Levels of climate-warming methane - a greenhouse gas 25 times as potent as carbon dioxide - rose abruptly in the Earth's atmosphere last year, and scientists who reported the change don't know why it occurred.

Methane, the primary component of natural gas, has more than doubled in the atmosphere since pre-industrial times, but stayed largely stable over the last decade or so before rising in 2007, researchers said on Wednesday.

This stability led scientists to believe that the emissions of methane, from natural sources like cows, sheep and wetlands, as well as from human activities like coal and gas production, were balanced by the destruction of methane in the atmosphere.

But that balance was upset starting early last year, releasing millions of metric tonnes more methane into the air, the scientists wrote in the Geophysical Research Letters.
 
'The thing that's really surprising is that it's coming after this period of very level emissions,' said Matthew Rigby of the Massachusetts Institute of Technology. 'The worry is that we just don't understand the methane cycle very well.'    

Another surprise was that the rise in methane levels happened simultaneously at all the places scientists measured around the globe, instead of being centered near known sources of methane emissions in the Northern Hemisphere, said Rigby, one of the study's lead authors along with Ronald Prinn, also of MIT.

A rise in methane in the Northern Hemisphere might be due to a year-long warm spell in Siberia, where wetlands harbour methane-producing bacteria, the scientists said, but had no immediate answer on why emissions also rose in the Southern Hemisphere at the same time.

There is considerably less methane than carbon dioxide in the atmosphere. Pre-industrial concentrations of methane were about 700 parts per billion - that is, for every billion molecules of air, there were only 700 of methane - but that level rose gradually to 1773 parts per billion by the late 20th century, Rigby said in a telephone interview.
   
The rise in 2007 was about 10 parts per billion over the course of a year, a real jump for such a short period of time.

By contrast, there are about 385 parts per million of carbon dioxide in the atmosphere. However, methane is much better at locking in the solar radiation that heats up the planet.

Methane is destroyed by reaction with an atmospheric 'cleanser' called the hydroxyl free radical, or OH. The researchers theorised that the rise in methane might be due in part to a decline in OH.

The researchers said it is too soon to tell whether the one-year rise in the amount of atmospheric methane is the start of an upward trend or a short-lived anomaly.-Reuters