Jan 10, 2012
Keeping tabs on Eurasian methane
As the second most important greenhouse gas, methane could have a considerable role to play in our planet's future. But it's not clear just how big some of the wetlands that are emitting the gas actually are and how climate change will affect them. With that in mind, a team from the US, Russia and China has used a terrestrial ecosystem model to investigate likely methane emissions from Northern Eurasia into the 21st century.
"Compared to the total global methane source, the strength and trends of the contributing sources are considerably less known," Xudong Zhu of Purdue University, US, told environmentalresearchweb. "Northern Eurasia accounts for 60% of the terrestrial land cover north of 40°N and contains vast areas of wetlands. There is a lack of comprehensive estimates of methane fluxes in the region that consider both emissions and consumption of methane within different wetland types."
Zhu and colleagues from Purdue University, Russian Academy of Sciences, Marine Biological Laboratory, US, Massachusetts Institute of Technology, US, and Chinese Academy of Sciences estimated that current net methane emissions in Northern Eurasia are 20–24 Tg per year, with around two-thirds of this emitted during the summer. The researchers project that the total will increase by 6–51% by 2100, depending on the wetland extent dataset and the climate scenario used.
"Water table depth and soil temperature were the two most important environmental controls on both methane emissions and consumption in this region," said Zhu. "The uncertainty in wetland extent had a larger effect on future terrestrial methane emissions than the uncertainty in future climate."
Wetlands are one of the most important natural sources of methane as they contain microbes that produce the gas in the absence of oxygen. In aerobic soils or aerated surface waters, in contrast, some microbes can consume methane. Both these processes are affected by water table depth, soil temperature and pH.
The team used three different datasets for wetland extent, all of which were extracted from global data. According to the researchers, satellite sensors may not detect a large part of wetlands, and these global datasets, which do not consider specific conditions of wetland distribution and diversity in the region, could "seriously underestimate the real extent of wetland areas".
The team used the Terrestrial Ecosystem Model (TEM), which is a process-based biogeochemistry model. During the 1990s, they found, some ecosystems in Northern Eurasia acted as a source of atmospheric methane (producing up to 10 g per square metre per year), while some dry ecosystems consumed up to 2 g per square metre per year. Typically boreal forests emitted and consumed the most methane, because of their vast areas of wetland; grasslands and wet tundra areas were also significant methane sources. Western Siberia was a major source of atmospheric methane whereas western and southern parts of Northern Eurasia acted as strong sinks, and other areas were weak sinks.
The study did not consider human-induced disturbances, which could either inhibit methane emission, for example by draining wetlands, or enhance it, by creating ditches or dams.
"Better characterization of the spatial distribution and the natural diversity of wetlands should be a research priority for quantifying methane fluxes in Northern Eurasia," said Zhu. "In future, better characterization of wetland dynamics, by incorporating hydrological and climate models, may further improve our estimates."
The scientists reported their study in Environmental Research Letters (ERL).
About the author
Liz Kalaugher is editor of environmentalresearchweb.