<P> In 2000, there were 18 megacities--conurbations such as Tokyo, Beijing, Guangzhou, Seoul, Karachi, Mexico City, Mumbai, São Paulo, London and New York City--that have populations in excess of 10 million inhabitants . Greater Tokyo already has 35 million, more than the entire population of Canada (at 34.1 million). </P> <P> According to the Far Eastern Economic Review, Asia alone will have at least 10' hypercities' by 2025, that is, cities inhabited by more than 19 million people, including Jakarta (24.9 million people), Dhaka (25 million), Karachi (26.5 million), Shanghai (27 million) and Mumbai (33 million). Lagos has grown from 300,000 in 1950 to an estimated 15 million today, and the Nigerian government estimates that city will have expanded to 25 million residents by 2015 . Chinese experts forecast that Chinese cities will contain 800 million people by 2020 . </P> <P> From a historical perspective, technological revolutions have coincided with population expansion . There have been three major technological revolutions--the tool - making revolution, the agricultural revolution, and the industrial revolution--all of which allowed humans more access to food, resulting in subsequent population explosions . For example, the use of tools, such as bow and arrow, allowed primitive hunters greater access to high energy foods (e.g. animal meat). Similarly, the transition to farming about 10,000 years ago greatly increased the overall food supply, which was used to support more people . Food production further increased with the industrial revolution as machinery, fertilizers, herbicides, and pesticides were used to increase land under cultivation as well as crop yields . Today, starvation is caused by economic and political forces rather than a lack of the means to produce food . </P> <P> Significant increases in human population occur whenever the birth rate exceeds the death rate for extended periods of time . Traditionally, the fertility rate is strongly influenced by cultural and social norms that are rather stable and therefore slow to adapt to changes in the social, technological, or environmental conditions . For example, when death rates fell during the 19th and 20th century--as a result of improved sanitation, child immunizations, and other advances in medicine--allowing more newborns to survive, the fertility rate did not adjust downward, resulting in significant population growth . Until the 1700s, seven out of ten children died before reaching reproductive age . Today, more than nine out of ten children born in industrialized nations reach adulthood . </P>

A period of heavy migration increases world population greatly