A warning shot is a global catastrophe that indirectly reduces existential risk by increasing concern about future catastrophes. 

Terminology

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]