Significant global climate change is occurring. The extent to which that is guided by mankind's actions is questionable, but it's certainly a factor. And regardless of its true level of influence on what is occurring, we should be making more efforts to limit our negative impact on the environment.
Yeah, this is a silly question because global warming is actually happening. The controversy is whether humans have caused it or not. I had a chemistry prof. who wasn't convinced that humans were causing it because there wasn't much evidence at the time, but now ~7 years later, a lot more evidence has come out, and I'm willing to bet he's more on the side of humans causing it.
We also can't discount how other factors can take even minimal influence and make it into something worse, or a certain level of influence can push a bad situation to be even worse. So, even when discussing whether human beings are significantly or even minimally impacting global climate change, both possibilities could have a major impact. We see that it is happening, we know something is at least some kind of a factor, and that's enough to inspire us to try to change that. It's like seeing that someone is sick and detecting that they have a bad infection. Maybe that infection is or isn't the biggest cause of their symptoms, but you know it's a problem that must be addressed either way.