Famously, Oppenheimer and co worked out how close a nuclear bomb test would be to causing a chain reaction of nitrogen fusion in the atmosphere. They made a lot of worst-case-scenario assumptions and still came to the conclusion that no, a nuclear bomb test wouldn’t scour the surface of the world.
But let’s say the atmosphere was twice as dense as it is. Or ten times as dense. At what point would that calculation turn very, very scary?
Edit: man, seriously, most of the people ‘answering’ this question didn’t even read it.
Fusion of two nitrogen-14 nuclei and a hydrogen nucleus. That was the feared chain reaction, since both elements are abundant.
Source
Thank you, the page you sourced references a 2024 paper inspired by the Oppenheimer movie that was super interesting to read
Prolly the most relevant paragraph from the linked article for this discussion:
Basically the temperature of the atmosphere is over an order of magnitude too low to have any chance of ignition (need 10s of millions of K), and the reaction rate is thus several orders of magnitude lower than the threshold.