IT鈥橲 a nightmare scenario 鈥� slowly rising sea temperatures trigger the release of massive amounts of methane that dramatically amplify the greenhouse effect, causing runaway global warming. Hopefully it won鈥檛 happen, but if it does it won鈥檛 be the first time. This exact chain of events was played out 55 million years ago.
Tremendous amounts of the powerful greenhouse gas methane are stored in icy hydrates under the seabed and in permafrost. The total amount of carbon in the hydrates is an estimated 10,000 gigatonnes, twice that in the reserves of all other fossil fuels combined and more than enough to dwarf the 750 gigatonnes of carbon in the atmosphere as carbon dioxide.
If today鈥檚 warming continues and deep-sea temperatures cross the threshold at which methane hydrates melt, huge amounts of methane could be released, triggering drastic global warming. It isn鈥檛 known how likely this is, but researchers have shown that something similar happened at the end of the Palaeocene epoch. Fifty-five million years ago, a gradual warming of the oceans preceded a dramatic shift in carbon isotope ratios and a steep jump in water temperatures 鈥� precisely the pattern expected if gradual warming melted the hydrate reserves. Looking at what happened in the Palaeocene helps to answer two crucial questions: how much of the methane can be released at once, and how that would affect climate.
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The temperature hike at the end of the Palaeocene is known to be one of the sharpest in the geological record. Within a few thousand years, sea surface temperatures soared by up to 8 掳C, while deep water warmed by 5 掳C. Now a team led by Deborah Thomas of the University of North Carolina at Chapel Hill has analysed levels of carbon isotopes in hundreds marine plankton shells, to pin down the sequence of events for the first time (Geology, vol 30, p 1067).
Crucially, they found that sea temperatures began rising gradually before the first evidence of methane release 鈥� a sharp drop in carbon-13 levels in surface-dwelling plankton 鈥� making it hard to find any other explanation for the isotope shifts (see Graph). Some 1200 gigatonnes of carbon was released as methane at the end of the Palaeocene, says Thomas, 鈥渃omparable to the size of the hydrate reservoirs at the time鈥�. Once sea temperatures crossed a certain threshold, all the hydrates were released in a cascade of bursts, each causing more warming that triggered further releases. Temperatures then remained high for the next 200,000 years. The drastic changes were bad news for deep-sea plankton, which suffered widespread extinction. But the rise in temperature benefited mammals, which evolved into new forms and spread around the globe.
Thomas is cautious about comparing the present to the past. In the Palaeocene 鈥渨e are dealing with a climate vastly warmer than we know today鈥�, she says. The planet lacked permanent ice caps, and the waters off Antarctica where she took her samples were about as warm as those off San Francisco are today. Yet today鈥檚 ice sheets pose their own risks. When large volumes of ice melt, this can disrupt the flow of deep waters and change climate significantly. 鈥淭he trigger is a lot touchier today than it was 55 million years ago,鈥� she told 快猫短视频.