Energy-efficient urban form.

The impact of cities—and urban design—on the global climate is becoming increasingly important. In 2008, urbanites will outnumber rural dwellers globally for the first time in human history (2). China’s population doubled between 1952 and 2003, but its urban population increased 7-fold; today, 170 Chinese cities have at least 1 million residents (3). The U.S. has 39 such cities (4). In coming decades, urban populations are expected to double while rural populations level off or decline. I mproving city layouts and transportation networks could reduce carbon emissions more than replacing all gasoline with corn ethanol (1). Although much attention on mitigating climate change has focused on alternative fuels, vehicles, and electricity generation, better urban design represents an important yet undervalued opportunity. Fortunately, such decisions are well within the reach of local governments and leaders and can reduce long-term carbon emissions. is to c k p h o to Interview