12th May 2014

Source of recent methane rise identified

A surprising recent rise in atmospheric methane likely stems from wetland emissions – suggesting that much more of the potent greenhouse gas will be pumped into the atmosphere as northern wetlands continue to thaw and tropical ones warm, according to a new international study led by the University of Guelph.

Prof. Merritt Turetsky is lead author of a paper in Global Change Biology that is one of the largest-ever analyses of global methane emissions. Her team looked at almost 20,000 field data measurements collected from 70 sites across Arctic, temperate and tropical regions.

One of the strongest greenhouse gases, methane is 86 times more potent than carbon dioxide over a 20-year time frame. It comes from agriculture and fossil fuel use, as well as natural sources such as microbes in saturated wetland soils.

The level of atmospheric methane had been relatively stable for about a decade, but concentrations began to rise again in 2007. Scientists believe this increase stems partly from more methane being released from thawing northern wetlands.

Until now, scientists had assumed that wetland methane release was largest in the tropics, said Turetsky. “But our analyses show that northern fens, such as those created when permafrost thaws, can have emissions comparable to warm sites in the tropics, despite their cold temperatures. That’s very important when it comes to scaling methane release at a global scale.”

The study calls for better methods of detecting different types of wetlands and human changes to those ecosystems. Fens are the most common type of wetland in Canada – but we lack basic scientific approaches for mapping fens using remote sensing products, she said.

“Not only are fens one of the strongest sources of wetland greenhouse gases, but we also know that Canadian forests and tundra underlain by permafrost are thawing and creating these kinds of high methane-producing ecosystems.”

Most methane studies focus on measurements at a single site, said co-author Narasinha Shurpali, University of Eastern Finland. “Our synthesis of data from a large number of observation points across the globe is unique and serves an important need.”

The team showed that small temperature changes can release much more methane from wetland soils to the atmosphere. But whether climate change will ramp up methane emissions will depend on soil moisture, said Turetsky.

In warmer and wetter conditions, much more of the gas will be emitted. If wetland soils dry out from evaporation or human drainage, emissions will fall – but not without other problems. In earlier studies, Turetsky found drying peatlands can spark more wildfires.

Another study co-author, Kim Wickland, U.S. Geological Survey, said, “This study provides important data for better accounting of how methane emissions change after wetland drainage and flooding.”

Methane emissions vary between natural and disturbed or managed wetlands, says Wickland, who has worked to improve methods for calculating greenhouse gas emissions from managed wetlands.

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