Droughts in west Africa over the past 20 years may have been caused by the destruction of rainforests in countries such as Nigeria, Ghana and Cote d'Ivoire, and further deforestation in the region "could cause complete collapse of the west African monsoon', says Xinyu Zheng of the Centre for Global Change Science at the Massachusetts Institute of Technology, Cambridge, US. Rainforests need high rainfall. But they also help generate rainfall elsewhere. At least half of the rain falling on the forest quickly evaporates from the forest canopy, providing moisture in the air to form clouds that produce rainfall. In this way, west African coastal rainforests, which receive rain from winds coming off the Atlantic Ocean, have helped generate rainfall in the drier lands of the interior. "The worst possible scenario for tropical deforestation in west Africa would see all forests replaced by the savannah,' says Zeng ( New Scientist , Vol 153, No 2065).