World human population

The current estimated world human population is 6,350,538,084. This figure is deceptively precise, however, since there is no complete database on the world's population, and humans are constantly being born (at the rate of about 3 per second) and dying. However, it is clear that the world's population continues to grow, and at rates that are unprecedented prior to the 20th century.

By some estimates, there are now one billion young people in the world between the ages of 15 and 24.

When was six billion reached?

The United Nations Population Fund designated October 12, 1999 as the approximate day on which world population reached six billion. This was about 12 years after world population reached five billion, in 1987.

(The term "billion" above is used to mean "thousand million", 10⁹, rather than "million million" as used in some countries. See billion for details.)

Rate of population increase

The last 70 years of the 20th century saw the biggest increase in the world's population in human history. The following table shows when each billion milestone was met:

• 1 billion reached in 1802.
• 2 billion reached in 1927.
• 3 billion reached in 1961.
• 4 billion reached in 1971.
• 5 billion reached in 1987.
• 6 billion reached in 1999.

From the figures above, the world's population tripled in 72 years, and doubled in 38.

The UN estimated in 2000 that the world's population was then growing at the rate of 1.2 percent (or 77 million people) per year. This represents a decrease in the growth rate in 1990, mostly due to decreased birth rates.

Forecast of world population

The future growth of population is difficult to predict. Birth rates are declining slightly on average, but vary greatly between developed countries (where birth rates are often at or below replacement levels) and developing countries. Death rates can change unexpectedly due to disease, wars and catastrophes, or advances in medicine. The UN itself has issued multiple projections of future world population, based on different assumptions.

The current projections from the UN's Population Division, from their 2002 World Population Prospects database , are:

Year	Population (billions)
2010	6.8
2020	7.5
2030	8.1
2040	8.6
2050	8.9

Other projections of population growth predict that the world's population will eventually crest, though it is uncertain exactly when or how. In some scenarios, the population will crest as early as the mid-21st century at under 10 billion, due to gradually decreasing birth rates.

In less optimistic scenarios, disasters triggered by a growing population's demand for scarce resources will cause a sudden population crash, or even a Malthusian catastrophe. (See overpopulation for more details.)

Doomsayers

In 1798, Thomas Malthus predicted that population growth would eventually outrun food supply, resulting in catastrophe. In 1968 Paul R. Ehrlich reignited this argument with his book The Population Bomb, which helped give the issue significant mindshare throughout the 1960s and 1970s. The dire predictions of Ehrlich and other neo-Malthusians were vigourously challenged by a number of economists, notably Julian Simon.

More recently, some scholars have applied bayesian probability to world population to argue that the end of humanity will come sooner than we usually think (see: Doomsday argument)