Istanbul, the only city in the world that spans two continents, is a perfect setting for a close look at the energy and sustainability challenges of our increasingly urban planet.

With 14 million people, Istanbul is the largest city in Europe. But energy demand here and across Turkey is growing at a rate more typical of much of Asia—about 45 percent per year, says Volkan Ediger, energy systems engineering department head at Istanbul’s Kadir Has University.

Ediger, who served as energy adviser to Turkey’s president under three successive administrations from 1998 to 2010, led the latest in a series of roundtable events convened by National Geographic as part of its Great Energy Challenge initiative, sponsored by Shell. (Read about our previous