There is one and only one reason why climate change and global warming are controversial. That's also why it's a partisan issue with the right on one side and the left on the other. Someone posted a pie chart which showed a very thin slice of climate change deniers. Honestly even it were 50/50 it shouldn't be controversial. We're talking about an extinction level event so it behooves us to take it seriously.
The reason that the right is still fighting it is, of course, $. Once people admit that there is a possibility of human caused global warming then we have to start spending shitloads of cash to change things.