Long-term global warming as seen during the PETM requires not ... of only a few billion tonnes of methane — the largest pulses expected from either methane hydrates or hydrothermal vents ...

Dickens, G. R., Down the rabbit hole: Toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events.

The PETM lasted more than 150,000 years ... hypothesis is that much of the carbon came from large deposits of methane hydrate, a peculiar, icelike compound that consists of water molecules ...