Tom Clancy movie plot: Apocalyptic extremist group gets hold of a nuclear weapon and plots to use it to release a large amount of methane. However, it doesn't stop there. They also ignite the methane to create an ongoing reaction heat the hydrate deposit to release additional billions of tons of methane.

Slightly more sophisticated movie plot: Market forces accomplish basically the same thing over the period of a few decades, which still results in a runaway positive feedback loop causing more methane hydrates to release, making it warmer, etc.