Without large scale energy usage (eg. heating/cooling), large parts of the world (anything outside say +/-10 to 30 degrees of latitude) are practically unliveab

Without large scale energy usage (eg. heating/cooling), large parts of the world (anything outside say +/-10 to 30 degrees of latitude) are practically unliveable for most parts of the year in modern standards of comfortable living. So you practically cannot reduce energy consumption without depopulating. In terms of population reduction, history suggests any culture that does not propogate will be pushed aside by one that does. It will be interesting to see if there will still be a Japan in 3 generations.