By Dani Cooper for ABC Science Online
ABC News Online, Posted 11 hours 50 minutes ago
Methane levels in the atmosphere have started to rise after almost eight years of near-zero growth, an international study says.
And the increase may lead to an acceleration of global warming, the scientists behind the study warn.
Dr Paul Fraser of the CSIRO, who co-authored the study that appears in Geophysical Research Letters, says samples taken from 12 stations across a global network showed a trend upwards in methane levels.
"After seven years [of zero growth] methane has started to rise again to growth rates of the early 1990s," he said.
The chief research scientist in the Division of Marine and Atmospheric Research says the increase in methane could lead to an acceleration in global warming.
"If methane concentrations continue to grow at the current rate then it will be once again the second-most important greenhouse gas to control after CO2 over the next few decades," he said.
Dr Fraser says methane accounts for about 20 per cent of all greenhouse gases since the industrial revolution.
It is 20 times more potent than carbon dioxide as a greenhouse gas and comes from sources such as natural wetlands, rice fields, fires, coal mines and natural gas reticulation.
Emissions have been balanced over the past decade by natural sinks that absorb the gas and through oxidation into the atmosphere.
Dr Fraser says sources of methane have been growing with drainage of tropical wetlands for agricultural use and increased fossil fuel use.
However, the destruction of natural tropical wetlands removes an important sink from the delicate equation.
"[The result is] in the past year the total sources have overwhelmed the total sinks, and methane has again started to rise," he said.
This imbalance has resulted in several million tonnes of extra methane in the atmosphere, the paper says.
The study, which also involved the Massachusetts Institute of Technology, Scripps Institution of Oceanography and the University of Bristol, says the rise in methane concentrations is due in part to increased methane releases in the high latitudes of the Northern Hemisphere.
Dr Fraser says this could be a one-off anomaly due to a very hot Northern Hemisphere summer in 2007 that would have warmed the wetlands of the tundra increasing methane production.
However if it is a long-term trend then it is a worrying development.
Surprisingly the researchers found the methane levels rose simultaneously at all measurement locations across the globe.
The researchers say further research is needed to understand why this happened.
The Intergovernmental Panel on Climate Change has identified the need to understand causes of the variations of methane growth rates as a priority area of research.
Dr Fraser says scientists have only a basic understanding of methane variations and continuing high-quality observations are required particularly in tropical and boreal locations.