For reasons why there might ever be fewer than 100M humans, see the Metaculus Ragnarok series. An important question is, if most people die, whether humanity will recover, and how long it would take.
If the human population declines to fewer than 100 million, how many years would pass until the population is above 1 billion again?
This question will resolve as the number of years between the date when the total population of humans is less than 100 million, and the following date when the estimate is greater than 1 billion. The estimates of population will include all humans in the observable universe. If there is no population decline to 100 million or less between January 1, 2000 to January 20, 2345, this question will resolve as ambiguous. If the human race never rebounds (ie. goes extinct), this question resolves as >3000.
For this question, "Humans" are defined as creatures who at least one 2020 Metaculus user would judge are humans if they magically met them.