics (where forest inventories are rare), the total net flux of carbon from changes in land use (2.2 PgC/yr) is consistent with recent estimates of flux based on atmospheric data.

Globally, terrestrial ecosystems are calculated to have been a net sink of 0.7 (±0.8) PgC/yr to the atmosphere during the 1990s. For the period 1850 to 2000, 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 emissions = Atmospheric increase + Oceanic uptake - Terrestrial net release
275 = 175 + 140 - 40

The terrestrial net release may be the result of different processes, however:

Terrestrial net release = Release from land-use change - Accumulation in undisturbed ecosystems
40 = 155 - 115

The last term may, again, be referred to as the missing carbon sink because it has not been observed.