I'm trying to calculate the ecological footprint my sexual activity will have on society 40 years into the future. What is the best amount of children to bring to the world to create a sustainable earth? How much is the life time pollution of a human being?
There are too many variables.
First: what we mean by sustainability in this case? It could mean "resources will never run out". If no other factors apply and sustainability is the only meaning of life, no children, collective suicide and controlled extinction of human race seems to be the best solution. This is clearly not what we really want (not only because nuclear warfare and other ways we would probably do to assure controlled extinction are much less sustainable than status quo). >=)
There's other definition of sustainability: sustainable society can exist for very long time (even for the rest of humankind's existence) without destroying its environment and running out of all its resources (some may run out if they are completely replaced by other - but it takes time, so they mustn't be depleted very quickly). This is easier with less people, but discipline is much more important than numbers. Many people really trying to live sustainably produce less waste than few who don't care.
There's also the social aspect: slightly more than two children are ideal to sustain the society forever. Assuming most children will grow up and have their families (which is often not the case), much more children will lead to overpopulation, while less children will lead to aging of the population, high percentage of old and frail and decreased stability of the society. This doesn't mean that every family should have exactly 2.1 kid, but that it's good to move the average closer to this value. In most rich countries, we have a dilemma between having no/few children and improve the average planetwise (and make the country's society demographically less sustainable), or having many children and improve the average countrywise (but adding to global overpopulation). So it's up to you.
While we can argue a lot whether it's better to have many or few children, we would certainly all agree that it's important how you raise your kids. Few families can really teach their children to resist the ads and other influences forcing them to buy a lot of things and not to care about sustainability very much. My personal opinion is that ten children with sustainable habits and strong belief that loving family or nice countryside are more important than things and money are much more sustainable than one kid who think that hapiness is a function of money spent on products advertised on TV.
To conclude, I'd say that the best is as much children as you can raise well.
What is a sustainable population growth?
On the long term, if the population is growth is constant and positive, we will have severe overpopulation at some point in the future (exponential growth). When the growth is constant and negative, the population will decay until there is no one left in the far future. So both a population growth and decay are not sustainable since both cannot continue indefinitely.
So a first order approach would be to strive for a growth factor such that each generation has as many people as the previous. If you translate that requirement to a single person, that means that everybody must get one exactly child. Since it takes two people to create a new one, as a couple you will be sustainable if you have two children, in which you both have half a share.
This of course neglects the fact that some people die before they have children, so if you want to compensate for that you may choose to have 2.05 children of course. However that still rounds to 2.
The second question would then be what is a sustainable population?
And is that a constant number or does it change due to better use of natural resources?
I think the world is currently overpopulated. And whether that is in fact true or not doesn't really matter in my opinion. Why should we strive to have as much people as we can feed on this planet? I don't think it makes a better living if we are with 9 billion instead of 6 billion. Actually the quality of life seems to suffer from having the maximum number of people.
So if you want to increase the quality of life, you need to have a slight decline in population. You don't want to decline too fast, since that has severe social and economic consequences. Who will look after you when you are old? Who will invest in new buildings if there is no one to buy them in the future?
I think 2 children is a nice figure to aim for. It keeps the population more or less constant, with a slow decline due to the fact that not everybody will have children.
You can only ever get a 1st order approximation to this question, which is something like the average pollution/person in your country. You can get vastly different answers depending on how you approach it. You could figure the average person in your state or city or neighborhood, they are all likely to be different. You could also figure an average for people who posts on this forum, and you might get a negative amount of pollution/person! (if they help other people reduce more pollution than they themselves generate)
There are many other responses too, such as:
- The Earth is already over carrying capacity, have no children
- If all the environmentalists stop having kids, we'll run out of environmentalists in a generation
- Having kids is natural, so its ok
- We're going to have to find a way to deal with over population no matter what, whats one more kid?
- Technology will save us, so have as many as you want/can afford
- We're going to populate space anyway, so who cares?
- We must go back to preagricultural populations and lifestyles, so no more kids
The list goes on ...