| World population milestones in billions (USCB estimates) | |||||||||
| Population | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| Year | 1804 | 1927 | 1960 | 1974 | 1987 | 1999 | 2011 | 2024 | 2042 |
| Years elapsed | — | 123 | 33 | 14 | 13 | 12 | 12 | 13 | 18 |
| World population milestones in billions (USCB estimates) | |||||||||
| Population | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| Year | 1804 | 1927 | 1960 | 1974 | 1987 | 1999 | 2011 | 2024 | 2042 |
| Years elapsed | — | 123 | 33 | 14 | 13 | 12 | 12 | 13 | 18 |
Milestones by the billions
It is estimated that the world population reached one billion for the first time in 1804. It was another 123 years before it reached two billion in 1927, but it took only 33 years to reach three billion in 1960. Thereafter, the global population reached four billion in 1974, five billion in 1987, six billion in 1999 and, according to the United States Census Bureau, seven billion in March 2012. The United Nations, however, estimated that the world population reached seven billion in October 2011.
According to current projections, the global population will reach eight billion by 2024, and is likely to reach around nine billion by 2042. Alternative scenarios for 2050 range from a low of 7.4 billion to a high of more than 10.6 billion. Projected figures vary depending on underlying statistical assumptions and the variables used in projection calculations, especially the fertility variable. Long-range predictions to 2150 range from a population decline to 3.2 billion in the “low scenario”, to “high scenarios” of 24.8 billion. One extreme scenario predicted a massive increase to 256 billion by 2150, assuming the global fertility rate remained at its 1995 level of 3.04 children per woman; however, by 2010 the global fertility rate had declined to 2.52.
There is no estimation for the exact day or month the world’s population surpassed one or two billion. The points at which it reached three and four billion were not officially noted, but the International Database of the United States Census Bureau placed them in July 1959 and April 1974 respectively. The United Nations did determine, and commemorate, the “Day of 5 Billion” on 11 July 1987, and the “Day of 6 Billion” on 12 October 1999. The Population Division of the United Nations declared the “Day of 7 Billion” to be 31 October 2011.
20世紀の人類の科学の発展は著しかった。飛行機、潜水艦、宇宙ロケットの開発により、人類の行動可能な範囲は、空へ深海へ宇宙へと拡大した。そして、北極点、南極点への到達などにより、地球上での人類未踏の地はほぼなくなった。科学の発展は産業の発展をもたらし、大量生産、大量消費の社会を生み出し、人々の生活を豊かにした反面、環境問題など多くの解決しなければならない、諸問題をも生み出した。さらに高度な科学技術は、極めて破壊力の大きい兵器をも作り出し、現在では人類を何度も滅亡させることの出来るほどの核兵器、化学兵器が存在する。
また、産業革命以降に増加のペースが早くなっていた世界人口は、20世紀に入り人口爆発とも呼ばれる急激な増加を見せた。20世紀初頭に約15億人だった世界人口は第二次世界大戦終結後の1950年に約25億人となり、それからわずか50年しか経ってない20世紀末には2倍以上となる約60億人にまで膨れ上がっている。