> How do you propose humane population control?

Actually, in many (most?) countries this is already happening, for better or for worse. It's just a limited subset of areas/countries that have quick population growth. This has different causes and the possible solutions are politically, socially, religiously and economically complex. Or rather, getting to a situation where a solution is applied is complex more than the actual solution.

Education, birth control and abortion access, and increased income levels. Each of these is associated with a lower fertility rate.

There is little pushback against the increase of these measures. It would be very interesting though if a fertility rate increase was warranted and folks were arguing against them in order to raise it.

I assume by "powerdown", the OP means reducing energy usage, especially of polluting energy sources, mainly fossil fuels.

This doesn't really require a lower population, it just means people need to stop expecting to live in huge McMansions with A/C blasting all the time and driving around in 3-ton SUVs. Living in dense, walkable/bikable city environments in energy-efficient dwellings, people could live quite well with a tiny fraction of the polluting energy usage.