The human population has skyrocketed in recent times. 2000 years ago, the Earth was home to just 300 million humans, roughly the population of the United States today. Two hundred years ago, 1 billion humans lived on our planet Earth. The world population is now at over 6 billion and growing rapidly. If current trends continue, we will add another 1 billion to the world population every 13 or 14 years.

This explosive growth came about because death rates fell faster than birth rates. The availability of antibiotics, immunizations, clean water, and increased food production yielded tremendous improvements in infant and child mortality. A rise in average life expectancy has also contributed to the surge in human numbers.

Fertility is the key to population growth over the long term. In order to maintain a stable population size, a total fertility rate (TFR) of 2.1 is needed. This represents the total number of children an average woman will bear over her lifetime. The worldwide average TFR is 2.9. The United States has a TFR of 2.1. Even with a TFR of 2.1, the United States and similar countries will still see its population increase, as children born in previous years reach adulthood and begin having offspring. Only after TFR values have remained low for a generation or longer will population size level off or decrease.

Many countries have low TFR values; for example, China, with a TFR of 1.8; Thailand at 1.74; Cuba at 1.55. Spain has the lowest TFR in the world, at 1.15; Italy and Greece are also very low, each below 1.3. All of these countries will eventually see their populations decline, if these trends continue.

Other countries have reduced their total fertility rates, although they still have relatively high TFR values. Kenya's TFR dropped from 8.1 to 4.4 over the past 20 years. Iran's TFR fell from 6.8 to 2.8, and Bangladesh reduced its TFR from 6.4 to 3.1, both over the past 15 years. These countries have made substantial efforts to ensure widespread availability of family planning methods.

In sub-Saharan Africa, the AIDS pandemic has reduced life expectancy, partially offsetting population growth caused by high birth rates. In the 29 African countries most affected by AIDS, life expectancy for infants born today is only 47 years, compared with 54 years without AIDS.

World population chartWhat will the human population size be in the future? The chart at the left has three curves, reflecting the uncertainties involved in predicting fertility rates. According to the "medium" projection, shown in green, the human population will reach 8.9 billion in 2050. This approach assumes that all countries will eventually reduce their total fertility rate to 2.1 or lower. In this scenario, the world population will level off at almost 11 billion in 2200. Using the "low" projection, shown on the chart in blue, world population will peak at about 7.5 billion in 2040 and then drop. The low projection assumes TFR values everywhere will drop to 1.6. Under the "high" projection (red curve on the chart), which assumes total fertility rates will stay at 2.1 to 2.6, human population will increase continuously, reaching 10.7 billion in 2050 and over 18 billion by 2150.