Sachs' book is about how to avert global economic catastrophe, which is obviously something worth worrying about. Basically, he seems to be saying that global cooperation is necessary, because "we're all in this together". He also argues, it seems, that population growth is bad; I found the article from Greg Mankiw's blog. Mankiw quotes the end of the review:
In an age when we don’t need to have lots of children to work the fields, or to compensate for high infant mortality, Sachs argues that it’s both economically rational — and crucial for a future of sustainable growth — for people to reproduce at a rate close to 2.1 children per family. In his acknowledgments, Sachs thanks his three children.
But Mankiw pointed out in 1998 that it's not necessarily true that population growth is bad. It seems like a lot of people like to quote Thomas Malthus on this, who said that population grows exponentially with time (true) but that food production ability only grows linearly. As far as I know Malthus had no reasonable argument for this; people talk about the Malthusian catastrophe. But if you actually look at the relevant section in An Essay on the Principle of Population, in chapter 1 Malthus just postulates these growth rates. In Chapter 2 he offers as justification for this basically that there's not enough room in England for the farms to feed an exponentially growing population. But there's not room for an exponentially growing population itself either!
It would be one thing if, say, farms fell from the sky at a constant rate. But farms are made by people; thus if people grow exponentially so will farms.
Now, I'm not saying that we aren't running out of room. It's obvious that some land is better for farming than other land, and people will tend to farm the better land first; as time goes on we will be obliged to farm more and more of the inferior land, therefore decreasing the amount of food grown per person.
But Malthus was writing in 1798. He assumes that the population is producing just enough food for itself in his time, and then goes on to say:
In two centuries and a quarter, the population would be to the means of subsistence as 512 to 10.Two centuries and a quarter is 2023; roughly speaking, now. We clearly have much more than two percent of the food we need.
Of course, it's possible -- as is pointed out in every introductory computer science class -- that polynomial growth can outstrip exponential growth over some short time, and we're not in the limiting regime yet. But I don't think anybody is seriously saying this. And anyway, Malthus made the stronger claim that food production is growing linearly.
(Does Malthus get more sophisticated than this? I'm just cherry-picking, but skimming his work it seems to continue in roughly the same vein.)
Now, this is an obvious criticism -- but somehow it rarely gets pointed out. Mankiw pointed it out, though -- sure, people use resources. But they also create resources. People will eat food. But they will also figure out how to grow more food. That is, if we don't just disintegrate into a society of virtual reality addicts first.
edited, 11:08 am: Also from the NYT, Deaths are outpacing births in the Pittsburgh metropolitan area. I'm not sure how meaningful this is on a national level, because people move around.