WWF Living Planet Report 2012 More than half of world's population already lives in cities, but that number is only about to get bigger. Especially, in Asia and Africa, states the World Wildlife Fund Living Planet Report.

Today's urban population is 3.3 billion and by UN calculations is expected to double by 2050. That means that two out of every three people will live in the city in 2050.

Seeing as megacities such as New York, London, Beijing, Mumbai, and Mexico City can only grow so much, most of the urban growth will take place in smaller cities. The WWF predicts that the highest growth rate of 4.19 percent will occur in cities with fewer than one million residents. Cities with more than one million residents will grow at rates less than two percent, as indicated by the chart on the right.

Currently, cities with less than one million residents account for more than 60 percent of the population.

To depict the current size of urban population around the world, the WWF put together this map: