Today, we are bombarded with such alarming statistics as these, suggesting a perilous future of constantly growing cities, fractured communities, increasing pollution and declining resources. While the specifics of the data might be debatable for some, the underlying realities are not: We live in a time of unprecedented global change. How can we preserve our way of life for generations to come? What will become of cities in the future?

As vividly demonstrated in the events surrounding this week's United Nations World Environment Day, there is growing agreement among environmentalists, urban planners, architects and elected officials that our way of life is simply not sustainable. We use too much energy, create too much waste and generate too much pollution. Urban sprawl forces people into their cars and away from their families and friends, resulting in serious environmental and social degradation, both locally and globally. With only 5 percent of the world's population, the United States consumes a quarter of the world's oil and generates the most solid waste and carbon dioxide.

We cannot, and should not, preserve our present way of life, but we can change it and make it better. Rather than perceiving those ominous statistics as harbingers of a grim, unavoidable future, we should see them as signs of great opportunity to make our cities better places to live -- to rebuild them as "living cities" that support both people and the natural world upon which we all depend.

While there are several names for this approach to revitalizing our cities -- smart growth, sustainable development, new urbanism -- the following are a few key components:

• Environmentally sustainable, energy-efficient buildings: In the United States, buildings account for 36 percent of total energy use, 65 percent of electricity consumption, 30 percent of greenhouse-gas emissions and 30 percent of waste output. Making new buildings that use recycled, nontoxic materials, recycle their own waste and derive multiple benefits from the renewable natural resources around them -- sun, wind, rain, etc. -- not only reduces their environmental impact, but also creates indoor environments that are healthier, more inviting and economical places to live and work. "Green" design is simply smart design.
  
  
• Adaptive reuse of existing structures: The creative reuse of existing buildings offers the multiple benefits of "harvesting" already built structures, reducing the demand on our overflowing landfills and allowing us all to maintain a stronger connection with our cities' evolving history.
  
  
• Renewable energy resources: We cannot afford to wait 20 years for the much-hyped hydrogen economy to meet our future energy needs. Simpler, less costly technologies, such as photovoltaic solar panels, wind turbines and bio- fuel generators, already exist and could significantly reduce the fossil-fuel consumption and pollution of our cities.
  
  
• Reliable public transportation: Efficient, convenient public transit gets us out of our cars, and into our neighborhoods, reducing congestion, pollution and the use of fossil fuels.
  
  
• Density and the pedestrian city: The desire to make our cities enjoyable places to walk and bicycle isn't a nostalgic urge to recreate an imagined 19th-century Main Street. Creating greater density in our cities, mixing retail and residential uses within human-scaled, tree-lined streetscapes, makes sense at many levels. Thriving pedestrian streets are safer, cleaner and more energy-efficient, and they foster a stronger sense of community than the auto-dominated commercial strips that surround us.
  
  
• Affordable housing: The creation of affordable housing close to public transit lines and workplaces is critical to creating diverse, dynamic cities around the world, not just in such expensive places as the Bay Area. Building a new generation of affordable, resource-efficient homes requires us to rethink the American dream of the mini-mansion with the three-car garage and replace it with more compact, but no less commodious, housing prototypes. Small really is beautiful.

In the ideal, a living city would behave more like an organism than a human construction. It would generate its own energy from the sun and wind, keep its air clean with minimal pollution and abundant vegetation, manage its waste stream and even grow some of its own food. But beyond these practical strategies for revitalizing our cities lies a deeper, more qualitative benefit: re-establishing fundamental connections between each of us, our families, our communities and the natural world around us. By making our cities more environmentally sustainable, we will also make them healthier, more humane and vital places to live.

The United Nations World Environment Day is an excellent opportunity to reflect on these challenges and opportunities and to begin enacting positive change for the future. Our nation has committed enormous intellectual and economic capital to such diverse efforts as exploring space, developing digital technology, curing disease and making war. If we chose to apply the same degree of commitment to making our cities livable and environmentally sustainable for generations to come, ominous predictions for an uncertain future would only sound like clarion calls to innovation. There's no telling what we could achieve.

William Leddy is a principal in the San Francisco firm of Leddy Maytum Stacy Architects (www.lmsarch.com).

© 2005 San Francisco Chronicle

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