One theory is that the meltdown of methane hydrates—icelike deposits that store massive amounts of potent greenhouse gases in the seafloor—was responsible.
According to the new study, pre-warming triggered the melt, releasing greenhouse gases into the atmosphere. Less clear is the nature of that pre-warming, study author Sluijs said.
One possibility, he pointed out, is a bout of volcanic activity that ripped Greenland from Europe, a theory proposed earlier this year in the journal Science.
Today Earth is also experiencing global warming, which scientists believe is largely driven by the burning of fossil fuels like coal and oil.
This warming could force a meltdown of hydrates on the seafloor as well, releasing methane into the ocean-atmosphere system.
"We really should know whether the [carbon dioxide] that's being added to the atmosphere now has the potential to generate some kind of unanticipated cascade of events," Wing, the Smithsonian biologist, said.
Though the Nature study does not solve the question, he added, scientists now have more reason "to start to worry about these kinds of unanticipated changes."
Hydrate deposits contain approximately as much greenhouse gases as will be released from current and projected emissions from fossil fuels, Sluijs pointed out.
"We are just at the beginning of the modern climate change," he said. "We are able to stop it, or at least keep the damage minor.
"But if we are going to keep burning fossil fuels for the next couple of centuries, then yes ... definitely at a certain point you will dissociate the methane hydrates, without a doubt."
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