サイレンが急変を知らせました。

siren

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

sudden turn; sudden change; (suddenly occurring) accident; emergency

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to notify; to advise; to inform