How do I lead a sustainability project when I lack formal authority?

This is where your real power shows up, and it has nothing to do with your title. Authority comes from being clear and being good at what you do, not from a position. When you don't have formal power, you have to be twice as thoughtful. Know your material deeply. Expect the objections. Respect the limits other people are working under. Build allies by listening first. Who needs this project to work, and why? Show them you understand their worries, not just your own vision. Then lead by example. Do excellent work where people can see it. Deliver what you promise. Be reliable. People follow that far more than they follow an org chart. The real test is convincing people when you can't command them. That forces you to sharpen your thinking. Why does this matter? What's the evidence? What real problem does it solve? When you can answer that clearly, people move. You'll also find that limits make you creative. Formal power makes leaders lazy, and you can't afford that. Let your lack of power push you to be smarter, not to give up.