Honestly I think American scientists not speaking in Fahrenheit units to an American audience is an incredibly simple and stupid, but very significant cause of the problem.

If I tell you the Earth is going to get 2 degrees C warmer, Americans are going to think, "2 degrees, not that much, barely noticeable." If you tell them it's 4 degrees F, they'll think, that's not a ton, but it's definitely noticeable.