‘Major ongoing changes in methane budgets are occurring,’ write the authors of a new report on the sources of methane. Together with the US National Oceanic and Atmospheric Administration (NOAA), a team of atmospheric scientists traced the biggest emitters of methane back to microbial sources. In other words: wetlands, rice paddies and livestock.
‘Our results go against conventional thinking that the recent increase in atmospheric methane must be caused by increased emissions from natural gas, oil and coal production,’ says Euan Nisbet, greenhouse gas researcher at Royal Holloway and co-author of the study.
Methane is a greenhouse gas with powerful effects. Though it only causes around a tenth of total emissions, it traps 30 times more heat than the better-known pollutant, carbon dioxide. ‘Throughout the 20th century, methane was emitted in large amounts by leaks in the coal and gas industries,’ says Nisbet. ‘But by the beginning of this century it appeared that the amount of methane in the air was stabilising.’
However, since 2007 the levels of methane started to grow again and in 2014 the growth rate doubled. ‘The year 2014 was extreme,’ says Nisbet, ‘with large increases seen across the globe.’ According to the research, the emissions have changed location though, now increasing from the tropics and other sources. ‘Tropical wetland or agricultural emissions, or a combination of both, are likely the dominant causes of global methane rise from 2008 to 2014,’ the study notes.
Methane comes from microbes fermenting in the guts of livestock and in the stagnant water of wetlands, both processes which are dependent on the local temperature and rainfall. As the scale and pace of methane rise has been so high – outstripping the pace of wetlands and agriculture expansion in the tropics – the findings point to weather as being the likely cause. Particularly as the tropics have seen increasingly warmer, wetter seasons. For the experts, understanding tropical emissions is vital to predicting the speed of methane rise: ‘is this merely a weather oscillation, or is it a troubling harbinger of more severe climatic change?’ asks Nisbet.