By David Reed, UT EditorLondon-German, Irish and Dutch researchers have shown that living plants can emit methane, by an unsuspected-and as yet unelucidated-mechanism. They further calculate, in a letter to Nature, the UK science magazine, that this means plants could contribute between 60 million and 240 million tonnes of annual emissions of the greenhouse gas (Nature 439, 187-191, 12 January 2006).This would make plants a source of between 10 and 30 percent of the global total for all sources of methane, suggest calculations in a separate article in Nature, written by David Lowe of the National Institute of Water and Atmospheric Research in New Zealand. Other key sources of methane include rice cultivation and cows and other ruminant animals which account for about two thirds of the 600 million tones of methane annually released into the environment, a report in The Guardian newspaper added.The researchers say they found significant methane emissions from both intact plants and detached leaves in incubation experiments in the laboratory and in the field. They point out that it has previously been assumed that methane emission would only occur in anoxic environments (where no oxygen is present).Methane is a more potent greenhouse gas than carbon dioxide, which is generally accepted as the main cause of global warming and hence has been the main target for reduction so far. Carbon dioxide would remain the predominant cause of the greenhouse effect but, the researchers conclude, "we suggest that this newly identified source may have important implications for the global methane budget … [and] … may call for a reconsideration of the role of natural methane sources in past climate change."Methane plays a central role in atmospheric oxidation chemistry and affects stratospheric ozone and water vapour levels, say the authors of the letter: Frank Keppler (Max-Planck-Institut für Kernphysik, Heidelberg, Germany) John Hamilton Department of Agriculture and Rural Development for Northern Ireland, Belfast, Marc Bra (Max-Planck-Institut für Kernphysik/ Institute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands) and Thomas Röckmann (also based at the two institutes)."