The two best-documented studies of annual rates of clearing of tropical forests appear to come to very different conclusions; the published FAO/UNEP estimate is 7.4 × 106 ha cleared annually during the late 1970s, and the NAS (National Academy of Sciences) estimate is 22.0 × 106 ha disturbed annually during the same period. Closer examination reveals that when corrections are made for differences in forest types included in the studies, and for differences in definitions of ‘deforestation’ (FAO/UNEP) and ‘conversion’ (NAS), there appears to be general agreement on the rate of deforestation in tropical closed broad-leafed forests: 5.9 × 106 ha (FAO/UNEP) and 7.5 × 106 ha (NAS) per year for the late 1970s. ‘Deforestation’ is defined according to the FAO/UNEP report and is the transformation of closed tropical-broad-leafed forests to either land in the shifting-cultivation cycle or permanently-cleared land.
‘Deforestation’ sensu FAO/UNEP is a more restricted term than the term ‘conversion’ used in the NAS report. ‘Conversion’ includes ‘deforestation’ sensu FAO/UNEP as well as: (1) the temporary clearing of fallow-cycle vegetation from land already in the shifting-cultivation cycle for short-term subsistence agriculture; and (2) the permanent clearing of fallow-cycle vegetation and thus the removal of land from the shifting-cultivation cycle.