A classic answer is Warfare, or
Societal Collapse, or perhaps
Lunatic release of Bioweapon (SmallPox?)
I do not see a path that is not catastrophic...
An interesting take. I had not considered this aspect. The liberal scientific and political elite have been wanting to significantly reduce the world's population. Not going to help property values though. If someone tries this they better hope no additional stressors occur as I believe small pox would take out half the population of the world.