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 p

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.