The Deforestation Debate: Estimates Vary Widely over the Extent of Forest Loss
Monastersky, Richard, Science News
As tales of burning forests captured headlines in the late 1980s, a string of rock stars, movie actors, and even ice cream makers joined the fight to save tropical woodlands, helping to transform the awkward term "deforestation" into a household word. But recent studies have produced markedly different estimates of the pace of clearing, raising questions about the accuracy of deforestation figures that have floated around policy circles in recent years.
While tropical forests are certainly vanishing at a disturbing rate, the widespread disagreement over deforestation estimates makes it difficult for government officials and scientists to assess the problem. That, in turn, hampers efforts to gauge the threat of related issues, such as habitat destruction and global warming.
Concerns about previous deforestation estimates emerged in the last few years as researchers from a number of countries looked into the problem, often using more reliable methods than before. Most recently. a study published in the June 25 SCIENCE confirmed suspicions that several earlier assessments had drastically overestimated the rate of forest destruction in the Brazilian Amazon basin, thereby inflating some global estimates.
The Brazilian case provides a dramatic example of how different researchers can arrive at markedly divergent conclusions concerning the extent of deforestation. In 1988, Alberto Setzer of Brazil's National Space Research Institute (INPE) used data collected by infrared sensors on a U.S. weather satellite to gauge the number and extent of fires within the legally defined Brazilian Amazon -- an area that includes only part of Brazil's tropical forests. Assuming that 40 percent of the fires occurred on recently cleared forest, Setzer's team calculated that 8 million hectares of forest were cleared during 1987 within the legal Amazon - an almost unfathomable amount equal to 2.2 percent of the forest.
Although contested by other researchers, that alarming number found its way into several global deforestation estimates at the time. In particular, the Washington, D.C.-based World Resources Institute (WRl) included Setzer's Amazon figure in a 1990 worldwide assessment. The high number for Brazil drove up WRI's global estimate for tropical forest loss, which was calculated at 16.4 to 20.4 million hectares per year.
Despite the controversy over the Brazilian estimate, WRI's global total seemed to agree with a provisional number issued by the United Nations Food and Agriculture Organization (FAO), which put tropical deforestation at 17 million hectares per year for the period 1981 to 1990 (SN: 7/21/90, p.40).
Brazil emerged from the WRI study and others looking like the ultimate forest destroyer, responsible for roughly one-third to one-half of the global deforestation total. That triggered a round of international finger-pointing, focusing criticism on Brazil for allowing such rapid clearing of the Amazon. Brazil, however, complained that the estimates were inaccurate and that deforestation rates had never reached such heights, says Jayant A. Sathaye, an energy and forestry analyst at the Lawrence Berkeley (Calif.) Laboratory.
More recent studies have backed up Brazil's claims. In the last few years, researchers at INPE and the National Institute for Research on Amazonia, based in Manaus, Brazil, challenged Setzer's fire-counting technique and began gauging deforestation by mapping cleared areas on images taken by Landsat satellites. Studies that relied partly on this technique indicated that deforestation within the Brazilian Amazon averaged 2.1 million hectares per year between 1978 and 1989 and 1.4 million hectares from 1989 to 1990.
The newest estimate for Brazil goes even lower. David Skole of the University of New Hampshire in Durham and Compton Tucker of NASA's Goddard Space Flight Center in Greenbelt, Md., studied some 200 Landsat images covering the entire Brazilian Amazon for 1978 and 1988, allowing them to map the extent of forest and cleared land for those two years. …