Nobody knows what started it, but there's no doubt Earth's temperature rose by as much as 6 degrees Celsius, which affected the planet for up to 150,000 years, until excess carbon in the oceans and atmosphere was reabsorbed into sediment. The ecosystem changed and many species went extinct during the Paleocene-Eocene Thermal Maximum when at least 2,500 gigatons of carbon were released into the ocean and atmosphere. 

A new report in Nature Geoscience suggests that even though methane-containing gas hydrates , the "ice that burns", occupied only a small zone of sediment under the seabed before the PETM, there could have been as much stored then as there is now. Anything that disturbs the stability of methane hydrate under the ocean and in permafrost could warm the atmosphere and prompt the release of large amounts of methane.

Some who study the PETM blame the worldwide burning of peat, volcanic activity or a massive asteroid strike as the source of the carbon, "but there's no crater, or any soot or evidence of the burning of peat," said Gerald Dickens, a Rice professor of Earth science and an author of the study, who thinks the new paper bolsters the argument for hydrates.

In the ocean, organisms die, sink into the sediment and decompose into methane. Under high pressure and low temperatures, methane molecules are trapped by water, which freezes into a slushy substance known as gas hydrate that stabilizes in a narrow band under the seafloor.

Warmer oceans before the PETM would have made the stability zone for gas hydrate thinner than today, and some scientists have argued this would allow for much less hydrate than exists under the seafloor now. "If the volume – the size of the box – was less than today, how could it have released so much carbon?" Dickens asked. "(Lead author graduate student Guangsheng) Gu's solution is that the box contains a greater fraction of hydrate."

"The critics said, 'No, this can't be. It's warmer; there couldn't have been more methane hydrate,'" says Chemical Engineering professor George Hirasaki. "But we applied the numerical model and found that if the oceans were warmer, they would contain less dissolved oxygen and the kinetics for methane formation would have been faster."

With less oxygen to consume organic matter on the way down, more sank to the ocean floor, Gu said, and there, with seafloor temperatures higher than they are today, microbes that turn organic matter into methane work faster. "Heat speeds things up," Dickens said. "It's true for almost all microbial reactions. That's why we have refrigerators."

The result is that a stability zone smaller than what exists now may have held a similar amount of methane hydrate. "You're increasing the feedstock, processing it faster and packing it in over what could have been millions of years," said Dickens. While the event that began the carbon-discharge cycle remains a mystery, the implications are clear. "I've always thought of (the hydrate layer) as being like a capacitor in a circuit. It charges slowly and can release fast – and warming is the trigger. It's possible that's happening right now."

That makes it important to understand what occurred in the PETM, he said. "The amount of carbon released then is on the magnitude of what humans will add to the cycle by the end of, say, 2500. Compared to the geological timescale, that's almost instant."

"We run the risk of reproducing that big carbon-discharge event, but faster, by burning fossil fuel, and it may be severe if hydrate dissociation is triggered again," Gu said, adding that methane hydrate also offers the potential to become a valuable source of clean energy, as burning methane emits much less carbon dioxide than other fossil fuels.

The calculations should encourage geologists who discounted hydrates' impact during the PETM to keep an open mind, Dickens said. "Instead of saying, 'No, this cannot be,' we're saying, 'Yes, it's certainly possible.'"