There can be a debate about how much global warming would happen or even, potentially, whether or not there are highly complex forces at work that could potentially cause it to be null. And, there's also the potential question of how much the climate would change given a certain amount of increased energy in the system (global warming just means that there's increased energy in the system). However, it seems like, from the measurements we have seen, there is quite a bit of warming from this and that the climate change is likely to be severe. We're already seeing quite a lot of it and we'll almost certainly see a lot more.

So, sure, there are questions about how bad it'll be. But those questions are just: will this be bad or really bad. They're not: is global warming occurring or not?