Preventing unwanted pregnancies is a cost-effective and ethical way to reduce environmental destruction and minimize population growth, and 45% of pregnancies in the U.S. are unintended. Of those, 58% will result in birth. Comprehensive sex education would go a long way, too, and many states do not include it in their curricula, even though comprehensive sex education has strong bipartisan support among likely American voters. Many women at high risk of unintended pregnancy are unaware of long-acting reversible contraceptive options, and many men don't know how to use a condom properly, which does actually make a huge difference. Besides that, it could help to ensure everyone has access to effective contraception, so consider advocating policies that improve accessibility of long-acting reversible contraceptives and help get the word out that it is ethical to give young, single, childless women surgical sterilization if that is what they want.

As for the rest of the world, it would help to donate to girls' education. It might also (perhaps counter-intuitively) help to improve childhood mortality by, say, donating to the Against Malaria Foundation.

All that said, population is not the most significant cause of climate change -- it's the market failure. That's why the single most impactful climate mitigation policy is a price on carbon, and the most impact you as an individual can have is to volunteer to create the political will to get it passed.

And returning the revenue from a carbon tax as an equitable dividend would help a little bit with inequality, while creating jobs and growing the economy.

Taxing carbon is in each nation's own best interest, and the IPCC makes clear carbon pricing is necessary.

Policy changes absolutely dwarf the magnitude of the impact of having one less child.