From Nuclear explosion:
In 1945 there was some initial speculation among the scientists developing the first nuclear weapons that there might be a possibility of igniting the earth's atmosphere with a large enough nuclear explosion. This would concern a nuclear reaction of two nitrogen atoms forming a carbon and an oxygen atom, with release of energy. This energy would heat up the remaining nitrogen enough to keep the reaction going until all nitrogen were finished. This was, however, quickly shown to be unlikely enough to be considered impossible . Nevertheless, the notion has persisted as a rumor for many years.