Reduced methane emissions from drying wetlands have temporarily offset increased emissions from human sources.
Credit: Derek Jensen
SYDNEY: Drying wetlands across the globe have been revealed as key players in the mystery of why atmospheric levels of methane – the second most important greenhouse gas – have stabilised in recent years, a new international study shows.
"We've known that methane hasn't been growing in the atmosphere for 7 or 8 years now, and we've been puzzled by that," said Paul Fraser from Australia's research agency, the CSIRO. "Our research has come up with a credible explanation of what's happening - and a suggestion of what may happen in the future."
The paper, published today in the British journal Nature, shows that declining fossil fuel use in the 1990s resulted in the recent slower release of methane into the global atmosphere. Since 1999, however, methane emissions from human activities - especially China's booming coal industry - have increased, but their effect on the atmosphere has been offset by reduced emissions from wetlands.
Methane - responsible for a fifth of global warming over the past 200 years - is emitted by both human activities and natural sources. Exploitation of fossil fuels and decaying landfill are the primary human sources, while rotting wetland vegetation is the major natural source. The digestive gases from the world's cattle and sheep herds also make a contribution.
According to co-author Paul Steele of the CSIRO's division of Marine and Atmospheric Research, prolonged drying of wetlands - caused by agricultural pressure and climate change - has resulted in a reduction in the amount of methane released, masking the rise in emissions from human activities in recent years.
"If the drying trend is reversed and emissions from wetlands return to normal, atmospheric methane levels may increase again, worsening the problem of climate change," said Steele.
The researchers used computer simulations of how the gas is transported in the atmosphere to trace back to the source of methane emissions, based on atmospheric measurements from the past 20 years.
The results suggest that the lower methane emissions in the 1990's were largely because of decreased use of natural gas in the Northern Hemisphere. The more recent rise stemmed from strongly increasing emissions from fossil fuel use in north Asia.
The scientists also showed how changes in emissions from wetlands and, to a lesser extent, bushfires accounted for variations in atmospheric methane from year to year.
"The end result will be a better understanding of the methane cycle in the atmosphere which will lead to more reliable predictions of climate change," said Fraser. In addition, the research will guide strategies aimed at reducing methane emissions from human activities.
To date, these strategies have included improved piping of natural gas, the capture of methane from landfill sites to generate electricity, and the development of vaccines to reduce methane production by cattle.
According to Fraser, "Although methane levels have stopped growing in the atmosphere over the last 7 or 8 years, that's no reason to become complacent."
Nice theory, but...
going out and measuring CH4 would make your computer model much more believable.