More than half of the world's expected nine billion people will live in giant urban expanses by 2030 as cities and their hinterlands occupy an additional 1.2 million square kilometers, thereby tripling in size. That's an additional 1.35 billion people living in cities, suggesting that urban areas that currently occupy roughly 3 percent of the planet's surface will continue to expand. By comparison, urban areas increased by just 58,000 square kilometers between 1970 and 2000.