Measuring tropical deforestation: Development of the methods

Estimates of the rate and extent of tropical deforestation differ widely. The accuracy of the estimates for many countries has improved greatly during the last decade, especially with increased use of satellite data and advancements in analysis techniques. A good basis for ongoing global monitoring of tropical forests has been established by the FAO in its Forest Resources Assessment for 1990 (though data for some countries are still inadequate). The FAO assessment estimates that over one million km(2) of the Earth's tropical rain-forest and moist deciduous forest was destroyed during 1981-90, representing an annual deforestation rate of 0.75% of such forests throughout the decade. This paper traces the controversy over measurements of deforestation in Brazil's Legal Amazon and the resulting confusion and exaggeration of research findings (in both directions). Some widely-cited estimates are highly inaccurate, indicating a need for critical evaluation of any estimates that are used. A substantial narrowing of the range of estimates is found in recent studies. Landsat data indicate that the deforestation rate in the Legal Amazon declined considerably from 1988 to 1991. Accurate estimates of world-wide tropical deforestation are needed to assess the impact of deforestation on the buildup of 'greenhouse' gases and to monitor compliance with any protocols to be negotiated under the 1992 Framework Convention on Climate Change. Ongoing collection and analysis of satellite data is needed, and methods are being developed to archive and analyse efficiently the vast amounts of data involved. In promoting such a monitoring effort, it is important that scientists be sensitive to the political ramifications in developing countries, involve local experts in planning and implementation, and emphasize the local benefits of the data to be obtained.