The world's population is unlikely to stop growing this century. Researchers from the United Nations and the University of Washington calculated that there's an 80 percent chance that the globe's population will rise to between 9.6 billion and 12.3 billion by 2100, up from just over 7 billion today. Much of the increase will occur in Africa, where birthrates are not falling as fast as once projected. (See "One Woman at a Time.") Asia's population, on the other hand, is expected to peak by midcentury. Here is how the probabilities break down by continent.