Nitrogen Fertilizer production is a base for the modern world that billions would die without, and with no identified economic replacement. It relies on fossil fuel to be produced. It is produced in the 100s of millions of metric tons a year.
The closest non-fossil fuel alternatives are various types of hydrolysis, which at current natural gas prices on the US are 3-10x more expensive per kg of hydrogen produced.
If you want world population to drop precipitously, make it expensive to produce THAT.
[https://en.m.wikipedia.org/wiki/File:Nitrogen_fertilizer_con...]
The closest non-fossil fuel alternatives are various types of hydrolysis, which at current natural gas prices on the US are 3-10x more expensive per kg of hydrogen produced.
If you want world population to drop precipitously, make it expensive to produce THAT.