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Causes of methane growth

According to the Intergovernmental Panel on Climate Change, 60 per cent of atmospheric methane is generated from human-related activities such as fossil fuel production, livestock, rice cultivation, and waste management. There are also natural sources, such as wetlands, termites, and wildfires.

Dr Etheridge said highly sophisticated analytical techniques are required to measure the changes in methane which exists at levels of only 1-2 parts per million in the atmosphere. This is particularly so in the tiny amounts of air in Antarctic ice. A further challenge was to measure the isotopic composition of methane, to pinpoint the cause of the changes and from which sources.

"We explored the causes of the 150% increase in global methane during the industrial period over the past 200 years and the relatively stable period beforehand by measuring air enclosed in Antarctic ice.

"Increases during the industrial period have come from elevated emissions from biogenic sources such as rice cultivation and more intensive livestock production, gas leaked during fossil fuel exploitation and pyrogenic sources such as the burning of forests and grass lands and wood fuels.

"The biggest surprise was how pyrogenic emissions revealed in the 2000-year record varied wildly.

"It was previously assumed that this source was smaller than today and quite steady. But we found that the amount of methane produced from burning was similar 1000 years ago to now.

"It then dropped to about half that amount, due to what we interpret as the effect of a cooler and wetter global climate, before increasing again. The recent increase is clearly from the human impact of the past two centuries" Dr Etheridge said.

"Warmer and dryer periods caused more fires, increasing pyrogenic methane emissions, while simultaneously decreasing the area of natural wetlands, reducing biogenic methane emissions."

The net result was approximately stable methane concentrations but a large swing in its isotopic makeup, as observed. He said there is also evidence that emissions from human activities affected global methane levels even before the industrial period (ie. between 0 and 1750 AD).

"Results from the Law Dome cores have been flowing since they were first drilled by the Australian Antarctic Program about 15 years ago. They have clearly shown the growth in greenhouse gases and ozone depleting gases in the atmosphere. The ability to measure isotopes is a new development which now allows us to understand the causes of the changes," he said.

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Last updated 12/11/08