Methane levels taper off - but why?

Posted by Emma Woollacott

There was a leveling off of the amount of methane in the atmosphere towards the end of the last century - and scientists can't agree on why it happened.

Methane is believed to be the second-largest contributor to climate change. And, after decades of increases due to worldwide industry and agriculture, the amount began to taper off during the 1980s.

But two papers from UC Irvine scientists have two different theories as to why, one citing less dependency on oil and the other new farming practices.

"It was an amazing mystery as to why this occurred," says earth system science professor Eric Saltzman, a co-author of one paper, who suggests that reduced use of petroleum and increased capture and commercial use of natural gas were the driving factors.

However, a second UCI paper has concluded that water efficiency and heavier commercial fertilizer use in the booming Asian farming sector provided less fertile ground for the soil microbes that create methane, while at the same time increasing nitrous oxide, another greenhouse gas.

Murat Aydin, lead author on the first paper, drilled into South Pole and Greenland glaciers to extract trapped air as much as a century old. The samples were analyzed for ethane, a chemical that has some of the same sources as methane but is easier to track.

"Levels rose from early in the century until the 1980s, when the trend reverses, with a period of decline over 20 years," Aydin says. "We find this variability is primarily driven by changes in emissions from fossil fuels."

The second team measured and analyzed the chemical composition of methane in the atmosphere from the late 1980s to 2005. They, though, found no evidence of fewer methane atoms linked to fossil fuel.

Instead, they say, the sharpest trend by far was changes in the Northern Hemisphere linked to new farm practices, mainly the use of inorganic fertilizers instead of traditional manure and drainage of fields mid-season.

"Approximately half of the decrease in methane can be explained by reduced emissions from rice agriculture in Asia over the past three decades, associated with increases in fertilizer application and reductions in water use," says lead author Fuu Ming Kai.

"It is indeed very remarkably rare that two differing studies about the same subject come out from the same department – I can't think of a similar case. But I think both analyses are scientifically sound and in themselves consistent," says Martin Heimann, director of Germany's Max Planck Institute for Biogeochemistry. "At this time I would not favor one over the other."

Heimann has invited members of both teams to a September symposium to  discuss the two studies.

"We will need to reconcile the differences," says professor James Randerson, a co-author on the second paper. "The important thing is that we must figure out – as scientists and a society – ways to reduce methane emissions."