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.
But the human need is more immediate. Of the 3 billion people who live in cities today, about 1 billion are in slums without clean water, adequate toilet facilities, or durable housing. Some 1.6 million urban dwellers - many if not most of them children - die each year due to causes associated with the lack of clean water and sanitation.
"For a child living in a slum, disease and violence are daily threats, while education and healthcare are often a distant hope," says Molly O'Meara Sheehan, project director of Worldwatch's 2007 report, a collection of articles and graphics produced annually since 1984.
This argues for a reassessment of global development priorities, advocates say, particularly the allocation of national and international aid. According to the Commission for Africa, launched by British Prime Minister Tony Blair in 2004, problems associated with urbanization are second only to HIV/AIDS on the world's most rapidly urbanizing continent.
Yet from 1970 to 2000, aid designated for cities in developing areas was just 4 percent of total development assistance worldwide. This was the period when many countries in Africa were transitioning politically and economically from European colonialism to independence.
"Too many of us were ill prepared for our urban future," notes Anna Tibaijuka, executive director of UN-HABITAT, the United Nations agency that promotes socially and environmentally sustainable towns and cities with the goal of providing universal adequate shelter.
"The promise of independence has given way to the harsh realities of urban living," writes Dr. Tibaijuka, an agricultural economist and native of Tanzania, in the report's foreword.
By 2015, there are likely to be 59 African cities with populations between 1 million and 5 million, 65 such cities in Latin America and the Caribbean, and 253 in Asia.
"Urban centers are hubs simultaneously of breathtaking artistic innovation and some of the world's most abject and disgraceful poverty," writes Mr. Flavin. "They are the dynamos of the world economy but also the breeding grounds for alienation, religious extremism, and other sources of local and global insecurity."
Cities also exemplify the challenges and promises of sustainability. China, for example, has 16 of the world's most polluted cities. But on an island in the Yangtze River near Shanghai, China this year plans to break ground on the Dongtan ecocity project designed to be nearly self-sufficient in food, water, energy, and waste disposal for its projected 500,000 residents.