sending vast clouds of dust into the atmosphere, starting around 2000 B.C. In West Africa's Sahel, experts trace the re- gion's weak plant life to both overgrazing and slash-and-burn agricul- ture centuries ago. "Killer droughts" such as occurred in 1973-74 were probably a recurring result.
Humans have cleared 7 million square kilometers of tropical forest alone, equivalent to half the Earth's present jungle area. Forty percent of the rain forests of Africa and Brazil have been destroyed....