World population size increased at a slow and uneven pace for centuries before the onset of the Industrial Revolution, and did not reach 1 billion until about 1800. The modern expansion of human numbers started then, but its pace was still modest for the next 150 years with the world total rising to 2.5 billion in 1950 . During the second half of the twentieth century, however, population growth rates accelerated to historically unprecedented levels, especially in Africa, Asia and Latin America. As a result, world population size nearly tripled to 7.3 billion by 2015. This ongoing population expansion is expected to continue for several more decades reaching 11.2 billion at the end of this century. The future addition of four billion more people to the planet will have wide-ranging and potentially adverse implications for human welfare and the natural environment.