By 2050 the global population will reach 9 billion. More than two thirds of that number will live in cities. In 1800, only three cities - London, Peking and Edo (now Tokyo) - had more than one million inhabitants. A hundred years later, the figure had risen to just 16. Today, there are more than 440 one-million plus cities.
There are now more "megacities" (cities with a population of more than 10 million) than there were one-million cities just over a century ago. In less than three years time there will be around 26 megacities worldwide - of which 22 will be in developing and newly industrialized countries.
The transformations being wrought in these cities are not purely local phenomena, but changes with real global implications. Radical patterns of economic migration, increased population density and the growth of informal communities - springing up across the globe as a by-product of the unrelenting march of urbanization - are asking new and difficult questions of a world in tumult. Increased infrastructural demands are created in terms of adequate water and energy provision, waste disposal and waste water treatment, mobility and transport infrastructure, and health.
According to the WWF's "Reinventing the City" study, USD 350 trillion needs to be spent on global urban infrastructure over the next 30 years - particularly in the world's small but fastest-growing cities and developing nations.