Introduction to special section on Synthesis of Recent Terrestrial Methane Emission Studies

[1] Methane (CH4) is a potent greenhouse gas, second in importance to carbon dioxide (CO2) [Intergovernmental Panel on Climate Change (IPCC), 2007]. Biogenic sources account for 70% of global emissions – wetlands, rice paddies, livestock, landfills, forests, oceans and termites. Natural wetlands and rice paddies are the largest CH4 sources, and account for 20–60% of the total natural and anthropogenic emissions. While the existing estimates of emissions from these known sources still have great uncertainty, recent studies reveal a number of new sources contributing to the atmospheric CH4 burden. For example, the bubble emissions due to thawing lake sediments from north Siberia alone were estimated to release 3.8 Tg CH4 yr−1 [Walter et al., 2006]. Permafrost thawing also increases the CH4 emissions from wet soils [e.g., Wickland et al., 2006]. To adequately quantify total CH4 emissions and reconcile atmospheric CH4 concentrations with the earth surface emissions, the mechanisms of and controls on these CH4 sources need to be further understood. [2] The last synthesis effort of studying CH4 emissions focused on data compilation and data analysis at site-levels [Ojima et al., 2000]. Our current synthesis continues the data compilation effort and incorporates new more sophisticated biogeochemistry and atmosphere transport models in quantifying regional and global CH4 emissions. Specifically, recent CH4 studies have focused on the following areas: (1) Understanding the processes and mechanisms of CH4 production and consumption in different environments through field observations, environmental manipulations, and using isotopic analyses; (2) Measuring the emission fluxes from natural sources and observing atmospheric CH4 concentrations and profiles using flask measurements and satellite instruments [e.g., Dlugokencky et al., 2001; Bergamaschi et al., 2007]; and (3) Refining the estimates of CH4 emissions and their effect on the atmosphere with process-based biogeochemistry models and atmospheric transport and inversion models [e.g., Zhuang et al., 2004, 2006, 2007; Walter et al., 2001; Chen and Prinn, 2005, 2006]. This section resulted from a project supported by the National Center for Ecological Analysis and Synthesis (NCEAS) and presents results from new field studies, new instruments, and new approaches to the above areas. The section specifically addresses the issues of methane emissions in both natural and managed ecosystems, which are undergoing anthropogenic and natural perturbations of water table, permafrost thaw, volcanic deposition, sulfur deposition, and manure/fertilizer amendment. To better quantify the regional and global CH4 emissions, these effects and controls need to be considered in biogeochemistry models. The continuous and long-term observations of CH4 fluxes impacted by those factors and processes should still be a priority for the CH4 research community. [3] Wetland constitutes the largest single source of CH4 emissions with emissions ranging from 100 to 231 Tg CH4 yr−1 [IPCC, 2007]. The climate variability and change modify the wetland distribution, soil wetness, water table depth, and soil temperature, affecting CH4 emissions. The first two papers in this section involve field manipulations of soil temperature and water table depth in northern peatlands [Turetsky et al., 2008; White et al., 2008]. The Turetsky et al. study involved an ecosystem-scale manipulation of water table and soil surface temperature in a moderate rich fen located in interior Alaska. Methanogen populations were found to respond rapidly to changes in soil moisture and temperature changes. White et al. studied the effects of water table, soil warming, and wetland type on production, oxidation, and emission of CH4 in northern peatlands. Acetate fermentation was found to be the principal methanogenic pathway in these systems. The White et al. paper does not appear in the print version of this section, but will be electronically linked to the section in the online version. [4] Arctic wetlands account for about half of the world's wetland area and store about one-third of earth's soil carbon [Gorham, 1995]. Thawing permafrost and fire disturbances change the conditions and transitions of wetlands and uplands, resulting in complex CH4 emission patterns. In addition, the presence of permafrost promotes the formation and persistence of lakes in the Arctic [Smith et al., 2007], and the CH4 emissions from these lakes have not been well quantified. Using data from Siberia and Alaska, a recent study estimated that arctic lakes emit 15 – 35 Tg of methane per year, most of it through bubbling [Walter et al., 2007]. To address some of these issues, the next three papers [Walter et al., 2008; Sachs et al., 2008; Gauci et al., 2008b] focus on CH4 emissions and their controls, including effects of permafrost thawing and bubble emissions from thawed lakes in northern high latitudes, ecosystem-scale field observations of CH4 emission from polygonal tundra in the Lena River Delta, Siberia, and the possibility of a large Icelandic volcanic eruption resulting in a large decrease in CH4 emission. [5] Rice paddies are another significant source of CH4 emissions ranging 31 to 112 Tg CH4 yr−1 [IPCC, 2007]. The large uncertainty of the emissions is due to incomplete understanding of mechanisms and controls of emissions and consumption affected by agricultural management, such as fertilization and irrigation. The next four papers [Khalil and Butenhoff, 2008; Khalil et al., 2008a, 2008b; Gauci et al., 2008a] investigated CH4 emissions from rice paddies and controls on emission controls through field and laboratory experiments. Khalil and Butenhoff [2008] showed that there are large natural variations in CH4 emission between rice plots, and that these variations must be considered in scaling-up plot measurements to larger areas. Khalil et al. [2008a] found that manure additions to two crops of rice in Qing Yuan, Guangdong, China, under hot weather conditions resulted in an increase in CH4 emission. Khalil et al. [2008b] found that high organic fertilizer additions increased CH4 production, but also decreased CH4 oxidation. Gauci et al. [2008a] showed that sulfate deposition through simulated acid rain decreased CH4 emission. [6] The Atmospheric Infrared Sounder (AIRS) on EOS/Aqua platform launched on 4 May 2002 provides a good opportunity to monitor atmospheric CH4. The paper of Xiong et al. [2008] presents their satellite retrieval methodology and the product of atmospheric CH4 vertical profiles based on the instrumentation of the Atmospheric Infrared Sounder (AIRS). [7] Under the auspices of the NCEAS, we have assembled in situ measurements, flask measurements, and satellite data of methane concentrations and fluxes and process-based and atmospheric transport models in our Working Group. The flux and ancillary data for wetlands, rice paddies, and Siberia lakes are archived in NCEAS Data Repository website (http://data.nceas.ucsb.edu). These data include site descriptors (location, ecosystem type and description, fractional inundation, elevation), daily / monthly climate, soil characteristics, and methane fluxes, monthly net primary production and net ecosystem carbon exchange. The data archive effort is continuing and these data will facilitate synthesizing the global methane cycle with process-based and atmospheric transport chemistry modeling approaches. [8] This special section is a contribution by the Working Group of “Toward an adequate quantification of CH4 emissions from land ecosystems: Integrating field and in situ observations, satellite data, and modeling,” which was supported by the National Center for Ecological Analysis and Synthesis, a center funded by NSF (grant DEB-0553768), the University of California, Santa Barbara, and the State of California. The research of this paper is supported by the National Science Foundation with projects of Arctic Carbon Synthesis (ARC-0554811) and Carbon and Water in the Earth System (EAR-0630319).