Shut down the growth plates and genetically produce vegetarians, etc., etc.
LIAO: "That's right. So, other things being equal larger people consume more energy than smaller people. They also, for example, it takes more energy to transport larger people. They -- you need more clothes -- fabrics to clothes larger people rather than smaller people. They wear out shoes, carpets, et cetera, et cetera more than smaller people. So think of the life time carbon footprints. That's quite a lot."
http://www.smatthewliao.com/