Methane, which is less abundant in the atmosphere than carbon dioxide but 20 times more potent as a greenhouse gas, is thought to have been responsible for up to one-third of global warming since the industrial revolution.
Unlike CO2, however, atmospheric methane concentrations stopped rising in the 1990s, probably as a result of the sharp decline of the Soviet Union's industrial power. Concentrations have remained relatively stable since 1999, giving the impression that human and natural methane sources and sinks may have reached a lasting balance.
But the most extensive study so far, reported in this week's Nature magazine, suggests this is not so: human-created sources are rising, but have been masked by natural declines.
The study has come too late to be included in the next Intergovernmental Panel on Climate Change (IPCC) report, due next year. This report actually reduces estimates of the effect of methane on future climate change as compared to the previous edition — a reduction that now seems to have been premature.
The team behind the report, led by Philippe Bousquet, an atmosphere researcher at the Climate and Environment Science Laboratory in Saclay, France, looked at air samples from 68 measurement stations worldwide, taken between 1984 and 2003. They then used complex atmospheric modelling, along with information about the isotopic composition of the methane and how its concentration varied with the seasons, to work out where the gas had come from. The information allows for a meaningful assessment on a rough continental scale, says Bousquet. "In some places we don't see very well, but we're not blind."
The decrease during the 1990s was indeed caused by a marked decline of emissions from human sources, such as coal mines and landfills, the study finds. But man-made emissions have resumed their increase since 1999, probably because of the economic boom in China and other southeast Asian economies.
This rise has been concealed by natural trends. The world has seen a 5 per cent decrease in flooded land, thanks to drought and land-use change. This has reduced the amount of methane produced by bacteria living in swampy lands.
Droughts also bring peat and forest fires, but such biomass burning seems to have released less extra methane than previous studies suggested.
"The big surprise is that wetlands, not fires, are the dominant contribution to changes in natural methane emissions," says Bousquet. "That wetland reductions have as yet entirely masked the rise in human emissions is really not something we would have expected."
The stabilization of atmospheric methane, although not a political achievement, has been a ray of hope amidst the growing concerns about global warming. The new findings may show that hope to be an illusion.
"Clearly, this is not good news," says Jos Lelieveld, an atmospheric chemist at the Max Planck Institute for Chemistry in Mainz, Germany. "Wetlands cannot decrease forever." Rather, wetlands are expected to become larger in northern regions, for example, as areas of permafrost melt. Scientists fear that this might release vast amounts of the greenhouse gas.
Fortunately, methane emissions are much easier to control than CO2. Siphoning off methane from coal plants and landfills, or feeding farm animals different food so they produce less methane in their guts, doesn't require large changes in our way of living, says Bousquet. Modernizing old coal mines could also help reduce deaths in industrial accidents in Asia.
"In my opinion the easiest and most time-effective way to control climate change is to start acting on methane," says Lelieveld.
Source: Nature, 27th September, 2006
© People & the Planet 2000 - 2006