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  Methane climate shock 'less likely'
Catastrophic climate change due to a massive release of underwater and underground stores of methane is less likely than previously thought, suggests new research.

CSIRO atmospheric scientist Dr David Etheridge and colleagues report their findings in today's issue of the journal Science.

"It think [the study] helps us have some confidence to narrow down the range of possible future methane levels," says Etheridge.

Methane is a potent greenhouse gas with current atmospheric levels close to 1800 parts per billion and increasing, says Etheridge.

He says the most common natural source of atmospheric methane is anaerobic decomposition of organic material in wetlands.

But a much greater potential source is trapped in methane hydrates, or clathrates, deep on the ocean floor or under permafrost.

Clathrates are ice-like lattices that can release methane gas as the temperature warms, pressure decreases, or land slips.

Some scientists have worried that catastrophic global warming could occur if warming triggers vast releases of methane from clathrates.

"If they were unstable, clathrates have the potential to overwhelm the atmospheric carbon cycle," says Etheridge.
Evidence

Etheridge says studies have suggested clathrates have previously caused a rapid rise in atmospheric methane.

But, he says 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°C.

"As the earth warmed, the methane increased quite significantly and it's not clear what the source of that methane was," says Etheridge.

He says 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.
Ancient bubbles

In an attempt to settle the issue, Etheridge and colleagues analysed 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 says.

Etheridge describes the study as providing "unambiguous" evidence on the source of the methane.

He says while the study focuses on just one warming event, the findings should reduce concern about the massive release of methane from clathrates.
Challenging work

Etheridge says analysing C14 levels is the most direct technique for distinguishing between modern and ancient methane.

But, he says it is challenging work because the amount of methane in the atmosphere containing C14 is only one part per trillion.

Etheridge says this meant the team had to excavate tonnes 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," says Etheridge.

The ice was melted on-site and large volumes of air were extracted for analysis. Each measurement required 1000 kilograms of ice, says Etheridge.

Etheridge's colleague, Dr Andrew Smith from ANSTO, used a technique called accelerator mass spectrometry to detect individual C14 atoms.
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