A Filipino mother carries her newborn baby on the eve of World Population Day in Fabella government maternity hospital in Manila. The United Nations' World Population Day is observed on July 11 each year.

Reuters

More than half of the world's seven billion people live in urban areas, with the top "mega cities" - with more than 10mn inhabitants - being Tokyo, Delhi, Shanghai, Mexico City and Sao Paulo, according to a UN report on Thursday.

That proportion is expected to jump, so that more than six billion people will be city dwellers by 2045.

Indeed, urbanisation, combined with overall population growth, will boost the number of people in cities by 2.5bn over the next three decades, with much of that growth in developing countries, especially in Africa.

India, China and Nigeria will make up 37% of the projected growth in the next three decades, with India adding 404mn city residents, China 292mn, and Nigeria 212mn, by 2050.

The key challenge for these countries will be meeting the needs of their growing urban population with basic services like education, health care, housing, infrastructure, transportation, energy and employment.

"Managing urban areas has become one of the most important development challenges of the 21st century," said John Wilmoth, Director of the Population Division in the UN's Department of Economic and Social Affairs.

The urban population has grown so rapidly that while in 1990, there were only ten mega cities, today there are nearly three times as many - 28 worldwide.

Sixteen of those are in Asia, four in Latin America, three each in Africa and Europe, and two in Northern America.

Tokyo is the world's most populous city with 38mn inhabitants, followed by Delhi with 25mn, Shanghai with 23mn and Mexico City, Mumbai and Sao Paulo, each with around 21mn people.

The New York-Newark urban area, the world's third-largest in 1990, fell to ninth place and is expected to drop further to 14th position by 2030 as cities in developing countries become more prominent, the report said.

Meanwhile, the world's rural population, which is now close to 3.4bn, is expected to reach its peak by 2020, after which it will decline to 3.1bn by 2050.

While Africa and Asia are urbanising rapidly, they are still home to nearly 90% of the world's rural population.

Low fertility, economic contraction and natural disasters are the most common factors that contributed to population losses in some Asian and European cities in recent years.