Yet for some reason, Marshall doesn't care about this by the end of the series. In Season 7, he briefly gets a position at an environmental firm, but the show abandons the storyline pretty quickly. And if you think that now that he's a judge, he'll be able to make real environmental change, that's absolutely not how judges work! They don't even get to pick which cases they want to oversee, and certainly can't dedicate their lives to pursuing a single cause.
Maybe in the universe of the show, the environment got totally cleaned up in the interim and rendered his dream career obsolete? If so, it seems like somebody would have mentioned it.