As urban leaders meet in Quito this week for Habitat III, the gathering marks the 20-year anniversary of the second Habitat conference, and the 40-year anniversary of the first. In between each of these historic events, we haven't simply turned the pages of the calendar. An entire generation has come of age. A technological revolution has unlocked the possibilities of a connected, interdependent world. And our urban population has rapidly increased, from 38 percent in 1976, to 45 percent in 1996, to 55 percent this year.

In some ways, our cities today face the same challenges they always have—from short-term shocks, like floods, fires, conflict, and disease—to long-term stresses, like poverty, inequality, strains on infrastructure, and scarcity of natural resources. But today—amid the intersecting forces of urbanization, globalization, and climate change—these disruptions come faster, stay longer, and have the potential to become full-blown crises at a moment's notice.