In 1950 2/3 of the world’s population lived in rural areas. Today, nearly half of the world’s 6 billion people live in cities. By 2030 the urban population will reach 4.9 billion—60% of the world’s population. Nearly all population growth will be in the cities of developing countries, whose population will double to nearly 4 billion by 2030—about the size of the developing world’s total population in 1990.
(Johns Hopkins University School of Public Health, http://www.jhuccp.org/pr/urbanpre.stm)