Methane spike sparks premature concern

The rise led to fears that the melting of Siberian permafrost had begun spiralling out of control. But the paper reported data up to only April 2008 and more recent figures show methane levels have stabilised again.

Methane is the second most important greenhouse gas and like carbon dioxide it is monitored at sites around the world. CO₂ has increased steadily over the years, but methane levels have varied, responding to a poorly understood interplay of emissions and sinks.

Between the late 1970s and 1997, the concentration rose from 1,600 parts per billion to about 1,780ppb, a level that was roughly maintained until late 2006.

Starting in early 2007, measurements from two monitoring networks showed an increase in methane levels.

A probable drop in levels of hydroxyl radicals contributed to the rise. Hydroxyl radicals, which destroy methane, are high-energy fragments formed by the action of sunlight on the atmosphere.

The researchers speculate that increased methane emissions from tropical and high-latitude wetlands also contributed to the rise. Siberia was 4°C warmer than the long-term average during 2007 which could have contributed to high wetland emissions, the authors note.

But suggestions that their findings are evidence for the start of catastrophic melting in arctic permafrost are premature.

"To say anything constructive [about the causes of rising methane] we have to wait," said one of the researchers Simon O’ Doherty of the University of Bristol.