It has recently been suggested that a wall be built around Rio de Janeiro's notorious shantytowns. It would have the same purpose as that infamous wall that once divided Berlin, and the wall that Israel is building to cut itself off from the Palestinian territories: to isolate people. In Rio, the people who would be isolated are the denizens of the city's vast and crime-infested favelas .
The wall, of course, will never be built. Even if it was, the problems of mass urbanization in the twenty-first century will not be solved by such crude methods. What the United Nations calls the "century of urbanization" will require creativity and imagination, innovations and not brute brick walls or even more subtle security frontiers.
Consider São Paulo, the world's fourth largest city and my home. Our past and present is the future for many others. Of the world's six billion people, more than 50% live in cities and towns. Even in a country as big as Brazil, over 80% of the population lives in urban areas.
The pace of urbanization is astonishing. In 1975, only 38% of the world's population lived in cities; by 1998, the rate had grown to 47%. By 2015, the number will reach or exceed 55%. In 1975, only 100 cities topped one million in population; by 2015, 527 cities will exceed that mark. More critically, while there were only two "mega cities" with populations over 10 million in 1960, and 18 now, some 26 will exist in 2015.