the end of the world, or a time when something very bad will happen

Ecologists predict a doomsday scenario (= a time when death and destruction will happen) if global warming continues to increase at the present rate. 生态学家预言，如果全球气候变暖继续以目前的速度发展下去，那么世界末日就要来临。

You could talk till/until doomsday (= for a very long time), but they will never change their minds. 你可以说个没完没了，但他们决不会改变主意的。