NASA: The second most important human-produced greenhouse gas after carbon dioxide, methane is colorless, odorless and can be hard to track. by Alan Buis and Robert Perkins
'The gas has a wide range of sources, from decomposing biological material to leaks in natural gas pipelines. In the early 2000s, atmospheric scientists studying methane found that its global concentration — which had increased for decades, driven by methane emissions from fossil fuels and agriculture — leveled off as the sources of methane reached a balance with its destruction mechanisms. The methane levels remained stable for a few years, then unexpectedly started rising again in 2007, a trend that is still continuing.
'Previous studies of the renewed increase have focused on high-latitude wetlands or fossil fuels, Asian agricultural growth, or tropical wetlands as potential sources of the increased emissions. But in a study published today in the early online edition of the Proceedings of the National Academy of Sciences, researchers at Harvard University in Cambridge, Massachusetts; Caltech in Pasadena, California; and NASA's Jet Propulsion Laboratory, also in Pasadena, suggest that methane emissions might not have increased dramatically in 2007 after all.'