It took more than a million years for the human population to attain the one billion mark. But it took only 12 years for it to rise from 5 billion to 6 billion. Studies show that the doubling time of the world population is reducing fast.
There is a great variation among regions in doubling their population. Developed countries are taking more time to double their population as compared to developing countries. Most of the population growth is taking place in the developing world, where the population is exploding.