Cities have always been considered engines of industrial growth, as they offer their residents opportunities for employment and prosperity. This fact has become particularly pronounced in modern times. Indeed as of 2009, for the first time in the history of humanity, more than half of the world's population lived in urban centers. Furthermore, by 2050, 70 percent of the world's people will live in cities, almost as many people as are alive today.
But the downside of urbanization is easy to see. The explosive growth in the number of city dwellers is posing a huge challenge for urban infrastructures, which are reaching their limits in many places. For example, today more than 50 percent of the world's population has settled on less than two percent of the earth's surface area. As a result, urban centers with their traffic, industry, and energy needs already account for up to 70 percent of global greenhouse gas emissions.
Cities literally generate stuffy air. And that air is increasingly unwholesome for residents. According to an analysis published by the World Health Organization (WHO) in May 2015, almost 90 percent of the world's urban population breathes in air with pollutant levels that are much higher than the recommended thresholds.