More than half of the world's population lives in cities, in constantly expanding urban areas that very often give rise to megalopolises of tens of millions of inhabitants, such as Tokyo, Shanghai and Mexico City. But this already striking proportion could grow further in favour of metropolises and at the expense of rural areas, with more than six billion people set to be "city dwellers" by 2045, according to the latest World urbanization prospects, the document on urbanisation published by the United Nations Department of Economic and Social Affairs.

The surge will be driven by two factors. On one hand, there is global population growth, which will rise from today's more than seven billion to the nine billion projected for 2050. On the other, there is people's desire to move to cities in search of a more economically comfortable lifestyle and greater security in terms of the services on offer. The combination of the two will increase the number of city dwellers by a further 2.5 billion compared with today over the next 30 years. The largest increase will be recorded in countries such as India, China and Nigeria, which will account for 37 per cent of total growth. In India alone, for example, the urban population will grow by more than 400 million.

This expansion will confront the administrators of these city-states with a number of challenges, such as ensuring universal access to education, healthcare, energy, transport and food that is healthy and sustainable in terms of its production. "What we need to be worried about in this situation is that governments will not make adequate plans to cope with growth that is already under way," said John Wilmoth, director of the department's Population Division. "Managing urban areas has become one of the most important challenges for sustainable development in the 21st century."

In the 1990s there were only ten megalopolises in the world. Today there are 28, of which 16 are in Asia, 4 in South America, 3 in Africa, 3 in Europe and 2 in North America. The largest remains Tokyo, the capital of Japan, with nearly 38 million inhabitants, followed by Jakarta, in Indonesia, with nearly 30 million, and New Delhi (India) with 24 million, according to the latest data from Demographia.

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The opposite fate awaits rural areas. Today 3.4 billion people live in the countryside, but by 2050 there will be no more than 3 billion, and they will be concentrated almost entirely (90 per cent) in Asia and Africa.