My dad has worked with an organization who works with people that conventionally deny climate change. They work with people like farmers and ranchers; people who have a working connection to their environment. Their approach is to starrt by asking them if they have noticed changes over the past years/decades. Overwhelmingly they get “yes” as an answer with examples. Don’t try to argue it on a macro level, try individual experiences and see how much you can get them to extrapolate from there.