Well, I'm not saying that modification of behavior is impossible, just that "don't have kids" is practically a non-starter. Stiff tax penalties for more than 3

Well, I'm not saying that modification of behavior is impossible, just that "don't have kids" is practically a non-starter. Stiff tax penalties for more than 3 kids is probably something which might become possible if things get pretty bad, for example (I'm sure conservatives would object strongly, however). Carbon taxes/credits are probably possible. Huge public works (build enough solar panels and energy storage station to meet most of our power needs). Big taxes on carmakers for making high emission vehicles, etc. There are lots of policies which can work, and I think the market will, eventually, also probably get there eventually. But just telling people not to have kids is silly.