Here is a paper providing a really useful survey of global human population trends. David E. Bloom and David Canning Global Demography: Fact, Force, and Future. Harvard School of Public Health, July 2006.
In the last fifty years, the world accelerated its transition out of long-term demographic stability. As infant and child mortality rates fell, population began to soar. In most countries, this growth led to falling fertility rates. Although fertility has fallen, population continues to increase because of population momentum; it will eventually level off. In the meantime, demographic change has created a “bulge” generation, which today appears in many countries as a large working-age population. This cohort will eventually become a large elderly population, in both developed and developing countries. Population growth has been the subject of great debate among economists and demographers. Until recently, most have agreed on a middle ground, in which population growth per se has no effect on economic growth. New evidence suggests that changes in the age structure of populations – in particular, a rising ratio of working-age to non-working-age individuals – leads to the possibility of more rapid economic growth, via both accounting and behavioral effects. The experiences of East Asia, Ireland, and Sub-Saharan Africa all serve as evidence of the effect of demographic change on economic growth (or lack thereof). Both internal migration (from rural to urban areas) and international migration complicate this picture. The overall implications of population growth for policy lie in the imperative for investments in health and education, and for sound policies related to labor, trade, and retirement. Understanding future trends is essential for the development of good policy. Demographic projections can be quite reliable, but huge uncertainties – in the realms of health, changes in human lifespan, scientific advances, migration, global warming, and wars – make overall predictions extremely uncertain.