A warning shot is a global catastrophe that indirectly reduces existential risk by increasing concern about future catastrophes.
The expression warning sign is sometimes used to describe any event that increases concern about a particular category of existential risk, regardless of whether the event itself constitutes a global catastrophe. For example, plausible candidates for an AI warning sign include a catastrophic failure by an AI system and public outreach campaigns or the publication of an exceptionally persuasive book on AI safety.[1]
A related notion is that of a fire alarm, a warning sign that creates common knowledge that some technology—typically advanced artificial intelligence—actually poses an existential risk.[2]
Note, however, that both "warning shot" and "fire alarm" are sometimes used as synonyms for "warning sign".[3][4]