The Fertility Implosion

A lot of people would be surprised to learn that the impoverished peoples in a many those countries are a lot happier we expect. All-in-all they aren't much less happy than we are.

Not saying that wealth isn't a good thing, but it doesn't seem to be a major determining factor in happiness. Not unless you're too poor to get your 1,800 calories or you think you're entitled to wealth you don't have, anyway.

Because of something called the "Dependency Ratio": http://en.wikipedia.org/wiki/Dependency_ratio

The basic concept is that in order for there to be a high quality of life for most people in a society, the majority of them need to be capable of doing economically productive work. Otherwise you have a large number of dependents (juveniles and/or retirees) whose sustenance must somehow be garnered from a relatively small number of workers.

In China, for example, demographers talk about the "4:2:1 generation", which is the long-term effect of improving longevity combined with the one-child policy. This means that there could be four retired grandparents and two retired parents to every one working individual. Somehow, the social services for six people would have to be supported off the tax revenue from one. That's just not possible, and could very easily lead to a bad outcome for society as a whole.

(This isn't to discount the problems that come from overpopulation -- I'm just pointing out that it cuts both ways).

Too-rapid population growth can also lead to high dependency ratios -- in the form of children rather than the elderly -- and this also makes it difficult/impossible to support adequate social programmes. Historically, the best-performing economies tend to be those with very low dependency ratios, which is typically the result of a baby boom that is quickly followed by a baby bust. That produces a bulge of economically productive workers, the revenue from which can be spent on things like infrastructure and investment rather than social-support programmes which have less of an impact on GDP.

(Incidentally, this is a large part of the reason why China is going so bonkers with infrastructure development, building high-speed rail and ghost cities for which there is arguably no market. They're doing this because now is the only time they can. In 15 years, China as a whole will be substantially wealthier -- with enough people who are genuinely able to afford high-end condominiums and high-speed rail -- but at that point their youth bulge will be gone, scarcity of labour will drive up wages, most government revenue will have to go to healthcare and social services, and they'll have no ability build that kind of infrastructure. So they're building it while they can, well in advance of their actual need for it, which is actually pretty damn clever.)

Runaway heating strikes me as highly unlikely. As temperature increases, blackbody radiation increases. Since solar input is very roughly constant, eventually a new equilibrium will be found where energy lost to space once again equals energy received from the sun.

I'm sure you think that, but the numbers are clear, and not really under any dispute. Global inequality has been falling steadily and rapidly since 1980; it's the biggest reduction in absolute poverty the world has ever seen. You can interpret those numbers however you like, but those are the numbers.

"And the environment might not recover eventually; it might go into runaway heating and boil off the atmosphere."

Not according to the IPCC and the "scientific consensus". :)

While we're talking arable land, we need to consider that the majority of the land that could be deployed as farmland is home to a whole host of ecosystem services and bio-resources that, if destroyed, won't be coming back. Temperatures can stabilize, pollution can go down, but if we can't stave off habitat loss and destruction, we'll be a lot worse off than today. More people means they all need somewhere to live, somewhere to grow their food, and somewhere to put their waste. That space is all at the expense of organisms we need. We can't just shrug our shoulders and assume everything will bounce back eventually.

As for the fertility implosion itself - it needs to happen, sooner or later. Those 20-year olds in the Middle East and the babies being born today? Yeah, they'll get old, too. Brooks' view on demographics is a pyramid scheme, we need to figure it out, and I'd sooner have it happen now while we still have a relative diversity of species and environments left.