<Tr> <Th> Year </Th> <Td> 1804 </Td> <Td> 1927 </Td> <Td> 1960 </Td> <Td> </Td> <Td> </Td> <Td> 1999 </Td> <Td> 2011 </Td> <Td> 2024 </Td> <Td> 2042 </Td> </Tr> <Tr> <Th> Years elapsed </Th> <Td>---- </Td> <Td> 123 </Td> <Td> 33 </Td> <Td> 14 </Td> <Td> 13 </Td> <Td> 12 </Td> <Td> 12 </Td> <Td> 13 </Td> <Td> 18 </Td> </Tr> <P> 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 . </P> <P> According to current projections, the global population will reach eight billion by 2024, and will likely 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 . </P>

When did the world population reach 5.9 billion