The world population just passed 6.6 billion in mid-June and is estimated to reach 7 billion in 2012.
The Conventional Thinking
The conventional thinking is that the world population will continue to grow to a peak of about 9 billion in 2050 before starting to decline. The chart below shows the number of years it took for each billion to be added to the population.
In reality, many factors could change how population growth will actually occur. The basic inputs to population growth estimates are fertility rates and mortality rates.
Fertility rates have dramatically dropped since 1960 (push play at this Gapminder link to watch the global decline in fertility rates and increase in longevity 1960-2005). The accelerating informationalization of the developing world and increasing literacy and participation by women in the economy could trigger even greater drops in fertility rates than have been included in the UN and other projections.
Mortality rates could see even more change than fertility rates. Disease treatment and prevention, personal genomics and radical life extension technologies could all trigger hugely discontinuous increases in longevity.
What would the world be like with huge populations of baby boomers and others living healthily into their 100s, 120s, and beyond? Six-seven generation families could be alive simultaneously.
At some point in the next few or several years, longevity will likely start to edge up in fast spurts and plateaus until immortality is ultimately achieved, probably synergistically with digital options such as uploading mindfiles (for backup at minimum!) and downloading into any variety of human and other corporeal forms (Hack your Body!).