According to a new study from the London School of Economics, eliminating unwanted pregnancies through family planning and contraception is four to five times more cost-effective than rolling out low-carbon technologies. Preventing unwanted pregancies (and the resulting overpopulation, which results in more environmental destruction and global warming) costs $7 per ton of carbon. Low-carbon energy sources go from wind at $24 per ton on up, for an average of $32 per ton.
Unfortunately, as Lydia DePillis notes, the Sierra Club and the Obama Administration are too nervous to talk about this, because family planning is controversial. Hurts to type that.