A sharp increase in methane, a potent greenhouse gas, in the Earth's atmosphere since 2007 is the result of higher emissions from biological sources such as rice paddies, cattle and swamps rather than fossil fuels, researchers said on Tuesday.
Methane traps heat, contributing to global warming. In 2014 the growth rate of methane in the atmosphere was double that of previous years, with emissions rising particularly in the tropics, said a study led by UK-based Royal Holloway, University of London.
"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," said lead author Euan Nisbet.
"Our analysis of methane's isotopic composition clearly points to increased emissions from microbial sources, such as wetlands or agriculture."
Methane increased through most of the 20th century, driven largely by leaks from the gas and coal industries, then appeared to be stabilising at the beginning of this century. But since 2007, levels of methane have started growing again, the scientists said.
The rise has likely resulted in part from changing weather patterns in hot and wet tropical areas of the world, according to the findings published in the journal Global Biogeochemical Cycles.
Nisbet's team, together with the US National Oceanic and Atmospheric Administration (NOAA), looked at measurements and samples of air taken from places like Alert in the Canadian Arctic, Ascension in the South Atlantic, and Cape Point in South Africa.
Tropical wetlands produce around a fifth to a quarter of global methane emissions, and those emissions respond quickly to changes in temperature and precipitation - for example, as wetlands expand by the end of the rainy season, the study noted.
In December, around 195 nations agreed a radical shift away from fossil fuels with a goal of limiting a rise in average global temperatures to well below 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times while "pursuing efforts" for 1.5C (2.7F).