By mid-century, according to United Nations demographers, the world will contain 9.6 billion people, some 300 million more than previously projected. The great majority of these unanticipated births resulting from higher than expected fertility will be in countries classified as least developed by the United Nations and thus the poorest in the world. Moreover, many are failed or failing states, namely those in the throes of chronic political instability and armed conflict, factors that mitigate against the provision of basic services, such as health care and family planning. This upward revision in fertility has a snowballing effect, so world population is now expected to peak in 2100 at 10.9 billion people, instead of the previously projected 10.1 billion.

The revised population projections include notable findings at the country level. The population of India is expected to surpass that of China around 2028, sooner than previously thought, when both countries will have populations of around 1.45 billion. Thereafter, India’s population will continue to grow for several decades and peak at around 1.6 billion before declining slowly to 1.5 billion in 2100. The population of China, on the other hand, is projected to shrink after 2030, possibly falling to 1.1 billion by 2100. Nigeria’s population is likely to surpass that of the United States before the middle of the century. By the end of the century, Nigeria could begin to rival China as the second most populous country in the world.

Most of the planet’s people will live in cities. Virtually all expected population growth will be concentrated in the urban areas of poor countries increasing the threat of pollution and epidemic. By 2050, the number of people living in cities will almost be equal to today’s world population. In 1950, only two cities in the world had at least 10 million inhabitants. Today, 23 megacities have more than 10 million inhabitants; by 2025, the number of megacities is projected to reach 37.

Faced with the opportunities and challenges associated with urbanization, many governments have traditionally sought to stem migration to large cities with a host of policies that have proven largely ineffective.