RAINFORESTS aren鈥檛 disappearing as fast as we thought. The first global survey of high-resolution satellite images focusing on deforested hotspots indicates almost 25 per cent less deforestation during the 1990s than would be expected from the widely used figures compiled by the UN鈥檚 Food and Agriculture Organization.
The FAO collects information from individual countries, which use satellites or local reports to estimate the amount and kinds of forest within their borders. With the help of additional satellite data, the FAO then calibrates this information and comes up with a global total.
These figures are considered to be the best estimates available, and are routinely used by the Intergovernmental Panel on Climate Change and researchers who want to calculate how much carbon dioxide is being released into the atmosphere.
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But Hugh Eva from the European Commission鈥檚 Global Vegetation Monitoring Unit in Ispra, Italy, points out that many countries, especially those embroiled in political conflict, find it difficult to compile accurate national figures. And each country has a slightly different definition of what a 鈥渇orest鈥� actually is. That, as well as other factors, mean there is a huge error in the FAO鈥檚 estimates 鈥� up to 50 per cent in some cases.
So Eva, Fr茅d茅ric Achard and their team collaborated with environmental experts from Latin America, Africa, South-East Asia and India to identify areas where deforestation is particularly bad. They selected 100 sites over the three continents, covering 6.5 per cent of the total forest, and tracked down high-resolution satellite images taken over these areas between 1990 and 1997.
They found that, on average, 4.9 million hectares of rainforest were lost each year to factors such as clear-cutting, fire and conversion to farmland. That is 23 per cent less deforestation than the 6.4 million hectares estimated by the FAO (Science, vol 297, p 999).
If the researchers are right, that would mean that less CO2 than we thought has been released as those trees rot or are burnt. A spokesman for the FAO says that Eva and Achard are relying on a relatively small set of data, which still leaves room for a large margin of error.
But a further review of the world鈥檚 tropical forests happening as part of NASA鈥檚 Landsat Pathfinder project should help resolve any discrepancies. Rather than focusing on deforestation hotspots, the Pathfinder satellite is taking images and data from coast to coast across each continent.