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This article is part of Yahoo's 'On This Day' series
On 31 October, 2011, the world's population was estimated to have reached seven billion.
Dubbed the Day of Seven Billion by the United Nations Population Fund (UNFPA), the day marked a milestone in the inexorable rise of the global population.
The milestone came just 12 years after the number of people in the world hit six billion, showing the speed with which populations were rising.
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Ten years on, population growth may have slowed but continued to creep upwards.
In 2019, a United Nations report predicted that the world’s population would increase by two billion people in the next 30 years, taking it to nearly 10 billion (9.7 billion) in 2050.
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However, some other estimates put the 10 billion milestone at anywhere between 2054 and 2071.
The World Population Prospects 2019: Highlights report, published by the Population Division of the UN Department of Economic and Social Affairs, concluded that the world’s population could reach its peak of nearly 11 billion around the end of the current century.
The report, which provided an overview of global demographic patterns and prospects, also confirmed that the world’s population is growing older due to increasing life expectancy and falling fertility levels, and that the number of countries experiencing a reduction in population size was growing.
The world's population is thought to have hit one billion for the first time in 1804, taking more than a century to hit two billion, which it reached in 1927.
After that, population growth sped up, with the number of people in the world reaching three billion in 1960, five billion in 1987, and then seven billion in October 2011.
Despite steady growth in the world's population for centuries, several European countries could see their populations halve by the end of the century, a report suggested last year.
Research compiled by the University of Washington, published in medical journal The Lancet, suggested that a total of 23 countries around the world, including Spain, Italy, Poland and Portugal, could see their populations drop by more than half of their current number.
Richard Horton, editor of The Lancet, said: "The 21st century will see a revolution in the story of our human civilisation.
"Africa and the Arab world will shape our future, while Europe and Asia will recede in their influence. By the end of the century, the world will be multipolar, with India, Nigeria, China, and the US the dominant powers.
"This will truly be a new world, one we should be preparing for today."
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