Your quest for a large impact is noble, but I encourage you to think more locally. Consider your community first, because it's where you have the most direct levers for change. Hell, you could have a book club. You could write up an e-mail to your friends with the climate policies of relevant political candidates, "just FYI!".
There's a middle ground between "I'm buying local produce for myself" and "I'm quitting my job and chaining myself to an oil tanker."