Associated Press: Greenpeace will keep up the pressure on leaders it believes let the world down on global warming, the head of the international environmental group said Thursday. Kumi Naidoo, Greenpeace’s executive director, spoke in an Associated Press interview in Johannesburg on a visit to his home country after last week’s U.N. climate change summit in Copenhagen. Back in Copenhagen, four Greenpeace activists will spend Christmas in a Danish jail facing trespassing charges because of a summit …