I'm not evangelical (or even Christian!) but I understand the rapture to be the belief that all Christians will simultaneously be transported to heaven to be with Jesus Christ. There is a Wikipedia article that might explain it better! But sinners/non-Christians get left behind on Earth, according to this belief; there is even a popular book series in the U.S. called Left Behind that follows this very idea.

But because certain very conservative Christian factions in the U.S. believe that the rapture could come at any day now, they tend to base their political choices on the notion that they could be sucked up into the clouds at any moment. Global warming (or any conservation issue) isn't a concern for them because, in a few years, they believe that they'll be sitting in Heaven. So why not exploit the Earth to the greatest extent possible?

So these people make it their business to fervently deny global warming as much as possible. After all, until the rapture comes, they want to enjoy their giant SUVs as much as possible!