I think this sentence mean that people have no time only to talk about how to save the Earth, people need to start doing it now. P.S. This film is really wonderful.

Pessimist means someone who is downbeat. For instance: He is a pessimist means he is always thinks bad things will happen to him or he is always see the dark side of something. The adjective of it is : pessimistic He is pessimistic. He is a pessimist. He is downbeat. About your question it means: It's late to think about bad thing or see a bad side of that. Hope it helps

What is the full context? A pessimist always thinks bad things will happen. Obviously something extremely bad has happened.