"The sudden renewed growth in atmospheric methane concentrations since 2007 has taken scientists by surprise, and no direct source of emissions or change in the removal of methane from the atmosphere has been identified," Benjamin Poulter of NASA’s Goddard Space Flight Center told environmentalresearchweb. "Individual lines of evidence related to emissions from oil and gas activities, and emissions from biomass burning have mostly been disproved because the methane concentration increase is related to a depletion of carbon-13, suggesting a microbial source."

To determine whether this pointed to wetlands, a team of more than 30 researchers from the US, Canada, Japan, the UK, Germany, France, China and Australia used a combination of satellite and ground-based mapping together with an ensemble of biogeochemical models. After analyzing all the results gathered as part of one of the most comprehensive studies so far, the researchers concluded that wetlands do not appear to be the main source driving the renewed atmospheric growth of methane concentrations from 2007 to 2012.

This is not to say that wetland emissions are constant – the scientists found that wetlands continue to dominate the interannual variability of atmospheric concentrations. And they did discover evidence for a longer term increase in methane emissions from wetland areas at higher latitudes, but this increase was offset by a decrease in atmospheric methane over the tropics, giving no overall change globally.

Building an accurate inventory of wetland areas was one of the main challenges, as satellite data only provided part of the picture. For example, one approach to identifying wetlands is to use satellite radar information to match soil brightness temperatures with the presence of water. This technique can work well in areas where vegetation canopies are relatively open, but struggles to capture the same level of detail in denser tropical forests.

There are other considerations too. "Wetlands do not always have water above the soil surface, and so satellite radar approaches may simply be unable to map [them]," adds Poulter. "New techniques using high-resolution radar and optical imagery from new satellite missions have potential to overcome these limitations, but we still lack a historical perspective."

The anaerobic nature of the methane emission process also presents complications when crunching the data, which is why the team took an ensemble approach.

"Methane is a gas that is emitted by microbes that use carbon for energy where oxygen is not available," said Poulter. "So modelling methane production, or methanogenesis, requires multiple strands of information – where and for how long soils are anaerobic; the pathways for methane to escape the soil and get into the atmosphere; and the consumption of methane by methanotropic bacteria or chemical reactions with hydroxyl radicals. Each process has high variability and a number of uncertainties, and using an ensemble of models, which make different assumptions on how to treat methanogenesis and methanotrophy, and the soil-to-atmosphere transport, allowed us to be transparent about these uncertainties."

The team believes that its study is the first to look comprehensively at the role of wetlands in the renewed growth of atmospheric methane, and provides a framework for future global wetland assessments.

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