Methane Levels to Rise Again After Slowdown

Scientists have uncovered evidence that levels of the greenhouse gas methane will rise sharply in the next few years, warming the planet faster than previously expected. The new data from an international team of scientists has revealed that while methane levels began to level off in the 1990s, emissions from human activity started to climb again before the end of the last century.

Phillipe Bousquet at the Laboratory of Sciences of Climate and the Environment in Paris joined scientists from the US, Australia, the Netherlands and South Africa to examine methane levels in the atmosphere from the early 1980s using a network of 68 ground-based tracking stations around the world. The upturn in man-made emissions was masked by a drop in the methane released naturally from wetlands, by unusually dry weather.

Writing in the journal Nature today, the scientists raise fears that inevitably wetter weather will return the wetlands to their normal state in the next three to five years, boosting the amount of methane in the atmosphere by 10m tonnes a year. Although methane levels are 200 times lower than the most widespread greenhouse gas, carbon dioxide, molecule for molecule, it is 20 times more effective at retaining heat in the atmosphere.

They discovered that methane levels fell from nearly 12 parts per billion in the 1980s to four parts per billion in the 1990s. But their calculations show that the slowing of emissions was only partly to do with strict limits imposed on industry. Since 1999, levels of methane from human activity have been rising in Asia, consistent with a surge in coal usage in China.

"The bad news is that the slowdown in global methane emissions in the past few decades was only temporary," said Jos Lelieveld, director of the Max Planck Institute for Chemistry in Mainz, Germany.