It's almost certainly caused by man as all of the evidence suggests that it is. But if it's not, that's a much more serious problem since if it's some unknown natural phenomena we probably can't do anything to stop or slow it from happening and we don't know how hot it will get or how quickly. Maybe humans have triggered a yet to be discovered tipping point and there's no stopping it.

Some reports are already saying that global warming is progressing faster than predicted... maybe we're on an exponential slope to higher temperatures and don't know it yet.