For the first time in human history, the world's population is about to become mostly urban.
Citing population growth rates and migration patterns, United Nations researchers and other experts predict that some time in 2008 more people will live in cities than in rural areas.
This demographic shift is mostly taking place in Africa and Asia, largely in low-income settlements in developing countries – much of it in the 22 "megacities" whose populations will exceed 10 million and in some cases grow to more than 20 million by 2015.
The environmental, economic, and social ramifications of such trends are enormous, according to the Worldwatch Institute's annual "State of the World" report released Tuesday. Among the major challenges are the mundane features of daily living: clean water and air, sanitary waste facilities, the cost of food, and the availability of shelter and transportation.
"Unplanned and chaotic urbanization is taking a huge toll on human health and the quality of the environment, contributing to social, ecological, and economic instability in many countries," warns the report, which is written by demographers, international program officials, and other experts from the United States and other countries.
But the news is not all bad. Researchers find examples of cities from Karachi, Pakistan to Freetown, Sierra Leone to Bogotá, Colombia with projects aimed at improving the lives of urban dwellers while reducing the environmental impact of concentrated populations. These include urban farming plots, solar water heaters, economic cooperatives, improved sewer facilities, and upgraded transportation systems.
"The task of saving the world's modern cities might seem hopeless – except that it is already happening," says Worldwatch president Christopher Flavin. "Necessities from food to energy are increasingly being produced by urban pioneers inside city limits."
Still, the challenges and the probable costs of addressing them remain daunting. Eight of the 10 most populous cities are on or near earthquake faults. Some two-thirds of the cities projected to exceed 8 million residents by 2015 are in coastal areas where sea levels may rise as a result of climate change.