After eight years of near-zero growth in atmospheric methane concentrations, levels have again started to rise. "This is not good news for future global warming," says CSIRO's Dr Paul Fraser, who co-authored a paper to be published in Geophysical Research Letters, a journal of the American Geophysical Union.

"Over recent years, the growth of important greenhouse gases, namely methane and the CFCs, had slowed. This tended to offset the increasing growth rate of carbon dioxide that results mainly from large increases in the consumption of fossil fuels, particularly in the developing world.

"Now that methane levels have resumed their growth, global warming may accelerate."

Methane is the second most important greenhouse gas in the atmosphere after carbon dioxide, accounting for nearly 20 per cent of global warming since the industrial revolution.

Methane is emitted to the atmosphere from natural wetlands, rice fields, cattle, forest and grassland fires, coal mines, natural gas leakage and use, and other sources.

"Over the past decade these methane sources have been close to balancing the absorption of methane through atmospheric oxidation and into dry soil," Dr Fraser says.

"This fragile balance has resulted in little growth of methane in the atmosphere. Apparently some sources have been increasing, such as from fossil fuel use, cattle, and rice, while others have been decreasing, particularly natural tropical wetlands. However, over the past year, the total sources have overwhelmed the total sinks, and methane has again started to rise."

Dr Fraser says that recent analyses of global data by CSIRO and collaborators at the Massachusetts Institute of Technology, Scripps Institution of Oceanography and the University of Bristol suggest that the methane increase is, at least in part, due to methane releases in the high latitudes of the Northern Hemisphere.

"Such increases have been predicted as rapid Arctic ice melting creates more high latitude wetland sources," says Dr Fraser.

"A possible additional cause of the methane increase is that atmospheric oxidation may be weakening, for reasons as yet unknown, although recovery from ozone depletion, which is predicted to have commenced, may be involved."

The Intergovernmental Panel on Climate Change (IPCC) has identified the need to understand causes of the variations of methane growth rates as a priority area of research. "The reality is that scientists have only a very basic understanding of these methane variations," Dr Fraser says.

"In order to predict the future contribution of methane to climate change, continuing high-quality observations, in particular in tropical and boreal locations, are required as input to, and verification of, sophisticated climate models."