Wiki mentions undersea volcanic activity, increasing levels of water vapor in the atmosphere, and increasing co2 levels.
Britannica mentions the melting of ice from a fork in the upper Angerman River in northern Sweden.
I spent a grand total of 5 minutes researching, so I'm sure there are many theories on it.
In my opinion, the problem is that most people who don't believe in GW are republicans, and if you ask most republicans, they'll tell you the Earth is only 5-6 thousand years old. I guess it doesn't matter if the whole world goes to hell when you're more concerned about the next world than the current one. That being said, no matter how much I disagree with some people, saying they have a bloated head on a matter such as this (come on, I can understand other political matters but not THIS one, and even then this forum is supposed to be friendly so don't do it anyway) is pretty out of line. Especially since the person who said that was even making jokes to ease the tension.