You turn video games into Captain Planet, Fern Gully, Avatar, or The Day After Tomorrow, and they’ll be even less ineffective than no ecologically sound video games at all.
I’m not saying that pro-environment games are bad; I’m saying that if producers set out first and foremost to preach to their audience, no one will be interested, and the game itself will suffer.
Final Fantasy VII has sold roughly ten million copies in the fifteen years since its release; it’s a pro-environment game, about what happens when man pollutes so much that the planet itself rises up against him in protest. While this informed the plot, setting, and characters, it wasn’t overly preachy, and Square didn’t set out to make a propaganda game. They set out to make a good game, and gamers were fine with the message because it wasn’t overbearing, or the focus of the story, or tacked on at the end obviously, or shoehorned in at the expense of quality gameplay.
Radiata Stories has the main character choose to side either with humanity against the faerie kingdom (who live in harmony with nature), or with the faerie against humanity, and the entire plot and ending changes depending on which choice the player makes.
If you’re making a game, you should set out to make a good game first, and tell whatever story works best with that game style. If you’re just going to shoehorn in propaganda for whatever cause you feel deserves it, then don’t bother.
No one will play it.