<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> 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>
when did the world population reach 5.9 billion
when did the world population reach 5.9 billion
six billion in 1999
six billion in 1999