 By the turn of the millennium, Attenborough's authored documentaries were adopting a more overtly environmentalist stance.In "State of the Planet" (2000), he used the latest scientific evidence and interviews with leading scientists and conservationists to assess the impact of man's activities on the natural world.He later turned to the issues of global warming ("The Truth about Climate Change", 2006) and human population growth ("How Many People Can Live on Planet Earth?