One argument I often hear from my environmentalist friends is that overpopulation is a big problem, and therefore they are generously foregoing having children for the sake of the environment. I don't believe them.
First, as I noted earlier this month, the maximum sustainable population of the earth is far, far higher than either our current population of just over seven billion, or the UN's prediction of around eleven billion at century's end. Given declining fertility rates, something around the latter figure is likely to be our maximum population unless there is some drastic political or economic shift. It is clear that from a physical and biological point of view, our population will never reach its constraints. Any ceiling on our population is instead defined by poor political policy and human ignorance, which lead to inefficiency and waste.
Second, even if overpopulation were a problem, there is a simple solution - baby offsets. What are they? Well, they work just like carbon offsets, of which I am an annual purchaser, and highly recommend. With carbon offsets, you pay a qualify provider an amount based on your personal emissions, and they use the money to fund clean energy sources, focusing as best they can on ensuring they are additional and not just paying someone to do what they would have done anyway, in an amount sufficient to offset any emissions you made.
Baby offsets work the same way, but instead of carbon offset providers like Terrapass or Native Energy (both recommended), you donate your money to a good international family planning organization instead, such as Pathfinder International. It takes about $100 for such organizations to prevent one unintended pregnacy in a developing country, so even if you assume that your child will be a typical American and pollute far more than the world average, you can simply donate $1000-$2000 and be sure that the net impact of your actions is an environmental and social positive. Better yet, teach your children to be a friends of the environment rather than typical American pigs, and make it a win-win situation! And before you ask, if you don't have a couple grand to spare, you probably aren't ready to have kids anway. $2000 is much, much less than the several hundred thousand your potential child is going to cost you over the coming years. In any case, it would be fair to pay for the offsets over time, say $100 per year, rather than as a lump sum.
So there you go, my environmenatlists friends. Have all the children you want. There is plenty of room, and offsetting any messes they make is easy.
Post-script: Strangely, every time I bring this up with environmentalists they get a bit huffy. It is almost as if they didn't really want stinky, annoying, extremely expensive children in the first place, and were just engaging in the time-honored tradition of rationalizing their self-interest as an act of sacrifice. Who would have guessed...