"Smart growth"—also called new urbanism—is an alternative to urban sprawl. This movement fights to make urban areas more people-friendly. Key elements are:  •  Careful planning on how to use the land  •  Enforcement of zoning regulations  •  Shops, schools, and workplaces within walking distance of homes  •  Good public transportation to reduce travel and pollution  •  Parks, trees, and green space. In Singapore, London, and other major cities, governments charge fees for driving on certain streets. Such policies contribute to sustainable development that allows urban areas to grow without destroying natural resources or increasing health risks.

It's easy to take cities for granted, but they are one of humanity's greatest achievements. Most medieval cities had no more than 50,000 inhabitants. In 1800, the 100 largest cities averaged 200,000 people. Only Beijing, China, and London, England, had more than a million. By 1950, more than 80 cities had populations over a million.

Today, lack of jobs in rural areas and small towns pushes more and more people into cities. Growth of metropolitan areas puts the squeeze on housing, sanitation, water supply, transportation, and health services. By the year 2020, cities are likely to be home to more than 60 percent of the estimated 7.5 billion people expected to be on Earth.

View of New York from the Empire State Building Source: Megacity Task Force, Benjamin Hennig

This phenomenal growth has put a new word into our vocabulary: megacity. A megacity is a huge urban center with a population of more than 10 million. In 1950, the world had only one—New York. By 1990, there were 13, and by 2015, there may be close to two dozen. Population figures reveal fascinating stories.

Megacities are the result of migration, the movement of people from one place to another. Sao Paulo, Brazil, for example, had 265,000 people in 1900; half of the 18 million people who now live there were born somewhere else.

Despite its growth between 1950 and 2015, New York City, which in 1970 was tied with Tokyo for 1st place, has now lost its lead to other megacities outside the U.S. Source: UN Population Reference Bureau

Most megacity growth is occurring in the developing world, where problems with housing, safe drinking water, and other basic services are most severe.

The bigger a city, the bigger its problems like traffic and pollution can be. Dense, dark surfaces—roads, roofs, parking lots, buildings—make cities heat up faster and hold heat longer than areas filled with forests and fields. This urban heat island effect causes warmer surfaces and higher air temperatures, which in turn create higher energy costs, ground-level ozone pollution, respiratory problems—and heat waves.

What happens when urban growth gets out of control? Journey to Mexico City and find out. Options: 56k | 220k (RealPlayer required) Source: Journey to Planet Earth

Some cities are finding creative ways to cool off and modify their own climate. Working with scientists, Chicago officials, for example, are analyzing whether rooftop gardens will lower temperatures, reduce energy demand, cut pollution—and health problems. See how Chicago is reaching new heights with creative environmental solutions.

20,000 plants in City Hall's rooftop garden, Chicago's laboratory for fighting the Urban Heat Island effect Source: City of Chicago, Mark Farina

Like weeds overtaking a garden, many things grow in a disorganized manner. Cities can be the same way. Urban sprawl occurs when uncontrolled, unplanned growth gobbles more and more surrounding land. Sometimes, the sprawl spreads so far that it's difficult to tell where one city ends and the next begins.

click to enlarge animation
	The sky doesn't lie. Satellite images tell the truth about the 200-year spread of Baltimore, Maryland Source: USGS

One serious effect of urban sprawl is loss of farmland. China, for example, has lost 20 percent of its fertile farmland since the late 1950s, in part because of urban sprawl.

Sprawl occurs most often in the U.S., where every year since 1970, one million acres of farms, forests, woodlands, and wetlands has turned into housing developments, shopping malls, and office parks. Fragmentation of undeveloped areas disrupts the migration and mating patterns of wildlife, further threatening biodiversity.

Between 1973 and 1998, Atlanta has lost more than 350,000 acres of forest to make room for city expansion—about 40 acres a day.

Cars fuel urban sprawl, especially in the U.S. The average American suburbanite makes 10 car trips a day. Many new housing developments don't have sidewalks but have plenty of parking. Nearly 20 percent of new homes in America have three-car garages.

In some cities, commuters spend the equivalent of six workdays in traffic jams each year Source: EPA

Since 1969, the number of motor vehicles on U.S. roads has doubled. Worldwide, they number about 600 million. At the current rate, this will soar to 1.2 billion by 2030.

When cars burn gasoline, they release compounds such as nitrous oxide, sulfur, carbon dioxide and other unburned hydrocarbons into the air. The last two of these chemicals react with sunlight and heat to form smog ozone that concentrates around roadways and parking areas.

Smog blankets Los Angeles in 1956. Since then, air quality has improved significantly but remains a major problem Source: EPA

	What is an air quality alert?
	How do automobiles and other motor vehicles cause water pollution?