Within five years, more people in the world will live in cities than in rural areas, a shift that will have big implications for the environment, according to a new study by the Johns Hopkins Bloomberg School of Public Health. Richard Blackburn, one of the study authors, said policy makers might do well to focus more on where people are living in the world, instead of just on the overall size of the world’s population. Cities, he said, “are consuming lots of resources.” For example, the study found that cities generate nearly 80 of all carbon dioxide emissions and account for 75 percent of industrial wood use. The World Health Organization says that most urban populations don’t have access to sanitation facilities and about half lack a reliable source of potable water.