Terrestrial sources and sinks of carbon inferred from terrestrial data

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). 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 US, Europe, and the former USSR show this circumstance to be correct. Accumulations of carbon in biomass and soil are 0.8 PgC yr^-1 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^-1) 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 0.8 ± 0.6 PgC yr^-1 during the 1980s.