In geography, "doubling time" is a common term used when studying population growth. It is the projected amount of time that it will take for a given population to double. It is based on the annual growth rate and is calculated by what is known as "The Rule of 70."

### Population Growth and Doubling Time

In population studies, the growth rate is an important statistic that attempts to predict how fast the community is growing. The growth rate typically ranges from 0.1 percent to 3 percent each year.

Different countries and regions of the world experience various growth rates due to circumstances. While the number of births and deaths is always a factor, things like war, disease, immigration, and natural disasters can affect a population's growth rate.

Since doubling time is based on a population's annual growth rate, it can also vary over time. It's rare that a doubling time remains the same for long, though unless a monumental event happens, it rarely fluctuates drastically. Instead, it is often a gradual decrease or increase over years.

### The Rule of 70

To determine doubling time, we use "The Rule of 70." It's a simple formula that requires the annual growth rate of the population. To find the doubling rate, divide the growth rate as a percentage into 70.

- doubling time = 70/annual growth rate
- Simplified, it is typically written: dt = 70/r

For example, a growth rate of 3.5 percent represents a doubling time of 20 years. (70/3.5 = 20)

Given the 2017 statistics from the U.S. Census Bureau's International Data Base, we can calculate the doubling time for a selection of countries:

Country | 2017 Annual Growth Rate | Doubling Time |
---|---|---|

Afganistan | 2.35% | 31 years |

Canada | 0.73% | 95 years |

China | 0.42% | 166 years |

India | 1.18% | 59 years |

United Kingdom | 0.52% | 134 years |

United States | 1.053 | 66 years |

As of 2017, the annual growth rate for the entire world is 1.053 percent. That means the human population on Earth will double from 7.4 billion in 66 years, or in 2083.

However, as previously mentioned, doubling time is not a guarantee over time. In fact, the U.S Census Bureau predicts that the growth rate will steadily decline and by 2049 it will only be at 0.469 percent. That is less than half of its 2017 rate and would make the 2049 doubling rate 149 years.

### Factors That Limit Doubling Time

The world's resources—and those in any given region of the world—can only handle so many people. Therefore, it is impossible for the population to continually double over time. Many factors restrict doubling time from going on forever. Primary among those is the environmental resources available and disease, which contribute to what is called the "carrying capacity" of an area.

Other factors can also affect the doubling time of any given population. For example, a war can significantly lower the population and affect both the death and birth rates for years into the future. Other human factors include immigration and migrations of large numbers of people. These are often influenced by the political and natural environments of any country or region.

Humans are not the only species on Earth that have a doubling time. It can be applied to every animal and plant species in the world. The interesting factor here is that the smaller the organism, the less time it takes for its population to double.

For example, a population of insects will have a much faster doubling time than a population of whales. This is once again primarily due to the natural resources available and the carrying capacity of the habitat. A small animal requires far less food and area than a larger animal.

### Source

- United States Census Bureau. International Data Base. 2017.