It took until about 1825 for the population to reach 1 billion. The most recent billion was added in just under 13 years. Another 2 billion is anticipated by 2050. Most of these will be added in the poorest parts of the world. Speakers at the 2011 American Association of the Advancement of Science meeting in Washington DC offer perspectives on the carrying capacity of the Earth. The Royal Society in London is also looking at population and will describe the range of consequences if certain routes are taken. John Sulston says the inputs to current methods of food production are unsustainable. He says education is the key to reducing population growth.
http://www.abc.net.au/rn/scienceshow/stories/2011/3203724.htm