Carbon and Changes in Land Use

Our work shows that for the years 1850 to 2000 about 155 PgC were released to the atmosphere as a result of changes in land use, 85% from forests either logged or converted to other uses, the rest largely from cultivation of prairie soils (One Pg [petagram]=one billion metric tonnes=1000 x one billion kg). The total loss of carbon from terrestrial ecosystems was 165 PgC, but about 15 PgC accumulated in wood products (e.g., buildings, furniture, paper, etc.). The net increment in these wood products is the difference between harvests (about 235 PgC) and oxidation of products (about 220 PgC) over this 150-year period. Conversion of natural ecosystems to croplands and pastures was responsible for net releases to the atmosphere of 107 and 21 PgC, respectively. The net effect of logging and regrowth was to release about 23 PgC. The annual rate at which carbon was released to the atmosphere generally increased over the period 1850 to 2000 as rates of deforestation increased [Figure 1].

Figure 1 (select image for larger version - JPG, 110KB)

For the decade of the 1990s the net release of carbon from changes in land use averaged 2.2±0.8 PgC/yr. In the 1990s the net flux of 2.2 PgC from changes in land use was the difference between a release of 5.2 PgC from oxidation of dead vegetation, soil organic matter, and wood products and an uptake by regrowing ecosystems of about 3.0 PgC/yr). For the decade of the 1990s, the global carbon cycle can be summarized as follows (units are PgC):

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)

For additional information on the "Missing carbon sink", see Missing Carbon.