Giant ocean farts will kill us all.
So here is how it goes, we warm the climate a bit with our cars and burning forests and coal power plants, that warms the ocean, which melts these crazy things called methane hydrates, and the earth lets out a giant fart that it has been holding for a hundered million years. Unfortunatly, since methane is a greenhouse gas 20 times stronger then Cabon Dioxide, we all cook like those wierd pink hotdogs they sell in New England.
Plan B: we quickly swim down, put a big bubble over the earths anus, and suddenly we all have a cheap source of natural gas. I have a snorkel mask, if you have some plastic tubing, I say lets get started!
For more info: http://www.usatoday.com/weather/climate/2006-07-21-undersea-gas_x.htm