There is an excellent article in the October 2011 National Geographic on the Paleocene-Eocene thermal maximum 56 million years ago. A quantity of CO2 was released in a few thousand years that was 10 times what we have released in the last 100 years, but in the ballpark of what will be released in the next couple of hundred years if we burn it all. The massive release 56 Ma is thought to be from methane near the sea floor (largely for lack of other reasonable choices). (The methane over time converts to CO2 the article says). Today there is frozen methane all over the oceans beneath levels controlled by temperature and pressure. If Ocean temperature at depth (say, near 400 m to 1 km in places like California offshore) increase, methane hydrate becomes unstable and turns to methane gas, which is then released to the water and atmosphere (sometimes by submarine landslides).

I suggest to anyone to read this article. It should be read by school students.

Far better science than what Nova puts out.
(Nova Science Now is different; they do a good job).

Chris