Can we hurt it?

I was recently reading a Rush Limbaugh book (that tells you a little bit). In the book he made the claim that no matter what we do we can not possibly harm Earth short of a nuclear war. Now I would have to say in the case of carbon emissions and pollution, I do not believe that humans are powerful enough to harm this planet. My reasoning is that volcano's put far more CO2 into the atmosphere and far more toxic chemicals into the water than we will. And that has not changed anything too much. But what is making me wonder if I may have to revise my theory is the oil spill in the Gulf. Nobody seems to be willing to admit just how much oil has been leaked into the ocean so far, but with the spill threatening every coast in that region, and thoughts going on toward it crossing into Europe at some point, I would say that this could drastically change Earth. But then I may be wrong too, maybe I too am being arrogant as some others in my belief that we could ever cause a change in this planet. Maybe it will surprise us all and prove that we are very small and do not really understand this great planet as much as we wish.