By next year, more than half the world’s population, 3.3 billion people, will for the first time live in towns and cities, a number expected to swell to almost 5 billion by 2030, according to a United Nations Population Fund report released today.

The onrush of change will be particularly extraordinary in Africa and Asia, where between 2000 and 2030 “the accumulated urban growth of these two regions during the whole span of history will be duplicated in a single generation,” the report says.

This surge in urban populations, fueled more by natural increase than the migration of people from the countryside, is unstoppable, said George Martin, author of the report, “State of World Population 2007: Unleashing the Potential of Urban Growth.”