Scientists are using satellites to study urban growth trends in Africa. By some estimates, Africa’s population will swell to 2.5 billion people by 2050, with more than 80 percent of the increase concentrated in cities, said Jody Vogeler, a Colorado State University researcher whose lab is using satellites to study urbanization trends in Africa. Vogeler and other urbanization experts expect to see some of Africa’s fastest growth rates in what they describe as “secondary cities”—those that are lar