“In 2018, 4.2 billion people, 55 percent of the world’s population, lived in cities. By 2050, the urban population is expected to reach 6.5 billion. Cities occupy just 3 percent of the Earth’s land but account for 60 to 80 percent of energy consumption and at least 70 percent of carbon emissions. 828 million people are estimated to live in slums, and the number is rising. In 1990, there were 10 cities with 10 million people or more; by 2014, the number of mega-cities rose to 28, and was expected to reach 33 by 2018. In the future, 9 out of 10 mega-cities will be in the developing world. In the coming decades, 90 percent of urban expansion will be in the developing world. The economic role of cities is significant. They generate about 80 percent of the global GDP”.