Question

What is a good rhetorical question to start a environment speech?

Answer 1

Well, it all depends on what point you're trying to get across. 
Like if it is an argumentative speech on cleaning up the environment you would say something like: "Do you enjoy living on Earth?" "We all need oxygen, right?"

Again, it's very situational, just ask a question that everyone obviously knows the answer to.

Answer 2

It honestly depends on what topic you're going for. If it's something like pollution you could say, "What makes people throw their trash on the ground and spray harmful chemicals in the air?"