gently, without mass starvation
Even more gently if you want to make sure there’s enough younger people to care for the elderly
A fuckton of people work bullshit jobs that should not exist. We could run the same society with much, much less people working.
Then fix that first instead of delaying it. Climate change is more directly caused by capitalism than it is caused by natalism. It’s easier to (proverbially) eat the rich than it is to tell people to stop having the children you need to wipe your grandparent’s ass.