Woods Hole Research Center

For the years 2000-2008, the global carbon cycle can be summarized as follows (units are PgC: one Pg [petagram]=10¹⁵ grams=one billion metric tonnes):

Atmospheric increase	=	Emissions from fossil fuels	+	Net emissions from changes in land use	-	Oceanic uptake	-	Residual carbon sink
4.1(±0.04)	=	7.7 (±0.4)	+	1.5 (±0.7)	-	2.3 (±0.4)	-	2.8 (±0.9)

 

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 "residual terrestrial sink," residual because the accumulation of carbon on land has not been measured but is calculated by difference from the other terms. The average annual emissions of 9.2 PgC during the period 2000-2008 (7.7± 0.4 Pg from combustion of fossil fuels and 1.5± 0.7 Pg from changes in land use) are greater than the sum of the annual accumulation of carbon in the atmosphere (4.1 ± 0.04) and the annual uptake by the oceans (2.3 ± 0.4 PgC/yr). An additional sink of 2.8 PgC/yr is required for balancing the budget. The terms in the global carbon equation can be shown graphically over the period 1850 to 2005 [Figure 1].

Figure 1

 

For the period 1850 to 2005, 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
336	=	208	+	158	-	30

 

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

 

Terrestrial net release	=	Release from land-use change	-	Accumulation in undisturbed ecosystems
30	=	155	-	125

 

The last term may, again, be referred to as the residual carbon sink because it is determined by difference (it has not been observed).