Fossil fuels have long been blamed for global warming because of the greenhouse gases they produce. However, researchers from Royal Holloway, University of London say that fossil fuels might not actually be to blame, at least where methane is concerned.

In a study published in the journal Global Biogeochemical Cycles, the researchers showed that recent increases in methane levels in the atmosphere are actually caused by biological sources like rice fields, swamp gas and cow burps, and not fossil fuel emissions. They also noted that the spike in methane levels was particularly evident in the tropics.

For the study, Euan Nisbet and colleagues worked with the U.S. National Oceanic and Atmospheric Administration to analyze measurements and air samples taken from various parts of the globe, like Ascension Island in the South Atlantic, Alert in the Canadian Arctic and Cape Point in South Africa.

"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 Nisbet, the study's lead author, adding that an analysis of methane isotopes pointed to a higher level of emissions from microbial sources, like agriculture or wetlands.

According to him, atmospheric methane is considered one of the most potent of the greenhouse gases. It largely increased in level during the 20th century because of emissions from the coal and gas industries, but it had started to stabilize at the beginning of the new century. However, methane in the atmosphere started rising again around the globe since 2007, reaching extreme levels (double that of the previous years) in 2014.

Changes in weather patterns were one reason why methane was increasing in the atmosphere, with natural processes that remove the gas slowing down, but it was likelier that there was an increase in gas emissions from tropical areas around the world.

In another study, researchers from the Washington State University have discovered another unlikely source of greenhouse gases: hydroelectricty. Hydropower has been in use for decades as a clean energy source, but it turns out man-made reservoirs and dams are contributing emissions that yield almost a gigaton of carbon dioxide each year. That much carbon dioxide is equivalent to more than the total of all of the greenhouse gases Canada can produce.

How exactly are reservoirs and dams contributing emissions? They create ideal conditions for microorganisms to thrive and generate greenhouse gases like carbon dioxide and methane.

According to study author John Harrison, bubbles rising from the bottom of lakes or reservoirs are usually methane.

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