It will be largely driven by a combination of automation (AI replacing almost all jobs) and changing immigration patterns caused by global warming. This is not mainly an Asian phenomenon; it will be global. The southern US states will, to a large extent, uninhabitable.

50% drop in global population is actually the optimistic model. The pessimistic model is human extinction. (It's alright to think I'm crazy, but I think humans are just carnivorous monkeys with big brains and a few neat technologies. Unfortunately for us, we are constrained by our behavior which is the same as 2M years ago. We are small monkeys riding a very big tiger which will turn on us and eat us if we fall.)

Human populations are subject to large rises and dramatic falls. Western Europe's population peaked in the 1350s, then the Black Plague came, and it did not get back to the original number until the 1700s. Same thing has happened in Chinese history more than once.