The 21st century may be considered the era of cities, as opposed to the 20th century being the era of countries.

As the driving force of world economies shifted from manufacturing to services throughout the last century, New York, London and Tokyo became financial hubs and signified the concentration of economic power in these global cities.

"Thus, cities have become as powerful as countries," said Hiroo Ichikawa, a professor of urban policy at Meiji University's Graduate School of Political Science and Economics and dean of Meiji's Professional Graduate School of Governance Studies. "Cities cannot survive without solving global issues."