It is as impossible to prove the existence of Global Warming as it is impossible to prove the existence of God.
I will argue neither point. This is simply an observation.
Advocates of both point to certain signs and symptoms that denote their existence, but sadly neither can conclusively prove or disprove their actual existence.
Ironic further is that (until recently) the inverse population seems to believe in each. Typical God-fearing Christian Republicans stereotypically disbelieve in the theory of global warming, while environmentalists and global warming fanatics (until recently) tend to be Democratic and are more often humanists and/or atheists.