Fifty years ago, most people lived in rural areas. But the world has changed. In the near future, more than half of all people will live in cities for the first time in history. City life is not always a bad thing, but many experts worry about this process of urbanization (城市化 ). A new report says that process is having a huge effect on human health and the quality of the environment. Of the three billion people who live in cities now, the report says, about one billion live in unplanned settlements. These are areas of poverty, slums that generally lack basic services like clean water, or even permanent housing. More than 60 million people are added to cities and surrounding areas each year, mostly in slums in developing countries. The international community has been too slow to recognize the growth of urban poverty. Policy makers need to increase investments in education, health care and other areas. The report talks about some successful efforts by local governments and community groups. For example, it says in Columbia, engineers have created a bus system that has helped reduce air pollution and improve quality of life. The link between urban poverty and the environment is serious, but governments also need to consider why people are moving out of rural areas. Climate changes, droughts, floods—there are many reasons forcing people to leave their farm land. The two issues of poverty reduction and the environment have existed side by side, but rarely have they connected—until now. Governments are starting to understand that environmental collapse is not a natural cost of economic development. Instead, it is hurting the possibility for growth. The main idea of the passage is about ______.