Levels of climate-warming methane 鈥� a greenhouse gas 25 times as potent as carbon dioxide 鈥� rose abruptly in Earth鈥檚 atmosphere last year, and we don鈥檛 know why.
The concentration of methane, the primary component of natural gas, in the atmosphere has more than doubled since pre-industrial times, but remained largely stable over the last decade or so before rising in 2007, researchers said.
This stability in methane levels had led scientists to believe that emissions of the gas from natural sources like livestock and wetlands, as well as from human activities like coal and gas production, were balanced by the rate of destruction of methane in the atmosphere.
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But starting early last year, that balance was upset as millions of tonnes of methane were released into the air.
鈥淭he thing that鈥檚 really surprising is that it鈥檚 coming after this period of very level emissions,鈥� said Matthew Rigby of the Massachusetts Institute of Technology. 鈥淭he worry is that we just don鈥檛 understand the methane cycle very well.鈥�
Worldwide increase
Another surprise was that the rise in methane levels happened simultaneously around the globe instead of being centred near known sources of methane emissions in the northern hemisphere, said Rigby, one of the study鈥檚 lead authors along with Ronald Prinn, also of MIT.
They say a rise in methane in the northern hemisphere might be the result of a year-long warm spell in Siberia, where wetlands harbour methane-producing bacteria, but they have no immediate answer as to why emissions rose in the southern hemisphere at the same time.
Pre-industrial concentrations of methane were at about 700 parts per billion, and levels rose gradually to 1773 parts per billion by the late 20th century, Rigby says. The rise in 2007 was about 10 parts per billion, a significant jump over such a short period.
Although the concentration of carbon dioxide in the atmosphere is much higher, at around 385 parts per million, methane is a worry as it is much better than carbon dioxide at locking in heat from solar radiation.
Methane can be destroyed, however, by reactions involving an atmospheric 鈥渃leanser鈥� called the hydroxyl free radical (OH). The researchers theorize that the rise in methane might be due in part to a decline in atmospheric OH.
They add that it is too soon to tell whether the one-year rise in atmospheric methane is the start of an upward trend or a short-lived anomaly.
Journal reference: , (in press)
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