1802, 3 billion in 1961, and 6.7 billion in 2008. As the human population continues to grow,
different factors may emerge limiting human population in different parts of the world. Space
may be a limiting factor or having enough clean air, clean water or food to feed everyone
are concerns we are already facing.
Figure 18.21: Human population from 10,000 BC through 2000 AD showing the exponential
increase in human population that has occurred in the last few centuries. ( 5 )
Not only has the population increased, but the rate of growth has also increased (Figure
18.22). It took all of human history for the population to reach the first 1 billion people,
in around 1802. The second billion was added 125 years later, in 1927. It took 33 years for
there to be 3 billion people in 1960, and only 15 years for there to be 4 billion people in 1975.
Another billion was added by 1987, just twelve years later, and it took only another twelve
years for the population to reach 6 billion people in 1999. Estimates are that the population
will reach 7 billion in 2012, 13 years after reaching 6 billion.
Figure 18.22: The amount of time between the addition of each one billion people to the
planet’s population including speculation about the future. ( 9 )
Although population continues to grow rapidly, the rate of growth has declined. Still, it is
likely that there will be between 9 and 10 billion people sharing this planet by the middle of