This problem is linked to that of the rise in population.

In the 1960s the author observed the number of children who could be seen in the rural areas of western Kenya and wondered what their future would be. Already it was clear that the land could not support them all. Farms were getting smaller and smaller as they were sub-divided. Now, in the 21st century we can see where they went. Round each of the small towns of the 1960s can be seen ever increasing informal settlements (slums) spreading further and further out, especially around Nairobi where huge slums circle the city.

London, Manchester, Glasgow, Birmingham and Leeds in the United Kingdom were the world's first industrial cities of the modern era. (Something similar may have happened in ancient times, such as at Mohenjo Daro, but on a smaller scale, and in Medieval Flanders). When industry began, the people left the poverty-stricken and overcrowded English countryside to look for work in the cities. Presumably the cities seemed less bad. But until the development of modern sanitation and medicine the cities were "sponges" of people - the death rate was higher in the town than the birth rate and they grew by absorbing the higher birth rate of the rural areas rather than by their own growth.

Modern cities, away from the industrial areas, grow both through their own increase and through immigration from the rural areas. Thus they explode even faster than the early industrial cities of England. The most notorious of these are Mexico City and Calcutta, but there are many others. The UN calculates that the proportion of urban to rural population will rise rapidly.

But these modern cities do not provide productive work for the people flooding in. Thus there is no money available to build the essentials of city life: sewers, piped clean water, electric power, reliable housing, city transport. Jobs could only be created by investing capital which these cities cannot accumulate from savings. The quality of life in these cities is at a low level compared with the industrial towns of today, but many of them show no signs of being able to develop into towns of the western standard. There is a question, moreover, of whether the world's resources could support such a change, as industry sufficient to support the people at even a level approaching the western standard would increase world pollution levels and increase the rate of depletion of resources.

Many of the world's largest cities are at or near sealevel and are threatened by climate change induced flooding - a surprising number are built on islands or swamp land bordering or reclaimed from the sea. The projected sea level rise will displace millions of people (even in such old cities as New York and London).

Some of the worst cities:

• Lagos
• Kibera (Nairobi)
• Mexico City
• Mumbai (Bombay)
• Kolkata (Calcutta)
• Kinshasa
• Sao Paulo

Large areas of these cities have been described as "mega-slums". But in many of these cities there are enclaves where the super-rich live often exist side by side with the poor.

At present (2008) about 50% of the world's population lives in cities. Of those in cities about 90% live in slums.

Endless City -
a useful book by architects on how to deal with the megacities

Edmund Glaeser - Triumph of the City

The Triumph of Cities: How Our Greatest Invention Makes Us Richer, Smarter, Greener, Healthier and Happier

As part of a population plan, it might become necessary to attempt to make cities once again a "sponge", not by increasing the death rate as in the 19th century and earlier, but by reducing the birth rate. That is, people might be required, as in China, to exchange the privilege of urban living for the duty to have only one child. Could this be done without infringing human rights? The result would be cities which have a low rate of increase of their own population but still able to absorb immigrants. A low ratio of dependents to workers would allow enough jobs to be provided and the accumulation of capital to provide urban services. This might be considered an emergency measure if the world's population continues to approach or exceed the limit that can be supported on income resources.Note Fred Pearce's book (see Population) that suggests birth rates are already declining.

Throughout human history large cities have been vulnerable to infectious disease - one reason why they were "sponges". Could infectious epidemics come again on a large scale? Western, developed, cities usually have efficient medical services; Third World cities usually have none. In the slums built with no sanitation a disease like Ebola could spread fast and infect many.