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Atmospheric levels of a potent greenhouse gas whose concentration levelled off in 1999 could rise in the near future, warns a study published in Nature today (28 September).

Its authors say China's booming economy could be responsible for an increase in methane emissions, and that this trend has been masked by a simultaneous decline in natural emissions from wetlands.

Methane is the most important greenhouse gas after carbon dioxide. Although significantly less methane is emitted into the atmosphere, the gas traps more than 21 times more heat per molecule than carbon dioxide.

Large yearly variations in the rate at which methane's concentration in the atmosphere rises are well documented, but poorly understood.

Now, a team led by French climatologist Philippe Bousquet has combined computer simulations of how methane is transported in the atmosphere with 20 years of atmospheric measurements to trace global sources of the gas.

They found that the annual variations are mainly due to natural fluctuations in methane emissions from wetlands during wet and dry periods.

The study shows that methane's concentration in the atmosphere rose less steeply during the 1990s, partly due to wetlands drying up.

The main reason for this recent trend, however, was a drop in emissions caused by human activities — for instance through initiatives that capture the gas emitted from landfills and use it to generate electricity.

But the researchers warn that we should not take comfort in this.

Indeed, they show that since 1999, extended droughts have reduced emissions of methane from wetlands and that this has masked a recent increase in industrial methane emissions, especially in North Asia.

Since 2003, human emissions have returned to early 1990 levels.

"This may reflect the booming Chinese economy," write the researchers.

"Had it not been for [the drop in methane coming from wetlands since 1999], atmospheric levels of methane would most likely have continued rising," says one of the team, Paul Steele from CSIRO Marine and Atmospheric Research in Australia.

Wetlands are responsible for roughly one-third of global methane emissions, according to 2001 estimates by the Intergovernmental Panel on Climate Change.

The team warns that if wetland emissions were to return to 1990 levels, overall atmospheric methane could rise again.

Link to full paper in Nature

Link to related commentary in Nature

Reference Nature 443, 439 (2006)