An African has taken over as director of Greenpeace, bringing experience honed as a teenage opponent of white rule in South Africa and a network of powerful contacts to the battle against global warming.Greenpeace was founded 38 years ago by environmental activists who wanted to stop the United States from conducting underground nuclear tests in a region off Alaska that harbored endangered sea otters. Kumi Naidoo, the new director, said Monday he still had much to learn about the group's current agenda, from protecting whales and forests to stopping nuclear tests and toxic dumping. But he has already grasped the issues around global warming, an increasingly overriding concern of Greenpeace and other environmental groups.