LAND-USE CHANGE AND THE CARBON-CYCLE

Changes in land use between 1850 and 1980 are estimated to have increa sed the global areas in croplands, pastures, and shifting cultivation by 891, 1308, and 30 x 10(6) ha, respectively, reducing the area of fo rests by about 600 x 10(6) ha, releasing about 100 PgC to the atmosphe re, and transferring about 23 PgC from live vegetation to dead plant m aterial and wood products. Another 1069 x 10(6) ha are estimated to ha ve been logged during this period, and the net release of carbon from the combined processes of logging and regrowth contributed 23 PgC to t he 100-PgC release. Annual rates of land-use change and associated emi ssions of carbon have decreased over the last several decades in tempe rate and boreal zones and have increased in the tropics. The average r elease of carbon from global changes in land use over the decade of th e 1980s is estimated to have been 1.6 +/- 0.7 PgC y(-1), almost entire ly from the tropics. This estimate of carbon flux is higher than estim ates reported in recent summaries because it is limited here to studie s concerned only with changes in land use. Other recent analyses, base d on data from forest inventories, have reported net accumulations of carbon as high as 1.1 PgC y(-1) in temperate and boreal zones. Because the accumulation of carbon in forests may result from natural process es unrelated to land-use change, estimates based on these inventories should be distinguished from estimates based on changes in land use. B oth approaches identify terrestrial sinks of carbon. The argument is m ade here, however, that differences between the two approaches may hel p identify the location and magnitude of heretofore 'missing' sinks. B efore different estimates can be used in this way, analyses must consi der similar geographical regions and dates, and they must account for the accumulation and loss of carbon in forest products in a consistent fashion.