best solution imho => raise wellfare of development countries and spread awareness, theres a distinct relationship between wellfare and the number of offspring one produces....
also looking at numbers from countries like japan who have a history of "population control" a huge factor is the social status and mindset of the people, if you know that on your old day you'll be taken care off, you won't feel the need to produce more offspring to secure your future....
so in basic => get africa and other shithole countries out of the depression they're in and i'm sure we'll level out our population ....
just my 0.02 $