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CHICAGO
population 2,862,244

Some 150 types of plants grow atop Chicago's City Hall.

From the rain garden on top of City Hall to the bike-commuter station downtown, green innovations are popping up all over Chicago. The Second City's green-roofs program is second to none, with more than 2.5 million square feet devoted to providing cooling and insulating cover. Chicago's 7,300 acres of parkland include 49 specially protected natural habitats. The first city to charge higher vehicle-registration fees for SUVs, Chicago also requires all new city-owned buildings to meet green-design and energy-efficiency standards.

NEW YORK CITY
population 8,104,079

Size matters in the Big Apple, where high-density, mixed-use development and primo public transit help residents consume far less energy per capita than the U.S. average. (Two-thirds of New Yorkers get to work without a car.) The city has one of the largest hybrid-bus fleets in the country and was one of the first to adopt hybrid taxis. There's calm amid the concrete too: Eighteen percent of the city's land area--one of the highest percentages in the country--is devoted to parks.

PORTLAND, OREGON
population 533,492

Dense development makes Portland walkable and green.

Portland has protected surrounding farms and open space by fitting new developments onto half the usual land area, making it a smart-growth pioneer--and one of the most livable places in the country. The first major city to tackle global warming, Portland creates less greenhouse gases than it did 15 years ago, while saving $2 million annually on city energy bills--and attracting new business with its efficiency expertise. Its green-building standards are the toughest in the nation.

SAN FRANCISCO
population 744,230
The first U.S. city to host a United Nations World Environment Day, San Francisco proves its worthiness with progressive purchasing policies (including phasing out toxic products and those from sweatshops), $100 million invested in solar power, and an innovative study of the potential for generating renewable energy from the waves off its shores. The city's acclaimed recycling program also contributes to its top-notch culinary reputation by sending compost made of food scraps to the region's famed vineyards and farms.

SEATTLE
population 571,480
Mayor Greg Nickels (D) brought Seattle into the national spotlight when he launched the U.S. Mayors Climate Protection Agreement to reduce global-warming pollution nationwide. But the Emerald City's local initiatives alone, including mandatory recycling, make it worthy of its name. The city government is retrofitting its heavy-duty diesel vehicles with devices that will cut particulate pollution in half, reducing its paper use by 30 percent, and creating "urban villages" that cluster offices, stores, and homes in walkable communities. By investing in renewable energy and efficiency programs to offset its contributions to global warming, the city-owned utility has become the first in the country to reduce its net greenhouse-gas emissions to zero.

Photos, from top: City of Chicago/Mark Farina, Lawrence Manning/Corbis, Scott Chapman