Another factor driving the spread of green roofs is our changing idea of the city. It's no longer wise or practical or, for that matter, ethical, to think of the city as the antithesis of nature. Finding ways to naturalize cities—even as nature itself becomes more urbanized—will make them more livable, and not only for humans.

Living roofs remind us what a moderating force natural biological systems are. During the summer, daytime temperatures on conventional asphalt rooftops can be almost unbelievably high, peaking above 150°F and contributing to the overall urban heat-island effect—the tendency of cities to be warmer than the surrounding region. On green roofs the soil mixture and vegetation act as insulation, and temperatures fluctuate only mildly—hardly more than they would in a park or garden—reducing heating and cooling costs in the buildings below them by as much as 20 percent.

When rain falls on a conventional roof, it sheets off the city's artificial cliffs and floods down its artificial canyons into storm drains—unabsorbed, unfiltered, and nearly undeterred. A living roof works the way a meadow does, absorbing water, filtering it, slowing it down, even storing some of it for later use. That ultimately helps reduce the threat of sewer overflows, extends the life of a city's drain system, and returns cleaner water to the surrounding watershed. London, for example, is already planning for a future that may well see more street flooding, and the city is considering how living roofs could moderate the threat.

Above all, living roofs are habitable. They recapture what is now essentially negative space within the city and turn it into a chain of rooftop islands that connect with the countryside at large. Species large and small—ants, spiders, beetles, lapwings, plovers, crows—have taken up occupancy on living roofs. The list includes Britain's black redstarts, a bird that colonizes the rubble of abandoned industrial sites, a habitat being lost to redevelopment. The solution fostered by Dusty Gedge, a British wildlife consultant and a driving force behind green roofs in the United Kingdom, is to create living rooftop habitat out of the same rubble.

And it's not just a matter of making new or replacing existing habitat. In Zürich, Switzerland, the 95-year-old living roof of a water-filtration system serves as a refuge for nine species of native orchids eradicated from the surrounding countryside when their meadow habitat was converted to cropland.