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ABSTRACT

Rates of land-use change, including clearing for agriculture and harvest of wood, were reconstructed from statistical and historic documents for 9 world regions and used, along with the per ha changes in vegetation and soil that result from land management, to calculate the annual flux of carbon between land and atmosphere. Between 1850 and 1990, changes in land use are calculated to have added 124 PgC to the atmosphere, about half as much as released from combustion of fossil fuels over this period. About 108 PgC are estimated to have been transferred from forests to the atmosphere as a result of human activity, 2/3 from tropical forests and 1/3 from temperate zone and boreal forests. Another 16 PgC were lost from non-forests, largely as a result of cultivation of mid-latitude grassland soils. About 800 × 106 ha of forest were cleared for agricultural purposes, and approximately 2000 × 106 ha were harvested. Conversion of forests to agricultural lands released 105 PgC; harvest of wood released about 20 PgC. These estimates of release include the accumulations of carbon in wood products (17 PgC) and woody debris (4 PgC), the losses of carbon from oxidation of wood products, woody debris, and soil organic matter (373 PgC in total), and the accumulations of carbon in forests recovering from harvest and in the fallows of shifting cultivation (249 PgC). Over the decade of the 1980s the annual net flux of carbon from changes in land use averaged about 2.0 PgC yr−1, higher than the 1.6 PgC yr−1 estimated previously. Almost all of this flux was from tropical regions, where rates of deforestation averaged approximately 15 × 106 ha yr−1. Outside the tropics, regrowth of forests logged in earlier years largely balanced the losses of carbon from oxidation of wood products.