In real life, people don't change easily. Even small adjustments in behavior or attitude often take years or more. In a movie, a character needs to change pretty dramatically in two hours. Getting him or her there in a believable, compelling way is one of the greatest challenges of being a writer, and by "greatest challenges" I mean, of course, "hugest pains in the ass."
One thing is for certain -- a character to have an important, defining trait that the audience wants to see change. That's a given. If the audience doesn't think there's anything wrong with the character, they're going to lose interest, no matter what kind of fancy plot mechanics and exotic locations we toss into the mix. "Cares a lot about the environment" probably isn't a trait that we would want to see change, unless we're Dick Cheney. "Cares a lot about the environment but neglects daughter," now we're in the ballpark.
With that trait defined, our next task is to come up with a mechanism for changing it (i.e., the plot). For our environmentalist, what could that mechanism be? Maybe he's presented with a great opportunity to spend more time with the kid, to be a truly meaningful presence in the kid's life. That sounds nice, except there's a big problem with it, because it's not believable that the environmentalist would choose that path. And if he does choose it, right now at the beginning of the movie, he's already made the big change and the story is already over.
As an alternative, then, consider this -- the environmentalist is presented with an opportunity to be an even better environmentalist, except it requires neglecting the kid even more. Is it believable that the character would choose this path? Sure, since we already know that the environmentalism is a lot more important to him than the kid is. So instead of choosing to change his/her negative trait, the character chooses to double down on it. And hopefully, what that will do is bring the consequences of his life choices into much sharper focus and force him to realize the truth that he's been studiously ignoring this whole time, whatever that may be. Only now is he truly susceptible to change.