A recently completed research program (TREES) employing the global imaging capabilities of Earth-observing satellites provides updated information on the status of the world's humid tropical forest cover. Between 1990 and 1997, 5.8 +/- 1.4 million hectares of humid tropical forest were lost each year, with a further 2.3 +/- 0.7 million hectares of forest visibly degraded. These figures indicate that the global net rate of change in forest cover for the humid tropics is 23% lower than the generally accepted rate. This result affects the calculation of carbon fluxes in the global budget and means that the terrestrial sink is smaller than previously inferred.

Daily global observations from the Advanced Very High-Resolution Radiometers on the series of meteorological satellites operated by the National Oceanic and Atmospheric Administration between 1982 and 1999 were used to generate a new weekly global burnt surface product at a resolution of 8 km. Comparison with independently available information on fire locations and timing suggest that while the time-series cannot yet be used to make accurate and quantitative estimates of global burnt area it does provide a reliable estimate of changes in location and season of burning on the global scale. This time-series was used to characterize fire activity in both northern and southern hemispheres on the basis of average seasonal cycle and interannual variability. Fire seasonality and fire distribution data sets have been combined to provide gridded maps at 0.51 resolution documenting the probability of fire occurring in any given season for any location. A multiannual variogram constructed from 17 years of observations shows good agreement between the spatial-temporal behavior in fire activity and the 'El Niñ o' Southern Oscillation events, showing highly likely connections between both phenomena.

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Definition of appropriate tropical forest policies must be supported by better information about forest distribution. New information technologies make possible the development of advanced systems which can accurately report on tropical forest area issues. The European Commission TREES (Tropical Ecosystem Environment observation by Satellite) project has produced a consistent map of the humid tropical forest cover based on 1 km resolution satellite data. This base-line reference information can be further calibrated using a sample of high-resolution data, in order to produce accurate forest area estimates. There is good general agreement with other pantropical inventories (Food & Agriculture Organization of the United Nations Forest Resources Assessment 90, World Conservation Union Conservation Atlas of Tropical Forests, National Aeronautics & Space Administration [USA] Landsat Pathfinder) using different approaches (compilation of existing data, statistical sampling, exhaustive survey with satellite data). However, for some countries, large differences appear among the assessments. Discrepancies arising from this comparison are here analysed in terms of limitations associated with each approach and they are generally associated with differences in forest definition, data source and processing methodology. According to the different inventories, the total area of closed tropical forest is estimated at 1090–1220 million hectares with the following continental distribution: 185–215 million hectares in Africa, 235–275 million hectares in Asia, and 670–730 million hectares in Latin America. A proposal for improving the current state of forest statistics by combining the contribution of the various methods under review is made.

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