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World population milestones, such as reaching particular population counts, have been unnoticed until the 20th century, since there were no reliable data on global population dynamics.〔Vaclav Smil, ("Global Population: Milestones, Hopes, and Concerns" ), ''Medicine & Global Survival'', October 1998; Vol. 5, No. 2, 105–108〕 It is estimated that the population of the world reached one billion for the first time in 1804. It would be another 123 years before it reached two billion in 1927, but it took only 33 years to rise by another billion people, reaching three billion in 1960. Thereafter, the global population reached four billion in 1974, five billion in 1987, six billion in 1999 and, by some estimates, seven billion in October 2011 with other estimates being in March 2012.〔 〕 It is projected to reach eight billion by 2024–2030. According to current projections, the world's population is likely to reach around nine billion by 2045–2050, with alternative scenarios ranging 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 which variables are manipulated 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 scenario predicts a massive increase to 256 billion by 2150, assuming fertility remains at 1995 levels. ==Billionth milestone days== There is no estimation for the exact day or month the world's population surpassed each of the one and two billion marks. The days of three and four billion were not officially noted, but the International Database of the United States Census Bureau places them in July 1959 and April 1974. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「World population milestones」の詳細全文を読む スポンサード リンク
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