Last fall, I climbed onto the roof of the 15-story Portland Building in downtown Portland, Oregon. My guide was Tom Liptan, the city's Ecoroof Program Manager and a self-confessed storm-water nerd, who began his experiments with green roofs by building one on his own garage in 1996. We walked to the parapet across plantings of sedums and fescues and looked down at the roof of Portland's city hall several stories below us. It has a conventional black tar roof, the kind of roof we have taken for granted for decades. But as part of Portland's Grey to Green project—a plan for sustainable storm-water management—that building will soon be retrofitted with a living roof. "The employees want it," Liptan said.

In the history of that municipal building, how often had the people who worked there ever thought about that black tar roof looming over their heads? Once the living roof is completed, they may visit it only rarely, but they won't forget that it's there, adding habitat to the city center, filtering the rain, moderating temperatures. It reminded me of something Stephan Brenneisen said: "People feel happier in a building where we've given something back to nature."

Think of the millions of acres of unnatural rooftops around the globe. And now imagine returning some of that enormous human footprint to nature—creating green spaces where there was once only asphalt and gravel. If a certain sum of human happiness is the by-product, who's to complain?