World population reached one billion shortly after 1800 AD, probably in 1804. It took just over 120 years to double to two billion, by about 1927. The next doubling was quicker, 47 years to 1974. The eight billion point, another doubling, will probably occur in 12-14 more years, for a doubling time of 48-50 years. The current rate of average world population growth is about 1.25% per year, which implies a doubling every 56 years. But there are pockets of higher growth, such as a major portion of Africa, where the yearly population growth is 2% or greater, just as there is a major portion of Europe in which population is actually declining. That's the world.
Let's consider the U.S., where I live. When I was born, the population was about 145 million. Today it is more than double that, just under 310 million. But population growth rates have declined since the founding of the Republic in a stepwise fashion:
- 1800-1860, 3.05%/yr
- 1860-1910, 2.26%/yr
- 1910-1960, 1.31%/yr
- 1960-2010, 1.09%/yr
I found this graphic in the Wikipedia article Demographics of the United States. Go there to see an active graphic that shows the population pyramid for each year from 1950 to 2010 (you'll need to scroll a bit).
The 2010 graphic shows a mature population, for which each 5-year cohort is about the same size (~20.5 million), until nearly age 60, then there is a steady decline to about age 90, with a small number of extreme elderly to age 100+. The raw shape of that pyramid indicates that the U.S. population is near stability. However, until the death rate matches the rate of birth plus immigration (minus emigration), the whole pyramid will slowly fatten.
Do you think the U.S. population will ever reach one billion? Can we feed that many? We are feeding that many now, because we export so much food. We'd have to reduce our exports dramatically, absent a significant improvement in farming technology. That may happen, but don't bet your life on it.
At the present growth rate of 0.98%/yr, in 100 years, U.S. population would grow by a factor of 2.65, to 820 million, and it would reach one billion in about the year 2130 (in 120 years). Can we push that date out to 200 years away, to 2210? We can, by immediately reducing population growth in the U.S. to 0.59%/yr. Ask your friendly local politician just how drastic that is! If we could somehow reduce population growth to 0.12%/yr, the one-billion point would be staved off by one thousand years. The trouble is, immigration is presently about 0.33%/yr, so if we could achieve genuine zero population growth among U.S. residents, we'd still have to reduce immigration to about a third of its present level. Your local politician won't even answer that one!
The fact is, we're in for hard times no matter what we do. Everyone with any power (and most of everyone else) is addicted to growth. We need a growing economy to avoid recession, so they say. But keeping this planet a habitable place will, sooner or later, require permanent recession.
The UN and other "world bodies" project that world population will peak in fifty years or less and then decline or hold steady. They are probably right, but it is quite unknown whether all the world's families can be convinced to confine their reproduction to two children per couple (or one per wife/husband in polygamous families). If they cannot, total world population will grow beyond whatever "sustainable" bounds the "world bodies" promote, then be held in check by starvation and disease.
A significant portion of the human race has had a hundred-year run of great prosperity. It can't last.