Whether we as humans are causing global warming is moot at this point for two reason:
1) Whether we're causing it or not, it's wise and the right thing to do to take care of our environment. To hide behind beliefs that we as humans aren't causing global warming is, frankly, asinine. The reason it is asinine is because even if we're not causing global warming, I'm still seeing smog in L.A., trash on the roadways and urban sprawl way out of control. We're not helping the environment.
2) The point is moot because, hey, this is a freakin' comic! Enjoy the humor of it and let's move on! =D
Let's not overanalyse this. It's just funny because it mines the inexhaustible vein of humour that is Dilbert's total (and I am starting to think entirely conscious, rather than ignorant) lack of tact.