Canary In a Coal Mine
= someone or something which warns of the coming of greater danger or trouble by a deterioration in its health or welfare
Origin: “Canary in a coal mine” came from when coal miners would take a canary down into the mines with them, canaries were very sensitive to any odorless noxious gases that might be released by mining, if the canary died the miners would know some gases had been released and they would evacuate the mine immediately.
Ex: So, if I get sick in there, you’ll know it’s not safe for the rest of you? I’m just a “canary in a coal mine” to you.
Ex: A melting glacier in Antartica, for example, may be described as a “canary in a coal mine” for global warming.
