Not forgetting the large stores of methane hydrate presently sitting at the bottom of the sea, under tundra and on the continental margins, all waiting for the temperature of the water to get high enough to release them.

Oh, and 'large stores' means approx 10,000 gigatons of carbon. The atmosphere's currently got about 730 gigatons of carbon in it. Not that all of the methane hydrate would end up in the atmosphere of course - a proportion of it'd react with the sea water as it bubbled up.

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Simon