Conservatives, for the most part, aren't very intelligent, and get confused easily.
Someone might say "We can't know for certain whether humans have caused global warming or not" (an absolutely true statement), then all of a sudden a bunch of crazies will go "OMG THERE'S NO SUCH THING AS GLOBAL WARMING'
Sorry, but global warming and climate change ARE facts, no expert debates that point. But when it comes down to why it's happening, well....it's like how a doctor can tell you that you have lung cancer. But (and people may not know this) there is no medical way to determine HOW you got lung cancer.
However, if you happen to be a heavy smoker, then one could make a reasonably safe assumption (by applying Occam's Razor) that your lung cancer was caused by smoking.
That's basically the same deal. No, no one can prove that human made pollution causes global warming, but by applying Occam's Razor, you find that it's a pretty reasonable assumption to make. And all those conservatives out there need to look at it like this....if global warming ISN'T caused by humans, and is simply a natural part of the planet's cycle, then whatever....we can't do anything about it. But if man-made pollution IS the cause, then it's probably a good idea to start bucking up our ideas, just in case.