We stand at a crossroads. Cities must change radically to achieve long-term sustainability. Energy, food and water sources, transportation systems and basic infrastructure, must all adapt to emerging pressures from climate change, dwindling resources and growing urban populations.

Mitchell Joachim is an architect and urban designer as well as a partner in Terreform, a New York–based organization for philanthropic architecture and ecological design. His design of a compact, stackable “city car,” developed with the MIT Smart Cities Group, won the 2007 Time Magazine Best Invention of the Year.