At the end of October 2011, the world officially crosses the 7 billion mark in global population. In recent decades, population has increased by a billion people approximately every 13 years. The UN projects that the next billion will be added by 2024 and that world population will reach 9.3 billion by the middle of the century. Fertility rates, however, are declining in most countries and overall world population growth should slow considerably. But this trend masks considerable differences among countries. Many countries have already gone through a demographic transition from high fertility and high mortality to low fertility and smaller family size. Forty-two percent of the world's population lives in countries where the fertility rate is below replacement level. Another forty percent lives in countries with slow-growing populations. The remaining eighteen percent are in high fertility countries, mostly in Africa and South Asia. It is these countries that will account for almost all population growth in years to come.
How quickly these high fertility countries transition to lower fertility will determine the world's overall population. UN projections are highly sensitive to small changes in fertility. The difference between its medium projection and its high projection is just half a child more on average per woman, yet the result is a world population of nearly 16 billion by 2100, versus 10 billion under the medium variant. For countries with high fertility rates, women's empowerment, female education, and access to voluntary family planning will determine the difference between these population projections.