From an episode of South Park featuring Al Gore. In the episode Gore is depicted as a paranoid guy who believes in a non-existent monster--"Manbearpig"--that is part man, part bear, and part pig. He believes the Manbearpig is a great threat.
South Park's creators, Trey Parker and Matt Stone, don't believe in global warming and the episode "Manbearpig" is their way of saying that when Al Gore talks about the dangers of global warming he is getting all worked up over something that doesn't even exist.
"The South Park guys think that global warming is a myth just like the Manbearpig, even though there's tons of evidence that it's happening, and they make fun of Gore for worrying about it."