Nature is the very ground on which cities and urban activities are built. The natural environment supplies resources such as clean air, water, trees, and species, as well as services, from water filtration to pollination, that are vital for city functioning and that make cities beautiful and livable. (See Chapter 17.) A sustainable city operates in harmony with nature, respecting and implementing the ecological principles of diversity, adaptiveness, interconnectedness, resilience, regenerative capacity, and symbiosis in its development and planning activities. In a sustainable city, nature is no longer an urban afterthought.1
Benefits of Nature in Cities
The extensive presence of nature in cities carries a range of benefits, starting with a more livable environment. Robust greening of cities can purify air and water, reduce artificial warming from buildings and streets (known as the heat-island effect), and increase biological diversity. Toronto, Canada, has calculated that vegetating 5,000 hectares of roofs would result in a reduction in ambient air temperature of 0.5–2 degrees Celsius, decreasing energy demand for cooling. As a result, since 2010, any building in the city with more than 2,000 square meters of floor space must devote up to 60 percent of its roof area to vegetation.2
A greened city provides clear economic benefits.
A greened city provides clear economic benefits. Property values often increase in beautified areas, and studies show that homes with trees fetch a higher market price than similar homes without trees. The High Line, a former elevated freight line in New York City that has been converted to a park, has attracted $4 billion in private investment to the area. Additionally, flood control from well-designed green areas makes houses and other buildings more secure and more desirable: preserved coastal wetlands provide an estimated $23 billion in hurricane protection alone.3
A healthy environment can improve human health as well. Green spaces in Denmark correlate with lower stress levels and lower levels of obesity. Purified air from trees and other plantings reduces exposure to pollution and contaminants. Green space is shown to attract people outdoors, as in Stockholm, where green urban features often entice people to walk or bike to work, incorporating exercise that they otherwise might not get. (See Chapter 18.)4
Managing Green Assets
“Green infrastructure”—the use of wooded areas, creeks or rivers, and other natural areas to provide economic services—can help cities avoid construction of costly new water management facilities. Aquifers can be recharged for water storage, sidestepping the need for new holding tanks. Parks and fields can be designed to provide flood protection, reducing outlays for concrete channels. Even at the scale of the neighborhood, green infrastructure can soak up and store water. As the U.S. Environmental Protection Agency observes, “bioswales, rain gardens, permeable pavement, green roofs, and other innovations help to channel, store, and filter water that would otherwise flow out of the city, sweeping up pollutants from streets in the process.”5
Parks and fields can be designed to provide flood protection, reducing outlays for concrete channels.
Management of a city’s natural assets extends beyond city limits. The Nature Conservancy notes, for example, that watersheds supplying the world’s 100 largest cities cover an area 12 times greater than the cities themselves. At no charge to cities, these watersheds collect, filter, and deliver water to nearly 1 billion people “before it ever enters a pipe.” Careful conservation of watersheds is therefore a smart way to ensure clean water for urban areas. New York City understands this, having opted to invest in conserving its upstate watershed rather than spend billions on an expensive plant to treat what otherwise would be impure water from a contaminated watershed.6
Cities also can collaborate with regional neighbors to secure water supplies. Consider San Diego, California, which pays farmers in the nearby Imperial Valley to conserve water and pump the savings westward to the city. Farmers line irrigation canals to prevent water loss, invest in drip irrigation and other efficiency enhancements, and fallow some fields every few years. By 2020, conservation in the Imperial Valley is expected to supply 37 percent of San Diego’s water supply.7
Indicators of a Green City
Timothy Beatley’s book Biophilic Cities paints a vivid and detailed picture of what an environmentally grounded city might look like, from urban infrastructures to municipal activities and city governance. Beatley has developed a set of indicators of “urban biophilia” and offers sample values that might serve as guidelines for cities. His values for infrastructure, for example, might vary by city, but they give a sense of the extent to which nature can be integrated into urban life. 8
Gary Gardner is director of publications at the Worldwatch Institute and co-director and contributing author of the State of the World project.
Read more in State of the World: Can a City Be Sustainable?