Scientists know levels of methane have skyrocketed since the 1850s, as the impacts of the Industrial Revolution rippled through the atmosphere.

Even today, though, it's difficult to determine how much of those emissions increases are coming from wetlands or from wellheads.

A study released yesterday in the journal Nature Geoscience takes aim at this problem, pointing out progress in quantifying emissions and areas that still need improvement.

Since methane is a potent, if short-lived, greenhouse gas, if scientists knew where these increased emissions were coming from, policymakers might be able to target certain sectors for emissions reductions.

"Methane is important. ... It is a gas on which we can take action," said study co-author Philippe Bousquet, a researcher at the Climate and Environmental Sciences Laboratory in France.

Bousquet's paper is an effort to comprehensively lay out what is known about methane sources and the sinks that take up methane, and to highlight ways to improve our knowledge of where methane is coming from.

Methane can be roughly divided into three categories, based on its origin. Scientists can distinguish these different sources by their chemical signatures.

Wetlands remain a mystery
One source is the methane that comes from microbes that emit it. This type of methane arises from oxygen-poor environments like wetlands, rice paddies and landfills. It also, oddly enough, comes from termites, which produce a lot of methane in the tropics, and cow stomachs. Methane produced by cows is actually as big a source as methane from fossil fuels.

Fossil fuels are the source of the second type of methane. This methane can be emitted when coal, oil and natural gas are extracted, and during transmission and combustion.

The third type of methane comes from biomass burning, most of which is caused by humans.

In the 1850s, global methane levels were at about 830 parts per billion in the atmosphere. As of 2010, concentrations were 1,770 parts per billion. But the increase isn't always steady.

The concentration of methane also significantly grew in the 1980s, but from 1999 to 2006 it hardly changed.

Since 2006, it's gone sharply up again. But the exact cause still needs research.

The paper authors speculate that, since 2006, natural emissions from wetlands may have increased due to higher temperatures in the higher latitudes and an abundance of rain in tropical wetlands during recent La Niñas. They also finger human-caused fossil fuel emissions from increased coal exploration in Asia and the oil and gas boom in the United States.

"Some combination of wetlands and fossil fuels is causing this increase in the last few years, but the exact split I don't think we can say right now," said Isobel Simpson, an atmospheric chemist at the University of California, Irvine, and a co-author of the paper.

The paper authors pointed out areas for improvement. Bousquet highlighted tropical wetlands, a significant natural source of methane, as a research need.

"It's the largest source, and it's also one of the most uncertain. We really need to put some effort to narrow down the spread of the wetland models," he said.

The authors also highlighted the need for more methane measurements in the tropics, which have a paucity of monitoring stations.

Another need involves improving inventories of human sources of methane, such as how much is emitted from the natural gas production cycle, an area of active research at the moment, Simpson noted.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500