"The scale of urbanization is unprecedented," the Worldwatch Institute's Molly Sheehan told OneWorld as the group prepared to launch its flagship annual report "State of the World 2007: Our Urban Future."

"We've gone from approximately 10 percent of the world's people living in cities in 1900 to half today--and if we continue on this course we're expected to top 70 percent in the next 20 or 30 years."

The highest rates of urban growth are expected in Asia and Africa, the report notes. Unlike previous periods of urban growth, however, this one is not necessarily tied to improved conditions for the poor.

"You don't see people going from cities to rural areas that much in search of economic opportunities," Sheehan said. "People who are moving to cities believe there's a better future for them there. However, poverty has been increasing in urban areas."

What this means, the report says, is that more urban denizens are living in slums that lack adequate sewage and sanitation and where health care is unavailable and schools are few and far between.

According to the report, 1 billion urbanites--or approximately one sixth of the world's total population--currently live in "slums," defined as areas where people cannot secure key necessities such as clean water, a nearby toilet, or durable housing. An estimated 1.6 million urbanites die each year due to the lack of clean water and sanitation, the report said.

Rapid urban growth also has implications for global warming, Worldwatch said. While cities cover only 0.4 percent of the Earth's surface, they generate the bulk of the world's carbon emissions.

Still, Worldwatch noted that many cities around the world are developing innovative solutions that, if replicated, could both fight poverty and save the environment.

In Pakistan's largest city, Karachi, where the local population recently passed 20 million, the government has linked hundreds of thousands of low-income households into settlements with good-quality sewers.

In the Chinese coastal city of Rizhao (population 3 million), a government program enabled 99 percent of households in the central districts to obtain solar water heaters. Most traffic signals and street and park lights are powered by solar cells, limiting the city's carbon emissions and local pollution.

"The fact that Rizhao is a small, ordinary Chinese city with per capita incomes even lower than in most other cities in the region makes the story even more remarkable," the Worldwatch report states. "The achievement was the result of an unusual convergence of three key factors: a government policy that encourages solar energy use and financially supports research and development, local solar panel industries that seized the opportunity and improved their products, and the strong political will of the city's leadership to adopt it."

As with other creative local solutions to global problems, the solar plan in Rizhao was led by a maverick mayor, Dr. Li Zhaoqian. Before becoming mayor, Dr. Li was vice president and professor at Shandong University of Technology and served as vice director general of the Economic and Trade Commission of the province, where he helped industries improve solar energy production technology and efficiency.

Likewise, the strong vision of the mayor of Bogota, Colombia, was key to the creation of a bus rapid transit system that has helped decrease air pollution and increase quality of life in that city of over 7 million. Curitiba, Brazil had developed a similar system in the 1970s, but until the late 1990s it was never replicated.

"A bus rapid transit system is basically a subway above ground," Worldwatch's Sheehan said. "So you're giving buses the right of way. In a lot of places that's politically [difficult] because the people with cars are the people with power and they're making the laws."

Sheehan said Bogota's bus rapid transit system owes much to the city's charismatic mayor of the late 1990s, Enrique Penalosa. His initiatives in the face of stiff opposition from private bus companies and urban elites have since inspired similar efforts elsewhere.

Indonesia's capital Jakarta launched Asia's first bus rapid transit system in January 2004, and in December 2004 Beijing followed suit. Seoul, South Korea also built an extensive bus priority system, and in 2005 Mexico City launched MetroBus.

Even Los Angeles, the international center of car culture, has launched a similar system.

"A city is a collective dream. To build this dream is vital," the former Mayor of Curitiba, Jaime Lerner, wrote in his foreword to the report. "It is in our cities that we can make the most progress toward a more peaceful and balanced planet, so we can look at an urban world with optimism instead of fear."