Population will decline at 9 billion?

I have periodically heard that the earth’s population will reach about nine billion, then it will start to decline.

My quesiton: Is this true? Why will it decline? When are we supposed to hit the nine billion mark? How will it decline?

Thank you.

Mass xenocide from intelligent koala bears.

WAG: By then we’ll have reached our carrying capacity. The earth will not be able to sustain us forever because of a high dependancy on non-replenishable resources.

Just a guess.
But then again, Bella’s might be true too… gets his koala hunting gun

WAG: The so-called ‘first world’ has a far lower birth rate than other nations. One can feasibly expect this to be repicated as more nations join this grouping.

As to the earth not being able, well, I think Malthusian inclined people have been playing prophet of doom on this long enough, and tey have still not been right

I’d say we should up the eucalyptus logging - hit them at the source…

{Raises and shakes Koalas}

Damn you all, damn you all to hell!

{Raises and shakes fists at Koalas}

Damn you all, damn you all to hell!

OK. Population growth (or decline) is caused by two factors…the death rate and the birth rate.

Improved sanitation and public health dramatically decreased death rates all over the world in the last few hundred years. World population had been growing before then, but it really started increasing. Eventually people started to get worried. Low death rates and high birth rates were increasing the global population dramatically.

But then people noticed that while death rates were staying low, birth rates were declining, especially in industrialized countries. Yes, we still have a positive global population growth rate. But in most industrialized countries the birth rate is only a little higher than the death rate, and in many countries it is lower.

If you simply took the current rate of growth and imagined that future growth rates would be exactly the same, then obviously population levels would increase forever. But a more sophisticated way to do it would be to project future death rates based on the current trend, and future birth rates based on the current trend. If you do that, then we can project that at some point the birth rate and the death rate will match…zero population growth. But there is no reason to suppose that it will stop there, the birth rate is likely to continue falling and global population will decline. The older the population gets, the higher the death rate becomes, and the lower the birth rate.

Obviously, how much it will decline, when it will decline, or even if it will decline all depend on future events. Maybe in 50 years a new religious movement will change first world countries dramatically, or asteroid strikes, or xenocidal koala bears. But, if we simply continue the trends that exist today, global population will level off sometime in the next century. Of course, declining population has some important demographic consequences…the proportion of elderly people will be much higher, the proportion of children and young people will be much lower.

A good website to view some of these numbers is www.nationmaster.com.

[C. Heston]
“Get you paws off my you damn dirty marsupial!”
[/C. Heston]

But the population may well start to rise again when and if life expectancy increases to beyond a century;
judging by medical and genetic advances, that stage may be reached in one or two hundred years.
So some sort of population control may be necessary after all, soon after the population begins to decline.

SF worldbuilding at

Birth rate vs Death rate isn’t the only factor involved. Total Fertility Rate (TFR) is an important factor as well. This is the average (mean) number of children that women in a particular group can be expected to have in their lifetime. Replacement level is about 2.2, 2.1 (I suspect that this number factors in infant mortality). All of Europe is below 2 (some are just barely above 1), Canada and the U.S. are a little above 2 (at replacement level). As mentioned previously, as a nation industrializes and gets wealtheir, the birth rate drops. Projections are that at about the time that the world’s population reaches 9 billion, TFR will be below 2.2.

Population growth is said to be exponential. Expect to see 9 billion people on earth within the next 80 years. I remember I did the calculations to 10 billion people and I was amazed at how soon 10 billion people were expected to live on earth. Don’t remember the exact numbers.

It will be interesting to see what immigration policies would be needed as a shrinking and aging population in the northern nations need more labor from southern nations to provide health care and taxes.

My own solution to some world poverty would be to start building schools for physical therapy, nursing, nurse’s aides and other such under-staffed jobs in various poor nations and start recruiting. The money all the friends who are immigrants send home must keep many families floating.

Population growth is only exponential if the growth rate is constant. At a 1% growth rate you have the population doubling every 70 years. Keep a constant 1% rate for several of those periods and you have exponential growth. But if the growth rate declines to 0.5% or increases to 2% then suddenly it isn’t exponential anymore. And if the growth rate becomes negative then we have an exponential decrease in the population.

You have to take into account not only the rate of change (the growth rate), but also the change of the rate of change. And the growth rate is never constant.

And extending lifespans to where 100 years are not uncommon is unlikely to drastically increase the population. Those people are not having children. The thing you have to remember is that an increased birth rate in itself increases the birth rate. Young people have the greatest fertillity, so lots of young people in the population means lots of people having kids. A baby boom leads to an echo baby boom. A baby bust leads to an echo baby bust. An young person living to 25 and then dying is likely to produce many more children than an old person living an extra 25 years.

Population structure is incredibly important here. The number of fertile young people is much more important than the number of infertile older people. The number of children is important if significant numbers of them are going to survive to be fertile young people. Since the birth rate is the number of children born per 100 members of the population, increased lifespan is going to drasticly reduce the birth rate, since very few old people have children even if they are healthy.

Population structure therefore lets you predict future birth rates more accurately.

This has been around a bit, but is a very cool site on population growth.

Those above hit it on the head: Poor nations pump out people like crazy for a number of reasons, especially because having lots of children is the way one ensures one’s security in retirement. Industrialized nations have much lower birthrates, including some with negative birthrates. As more nations are brought into better economic environments, they tend to decrease and eventually eliminate their poplulation growth.

Oh, and not to take this to GD, but the notion that the earth is even close to being unable to sustain more people is well-established as inaccurate.