Natural tropical wetlands were the main source of a marked rise in global atmospheric methane levels observed during the past few years.

After a decade of stability, atmospheric concentrations of the potent greenhouse gas began to climb in 2007. Philippe Bousquet at the French National Centre for Scientific Research in Gif-sur-Yvette, France, and his group used atmospheric chemistry and transport models, along with an ecosystem model of wetland methane emissions, to infer the contributions of natural and anthropogenic sources to the rise.

They found that, in 2007, emissions from tropical wetlands accounted for two-thirds of the 21-million-tonne increase in methane over the 1999–2006 average. And, in 2008, the wetlands contributed about 50% of the 18-billion-tonne excess. The authors suggest that growth in wetland area due to increased rain is driving these elevated emissions.

Atmos. Chem. Phys. 11, 3689–3700 (2011)