The sentence is not correct. When "Extinction...... It needs a "when" at the beginning.

if you are referring to this entire sentence, which i think you are then it does need a conjunction... "Extinction is forever" and "There is only one world" need no longer be heard [once] one becomes conscious of preserving the beauty of the natural world around us. [as, when, after, as, if] are possible conjunctions. depending on the meaning, these conjunctions work if youre trying to say that if you are already conscious of the world'd beauty then you dont need to hear "extinc...etc". also: Why does this sentence [have] no conjunction between the two clauses?

Seems like a mistake. Missing dash or semicolon.