Tim O'Reilly makes the most cogent argument I've yet seen on why we should believe in global warming and act accordingly, even if we don't fully buy into the hype: The downside to belief is quite small. The upside is quite big.
The downsides to a better environment, more fuel-efficient cars, etc., are not downsides at all. They're things I'd like regardless of whether the world is burning up or not. So why not?