Certainly, Earth has gone through many warming and cooling cycles over millions of years. Some evidence suggests that the global warming seen today may be due, at least in part, to natural causes. However, there is also compelling evidence suggesting that greenhouse gases released by human activities are playing a large role in the current warming.
Besides, how ridiculous does it sound to say "oh, our pollution really isn't causing global warming, so let's keep polluting"? Sooner or later, through global warming or through other means, we will suffer catastrophe if we keep abusing our Mother Earth.
Simply put, we are committing matricide! We are slowly but surely killing our planet, the very Earth that gave us life and continues to give us shelter and nourishment. We still cling to mother's teats, sucking what we can out of her breasts. If we lose mother's precious milk, if we make this planet uninhabitable for human beings, we are dead. There is no other. There is no conveniently inhabitable planet nearby. There is no wet nurse waiting in the wings.
We only have one home planet, one mother, one Earth.