Supercities.

Imagine a planet inhabited by cities like 'Mega-City One,' a megalopolis of over 400 million people across the east coast of the United States, featured in the Judge Dredd comic or 'San Angeles, formed from the joining of Los Angeles, Santa Barbara, San Diego, and the surrounding metropolitan regions following a massive earthquake.

Don't hold your breath: the 21st century will soon have 19 cities with populations of 20 million or more.

The history of the human species is a history of migration. In 1000 A.D. Cordova, Spain was the largest city. By 1500, Bejing began its rise to power, and 300 years later it was the first city to be over a million people. By 1900 London emerged the world's supercity with over 6 million people. In 1950 New York was proclaimed the first "megacity" with a population of over 10 million people in the greater metropolitan area.

How is increasing mass urbanization affecting the quality of life? 1.4 million people are moving into cities each week. How will this vast migration change the way we live and die; how we treat the elderly, the poor, the way work, trade, learn, the way we eat, consume, recycle, power, engineer, innovate?

While some say the world is flat, supercities are rising - vast, intensely urban hubs will radically redefine the world's future macroeconomic and cultural landscape. Most of the world's population right now lives and works in cities. Many more will. It's critical to gain a truer understanding of what's happening: the rise of supercities is the defining megatrend of the 21st century.

In 1800 only 3% of the world's population lived in cities; 47% by the end of the twentieth century. In 1950, there were 83 cities with populations exceeding one million; by 2007, this had risen to 468 urban areas of more than one million.

If the current trend continues, the world's urban population will double every 38 years. The UN forecasts that today's urban population of 3.2 billion will rise to nearly 5 billion by 2030, when three out of five people will live in cities. By 2050 two-thirds of the world’s population will live in cities, up from about 50% right now.

The world’s population of slum dwellers increases by 25 million every year. The majority of these numbers come from the fringes of urban margins, located in legal and illegal settlements with insufficient housing and sanitation. This has been caused by the massive migration, both internal and transnational, into cities.

The greatest population increase will be most dramatic in the poorest and least-urbanized continents, Asia and Africa. Surveys and projections indicate that all urban growth over the next 25 years will be in developing countries, where one billion people, one-sixth of the world's population, now live in slum-level conditions; breeding grounds for crime, addiction, alcoholism, poverty and unemployment. By 2030, over 2 billion people in the world will be living in these mega-ghettoes.

If current trends continue, nearly 1-billion people will live in China's cities by 2025. Click here to see dynagram.