Methane Mystery Just Got Murkier

August 11, 2011

Anna Salleh ABC

Some researchers say changes in rice cultivation were responsible for a plateau in methane emissions in the late 20th century

Two papers investigating a puzzling decline in the rate of growth of the powerful greenhouse gas methane have come up with two different explanations.

The papers, appearing in today's issue of the journal Nature, use different analytical approaches to explore why atmospheric methane levelled off at the end of the 20th century, before continuing its upward trend.

Methane is second in importance only to CO2 among the greenhouse gases that humans contribute to, and about 60 per cent of atmospheric methane is estimated to come from human activities.

Fossil fuels and agriculture (especially involving rice and cattle) are the two main sources of human-induced emissions of methane.

Accounting for the plateau in methane emissions in the late 20th century will help scientists better work out where methane is coming from, and how best to reduce it.

In one of the new papers, Dr Murat Aydin from the University of California, Irvine, and colleagues, say the main reason for the plateau is a fall in methane from fossil fuels, as it became an economically valuable commodity in natural gas production.

Aydin and colleagues measured levels of fossil-fuel-derived ethane, recorded in Greenland and Antarctic ice, as a proxy for methane from fossil fuels, allowing them to distinguish it from methane emitted from other sources.

Agricultural change

In the other new paper, Fuu Ming Kai, also from the University of California, Irvine, and colleagues, say that the mystery levelling off in methane was mainly due to changes in agricultural practices.

Using the different isotopic signatures of methane from microbial versus fossil fuel sources, Kai and colleagues argue the use of less water and more fertiliser in Chinese rice-growing is responsible for around half the decrease in Northern Hemisphere methane emissions seen in the late 20th century.

While on the surface this could appear to contradict the findings by Aydin and colleagues, Australian experts from CSIRO Marine and Atmospheric Research, says the findings are potentially reconcilable.

"What could be happening is a combination of both. Both explanations could be correct," says Dr Paul Fraser.

Uncertainties

Both Fraser and colleague Dr David Etheridge point to the uncertainty surrounding both sets of findings.

"There is actually quite a large uncertainty," says Etheridge, who like Fraser studies the changes in atmospheric greenhouse gas concentrations over time.

Both experts say the uncertainties mean that further research is required to improve both methods of tracing the sources of methane emission.

But, says Etheridge, there is a silver lining to these papers.

"One thing that both of these studies show is there are decreases in both fossil fuel and agricultural methane because of changes in human practices," he says.

"This shows that we have some control over these processes."

Etheridge says humans can consciously reduce methane levels by continuing the activities in agriculture and fossil fuel management that incidentally led to reductions in the late 20th century.

Source: ABC Science