The 2010 TIME 100

Cities are where people live — by the billions. So they're also where the planet's environmental problems need bold solutions. No one understands that better than Jaime Lerner, 72, a former mayor of Curitiba and an ex-governor of Paraná, in Brazil.

Over the past 40 years, Lerner has left a magnificent legacy of urban sustainability. He turned Curitiba's main downtown street into a pedestrian corridor. His decision in the early 1970s to allow only nonpolluting industries presaged modern models of green business. He pioneered the bus-rapid-transit systems now in use around the world.

"There is no need to be scared of simplicity," Lerner has said. Especially when it pays such rich rewards.

Robertson is the mayor of Vancouver

TIME 100 Social-Networking Index: 905