The Doctor in “Orphan 55”

Look, I know what you're thinking, but it's one possible future. It's one timeline. You want me to tell you that Earth's going to be okay? Cos I can't. In your time, humanity is busy arguing over the washing-up while the house burns down. Unless people face facts and change, catastrophe is coming. But it's not decided. You know that. The future is not fixed. It depends on billions of decisions, and actions, and people stepping up. Humans. I think you forget how powerful you are. Lives change worlds. People can save planets, or wreck them. That's the choice. Be the best of humanity. Or...