Studies have suggested clathrates previously caused a rapid rise in atmospheric methane. But, said Etheridge, the evidence to date is inconclusive and most previous studies have looked at periods so deep in the past they are not representative of current climate dynamics on Earth. Etheridge and colleagues studied the most recent pre-industrial increase in atmospheric methane 12,000 years ago, at the end of the last ice age. At that time, the methane nearly doubled to 750 parts per billion over a period of hundreds of years, coinciding with a warming of about 5 degrees Celsius. "As the Earth warmed, the methane increased quite significantly, and it's not clear what the source of that methane was," says Etheridge. One hypothesis suggests that the methane came from increased productivity in wetlands, boosted by global warming. But a competing hypothesis is that the methane was released from clathrates. In an attempt to settle the issue, Etheridge and colleagues analyzed the methane in ancient bubbles of air trapped in Greenland ice. They checked the levels of naturally occurring carbon-14 (C14) isotopes in methane over a period of 300 years at the start of the warming. Methane from clathrates is millions of years old and contains no C14, whereas methane from wetlands is more recent and has a higher level of C14. "The increase in the methane was not accompanied by a decrease in the C14, which tells us the origin was the wetlands not the clathrates," he said. Etheridge described the study as providing "unambiguous" evidence on the source of the methane. While the study focuses on just one warming event, the findings should reduce concern about the massive release of methane from clathrates, he added. Analyzing C14 levels is the most direct technique for distinguishing between modern and ancient methane, said Etheridge. But, he said it is challenging work because the amount of methane in the atmosphere containing C14 is only one part per trillion. Etheridge said this meant the team had to excavate tons of ice to get the measurements they needed. This was made easier by taking a chainsaw to an outcrop of ice that had been turned up on its side, revealing past layers on the surface. "You've essentially got the whole profile of the ice sheet tilted into the horizontal. So by going along the surface you can actually walk back in time," said Etheridge. Related Links: |
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