[actually_a_dog]

What evidence would you consider sufficient, besides globally falling birthrates?

Again, I will remind you, the comment says we are "starting to see evidence," of population collapse, not that population collapse is happening or is inevitable. Globally declining birth rates is certainly evidence that it may be happening.

[nl]

Well population dynamics are pretty predictable for the next ~40 years or so.

The inputs are the number of people of childbearing age over that time period and the number of births per adult. We know the maximum number of people (since new people don't get born at ages above zero). The number of births per adult tends to change very very slowly and pretty predictably.

A population collapse would be caused by one or both of these things changing dramatically.

There's no evidence of this. Instead there is evidence of a slow decline in population as the birthrate (especially in Africa) slowly decreases.

> What evidence would you consider sufficient, besides globally falling birthrates?

Something that indicates that the current models showing a slow decline are wrong.