We love superheros (our superhero themed office is proof). Comics are one of the best ways to creatively communicate how you feel about a cause. Let people know what problems exist, then help them imagine a way to use their superpowers to find solutions.
Comic stories are flexible enough that absolutely any cause will fit into one.
In a comic, there's usually a conflict between something bad and something good. If your cause is pollution, your villain can be the CEO of a company that pollutes or a crazy sludge monster. You have the same choice with the "good guy," who can also be a team of people.
Figure out what the villain does wrong that the hero has to put a stop to. Remember that every part of your story sends a message. Is the villain defeated with violence, smarts, the power of a team working together.
Plan out what's going to happen in the comic. Figure out what happens in each frame, then figure out how to represent it. Your comic can be five pages of squares, or one page of triangles and rectangles, or anything else you can imagine.
Comic books are known for their snappy dialogue that appears in speech bubbles. Think of a great catch-phrase for your superhero and unique onomatopoeia like "Whomp," "Krrrshhh," and "Biiiing."
Make a catchy title with your hero's name. How about, Eco Man's Revenge: The Demise of Sludgey Monster.
Draw up your comic, but don't stress about your art skills. Comics can be totally realistic, cartoonish, or stick figures. The important part is that you stay consistent. Draw the same character the same way every time. That way, your hero will be recognizable, like an icon.
Comics are super easy to share, especially if they're brief. You can pass them out like fliers, hang them like posters, or scan them and distribute them on the internet.