I'm always sad when I read something like this. If believing in God and protecting the planet or accepting basic science are contrasting ideas, then your wife's grandparents are believing in a caricature, not God.
But I think you can do better than just saying "these are fairy tales". I recently read a book from a former atheist (Jennifer Fulwiler) who came from a very loving and intellectually honest family. Her father said this when she was young: "Make sure you don't start believing things just because someone says it's true, even if it's coming from me. Question everything." I wish more people would be like that, atheist and religious alike.
That first paragraph is emblematic of the thing that most worries me about using religious groups as the core of one's social life. It may come from a well-intentioned place, but people are way too quick to resort to outright denigrating other people's beliefs in a way that, at best, infantilizes them with descriptions like "fairy tale" and "caricature". (There's not a whole lot of semantic distinction between these two characterizations.) Because of the ways that group behavior and peer pressure work, that will only get magnified if most your social life revolves around people who share identical beliefs.
There need to be more social institutions that encourage different people from different backgrounds to come together and learn to be more respectful of one another. How about a renaissance for bowling leagues?
But I think you can do better than just saying "these are fairy tales". I recently read a book from a former atheist (Jennifer Fulwiler) who came from a very loving and intellectually honest family. Her father said this when she was young: "Make sure you don't start believing things just because someone says it's true, even if it's coming from me. Question everything." I wish more people would be like that, atheist and religious alike.