Maybe it's cyclical, and not linear (it seems to be due to small life scale of average human). In evolutionary stages humanity has gone through multiple cycles of population explosion and then decline. Reasons may vary from one to another but I think once humanity gets reduced to the bring of extinction, they'll adapt and again start increasing in numbers. Most of the countries in next 100 years will hit the fifth stage of the Demographic Transition Model (low birth rates and moderate to high death rates); once it hits criticality new society along with new societal order will rise.