The current growth trends suggest that the global population will increase by a billion in the next few decades, with perhaps 500 million additional people in Africa and the same number in Asia [see http://www.un.org/esa/population/publications/longrange2/WorldPop2300final.pdf%5D. Another observable trend is urbanisation. Thus, taking these together it is not unreasonable to expect most of the population growth to occur in cities. The typical size of cities in Africa is 0.5 million people and so we might expect to see 1000 new cities in Africa and perhaps around 500 in Asia where the average size is 1 million.
The challenge for engineers is to provide an acceptable quality of life in these cities. This involves providing a built environment, food, energy, transport and health care using scientific advances in novel materials, information communication technology, biosciences, electronics and photonics.
Can it be done? Probably, but it will require a higher level of innovation than is the norm at the moment, otherwise the population crunch might take many forms.