I think we don't know, but we won't know until it's too late to do anything if there is something to be done. So why not err on the side of caution?
The thing with global warming is, it's probably not worse than many things that happened in the past on this planet. The difference is that we now have - how many? - something like 6 billion? - people on it to feed and house. A very small difference can have devastating effects.
It's like volcanoes, meteors and earthquakes. Not so long ago, a meteor destroyed a huge area somewhere in Siberia. Nobody noticed at the time. If it happened now, what would be the chances of nobody, or only a few people, getting hurt? There are people everywhere. We don't have that much margin for error.
My answer to the question would be, it's probably happening, it's probably at least partly our fault, and we need to do something about it, and try not to create even more problems while we do it
