There's a difference between global warming and anthropogenic global warming. Global warming is a natural process that would happen with or without humans.
Humans play an almost insignificant role in the warming of the globe in the long term really. We are just in an interglacial period and the earth is going to warm itself up for a bit longer before the next ice age. Even natural occurrences as simple as volcanoes and sunspots change Earth's climate on a much larger scale than humans will.
That said I love all these doomsday pseudo-environmentalist attitudes that seem to be in fashion these days because, hey.. if people start being more concerned about our planet - I see that as a good thing - even if it is done for the wrong reasons
It's natural, it's happened before it'll happen again. As to whether humans are contributing to it or not... who knows. Lets spend our tax payers research money on more useful things... like where does lead pencil go when it's rubbed out?
its both, this question is far too complex to only have two answers. i think the best answer is that natural forces are exacerbated by human activity (this is coming from an environmental geoscience student)