Human-caused global warming?

I can't get through a single month without running into something or someone that claims that "scientists all agree that people are causing global warming by putting greenhouse gasses into the air." I note, however, that what a man believes, he will act on. If these people really believe that "greenhouse gasses" in the air are causing global warming, why are they not looking for ways to remove the "excess" already present? It seems straightforward to me.