Methane-fuelled climate catastrophe 'less likely'

ABC News Online, Fri Apr 24

• http://www.abc.net.au/news/stories/2009/04/24/2552225.htm
• Audio: Stored methane reserves don't contribute to warming (The World Today)

Carbon dioxide is not the only problem for the world. A bigger problem could well be methane.

The gas emerges from swamps and in the burps and farts of animals, including us humans, and it is a big contributor to global warming.

Now there are fears that stores of the gas trapped at the bottom of the ocean could be released by warming temperatures.

It is something that is exercising the minds of scientists in Denmark, the US, New Zealand and Australia.

What has worried climate scientists about methane is that there is so much of it.

It is a far more potent greenhouse gas than carbon dioxide but thankfully, most of the planet's methane is locked up, stored at the bottom of the ocean or in ice sheets. It is known as clathrate methane.

The concern has been that as the world gets warmer, some of the clathrates would escape into the atmosphere and have a dramatic amplifying effect on global warming.

The circumstantial evidence has not been good, says Dr David Etheridge from the CSIRO.

"There's evidence in the long-term past, millions of years ago, that this may have occurred. It is circumstantial evidence only," he said.

"What we needed to know for the future is whether the warming that we are currently seeing, and which will increase in the future, will destabilise these clathrates."

Dr Etheridge and colleagues in the United States, New Zealand and in Australia have managed to work out whether the methane is something to be worried about.

They looked at an event about 12,000 years ago known as the Younger Dryas period. Temperatures got suddenly warmer in the northern hemisphere and were accompanied by a big increase in methane.

By mining ice sheets in Greenland and analysing the gas bubbles trapped in there, they have been able to prove that the methane was not there because it came out of the clathrates.

"Just to give you an idea of the technical challenges involved, the amount of carbon 14 methane in the Younger Dryas atmosphere amounted to about 1.25 kilograms globally. Because carbon 14 is radioactive that has decayed away over two half lives," said Dr Andrew Smith, from the Australian Nuclear Organisation ANSTO at Lucas Heights in Sydney

"It means that today there is only about one-third of a kilogram of that original radio methane left on the Earth."

Therefore, finding enough 12,000-year-old air to analysis was the challenge.

Tonnes of ancient ice was carved from the Greenland sheet, then melted in vacuum containers.

The gas that emerged was trapped and bottled and shipped to New Zealand and there it was converted into carbon dioxide and sent across to Australia.

At Lucas Heights it was condensed into tiny specks of graphite that Dr Smith carbon dated using a technique known as accelerator mass spectrometry.

"And from that 1,000 kilograms of ice, we ended up with just 20 micrograms of carbon. In that 20 micrograms of carbon, the carbon 14 was present at the level of about one part in a trillionth or less," he said.

The results were good news - the big increases in methane in the air were not coming out of the clathrates.

It means one less potentially significant contributor to future global warming.

The CSIRO's Dr Etheridge says it is an encouraging result.

"Clathrates contain several thousand times the amount of methane than is in the atmosphere presently, so there is a huge potential there and these clathrates can destabilise with temperature," he said.

"I think this confirms that that source of methane, that potential source of methane, is more stable than we previously thought and that gives us some upper bounds to the future releases that we might expect with a warming world."

Based on a report by Shane McLeod for The World Today.