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.