Of course it is both. The right question would be "Is human activity speeding up the process?" - because that seems to be what people don't realize. They just say "oh well the earth has been through similar shit a billion times" - not in fucking 50 year period it hasn't. It seems I have got the Bush virus, i voted "Caused by human activity", and my voting failed it said.
True, but the point is that, while it would happen it would be happening really really really slowly, we just add to it, and speed it up really dramaticaly, we just tip it over the edge. So really we are to blame for the way that global warming is effecting the world, the damage.
Other scientists and climatologists point out, however, that these changes are not slow at all and in fact happen relatively rapidly. There are several examples of rapid climate shifts in recorded history, for example, that you could easily research yourself.
Some have even pointed to evidence of a cooling trend in the past decade, saying it's largely been ignored by the media.
To be honest global warming isn't high on my list of intellectual priorities, i.e. I don't have piles of statistics lying around at my fingertips to quote at you. I get the feeling you don't either. Were we to talk metaphysics, things would be different.
That said, some general sources I know of are junkscience.com, and a pair of books entitled Cool It (by Bjorn Lomborg) and The Politically Incorrect Guide to Global Warming and Environmentalism (by Christopher C. Horner). As always, there are almost certainly counterarguments to the arguments those will present you, and counterarguments to those counterarguments, and so on in a viscous cycle. Use Google. My point is that you can't afford to be dogmatic or intellectually complacent when the stakes are this high, and the level of scientific agreement is largely exaggerated by certain sectors of the media.
Be warned that most of the people who disbelieve anthropogenic global warming are very conservative and very unapologetic about it. I, being neither socially nor economically conservative for the most part, am not one of those people. But don't dismiss them out of hand for their political views; that would amount to ad hominem.