Politicians and planners may be getting the message. In 2003, the U.S. Conference of Mayors passed an urban forestry resolution to promote the preservation and new growth of trees and forests in city environments. Two years later, 50 city leaders from around the globe signed a Green Cities Declaration at the United Nations World Environment Day in San Francisco. Mayors from Delhi to Dakar, Moscow to Manila, resolved to chart a bold new course for the urban environment, launching efforts to reduce waste and pollution, ease traffic congestion, and—by the year 2015—to ensure an accessible public park or recreational open space within a third of a mile of every city resident.

"Reclaiming space so that a city can 'breathe' is an integral part of the challenges confronting urban civilization today," says Bertrand Delanoë, the popular mayor of Paris. "A modern city needs areas free from density, noise, and the frenzied urban pace. We must re-create the kinds of spaces that lend themselves to talking, walking, discovering, relaxing."

When Delanoë ran for office six years ago, a centerpiece of his campaign was a pledge to find, within city limits, 75 acres (30 hectares) for new parks and public spaces. In a metropolis as densely settled as Paris, this is no easy task. But Mayor Delanoë and his staff are recycling land with characteristic Parisian creativity and verve, rescuing bits and pieces of the city to create new parks.

Among them is Un Tracé de Verdure sur les Maréchaux, a linear greenway to be planted along a tram route in the south of Paris, and the Jardins d'Eole, a soon-to-open informal ten-acre people's park where residents of the working-class neighborhoods of the northern 18th arrondissement can picnic and play on fields that were once train yards. With these and other small parks and public spaces, including some of the 40 or so vibrant community gardens that have cropped up on vacant lots all over the city, Delanoë's promise will likely be fulfilled.

Champions of urban parks hail recent progress in the greening of cities but warn that much remains to be done. Some leaders consider their cities all built up, with no room for more parkland, says Peter Harnik, director of the Center for City Park Excellence at the Trust for Public Land in Washington, D.C. But if a city has space for one more building, Harnik posits, it has room for one more park.

As for footing the bill: Cities have traditionally reserved funds for such requisites as police, sewers, and fire trucks, and considered parks and green space as pleasant amenities—investments for leftover money. But researchers such as Frances Kuo argue that parks in cities represent a minor public investment with a huge payoff. "Parks help people take care of themselves so cities don't have to spend as much on social, medical, and safety services trying to fix their problems," she says.

What then should be the goal of city planners? A park near every doorstep where people can gather and gain a healthy dose of that remedy Henry David Thoreau said we can never have enough of: nature.