EcoWatch: With more and more of the world`s population moving into urban centers, the number of such areas with more than 10 million people has exploded in the last quarter century. In 1990 there were only 10 of these "megacities," with about seven percent of the global population. Now there are 28, according to the UN report on World Urbanization. The total number of people living in megacities has grown from 153 million to 453 million during that period, says the UN report. About 12 percent of the world`s...