This strip is a common misconception. Yes, Fermi did one of his famous orders-of-magnitude calculations to see if the atmosphere would ignite, but it was followed up with very careful calculations afterward--prior to the a-bomb test. The probability of atmospheric ignition was on the order of the probability of causing an atomic blast by scratching your head.
Source: R.P. Feynman, Idiosyncratic Thinking, 1985
