That ancient seabed methane is the real old fart. Our contributions (personally and via cattle) may pale in comparison.
Agreed that methane releases from the tundra and seabed may turn out to be the Black Swan that trumps our other concerns.

IMO, we are just marking time until the next repeat of a Storegga or Unimak event:

http://www.semparpac.org/tsunami.jpg

EDIT: Won't be much fun for a city where the tsunami throws/break ups a giant LNG tanker against the docks or rocks.

it's happened before. Though the last time it happened was in the Permian era and is known as the great dieing. Looks like we are playing the remake here, just like the original except that instead of the formation of the Siberian traps dumping carbon into the air as the first part of the ball rolling it's us.

It's Gaia saying "pull my finger".