One thing I looked into during my research for the earlier Jericho post was the claim that Manhattan Project physicists were worried about atomic bombs igniting the atmosphere. It’s true, though they were worried about a run-away nitrogen fusion reaction, not actual ignition of the atmosphere. The scientists decided it wouldn’t happen. But don’t take my word for it. Take a look at the original summary report by Konopinski, Marvin, and Teller, which was declassified back in 1979. The report, number LA-602, concluded that you just couldn’t get a self-sustaining fusion or fission reaction out of the atmosphere no matter how hot you make things. You lose too much energy to various forms of radiation, including light via Compton scattering.