I would not be too dismissive of Methane, it may degrade relatively quickly but there is a lot of it and when it starts to be released from methane hydrates it is in a feedback loop that would accelerate as the temperature increased. Also bear in mind it is proposed that a partial release mechanism of the storegga slide was destabilised by methane hydrates and shot a tsunami over a chunk of northwest europe. The death toll in Doggerland alone would have been catastrophic.

http://worldoceanreview.com/en/wor-1/ocean-chemistry/climate-change-and-methane-hydrates/

https://en.wikipedia.org/wiki/Storegga_Slide