It is pretty obvious that if every two people have exactly two children, the population will not grow. Two people have two children, then die. Their children have two children, then die. Those children have two children, and then die themselves.
Unfortunately, it is not a simple matter to ensure that every couple has only two children. Many of the countries in the world have very high population growth -- the average couple produces significantly more than two children. This is why overpopulation is such a huge threat.
Suppose that, rather than two children, each couple produces eight. This is a fairly extreme scenario, but serves to demonstrate:
Generation 1: 2 People
Generation 2: 8 People
Generation 3: 32 People
Generation 4: 128 People
In four generations, perhaps 60 or 70 years, the population has increased 50-fold. Okay, this is bad. We need to stop it. So, we develop a
horrible nerve gas that tends to render the target
sterile and deploy it all over the place. It's not pretty, but better than the
alternative. We've effectively reduced the population growth to
ZPG: every couple has two children.
Generation 5: 512 128 People
Generation 6: 2048 128 People
Unfortunately,
it isn't over yet.
The parents don't die as soon as their children are born. They stick around for a while. Lets say they remain alive for two generations for the sake of this demonstration. This means that, while the fourth generation is just getting ready to
make babies, the second generation is getting ready to
rest in peace. But they're still consuming resources. Not only the 128 people in the fourth generation must be supported, but the 40 people in the two previous generations.
Even if the fifth generation only consists of 128 people (because of all that
nerve gas), the population has STILL grown: 8 people have died off, but been replaced with 128 new people. It's better than the 512 people they would have been replaced with before the gas, but it's still a net growth of 120 people... and the next generation will increase the total by 88 before the population finally stabilizes.
"
Yay!", we exclaim. It might have been tough, but we've finally stabilized the population. While
food may be a bit tight divided among 384 people, it's much better than the 2688 people we would have had without attending to the population growth problem.
It's still not over.
Within a few generations of Zero Population Growth, things calm down as better and less wasteful farming methods are developed. Everyone has enough to eat... but, hey, here's the
longevity vaccine /
cure for cancer /
magical gene machine. People are living for THREE generations now.
Fuck, time for more nerve gas.
In short, overpopulation
is as huge a problem as it seems. It has an obvious solution: make everyone stop having so many kids. However, implementing that solution (at least, in a humane manner) is very difficult, and the longer we take to implement it, the longer and harder the adjustment to an eventual stable population growth will be. We can’t just get on the TV and say “
Hey, everyone, stop having kids!”, because the people having the most kids don’t have TV’s. They probably don’t have toothbrushes or flush toilets, either, and they definatley don’t have
condoms. And, to my knowledge,
we don’t have any
surefire sterility-causing nerve gas. Our methods are generally more along the lines of education about
birth control, and are at a very local level, not a global one.
And hamster bong, you're absolutely correct that the earth can support more people if they each use a smaller quantity of resources. However, it can not support continued exponential growth, no matter how environmentally conscious people are. (And the countries with the highest population growth tend to be third-world, where recycling your empty beer cans isn’t usually an issue).