There are mega amounts of methane locked up in permafrost
and under a lot of continental shelves - it's thought that it sometimes
gets released in catastrophic events - causing undersea landslides,
tsunami and such.
The methane itself isn't frozen but it's encased in lattice-like structures
of frozen water. Russian scientists have been warning for years about this
because methane is hugely more potent greenhouse gas than CO2...
Methane release is considered a likely cause of the Storegga
Slides where a 180 mile long section of the Norwegian continental
shelf collapsed around 6000 years ago:
Shakhova et al. (2008) estimate that not less than 1,400 Gt of Carbon is presently locked up as methane and methane hydrates under the Arctic submarine permafrost, and 5-10% of that area is subject to puncturing by open taliks. They conclude that "release of up to 50 Gt of predicted amount of hydrate storage [is] highly possible for abrupt release at any time". That would increase the methane content of the planet's atmosphere by a factor of twelve.
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