B52C-01
Comparison and Synthesis of Gridded Inventories of Methane Emissions From Rice Agriculture
Rice agriculture contributes some 30 to 100 Tg of methane per year to the global atmosphere. It may be the single largest anthropogenic source of atmospheric methane. The production of rice is in transition as the availability of inexpensive inorganic fertilizers increases and their use rapidly replaces traditional organic types. Water management of paddy soils is also changing as irrigation water becomes less available. Production of rice is forecast to increase in the foreseeable future to meet growing world demand, as strategies continue to evolve to help mitigate the release of methane from rice crops. These factors make the study of rice emissions especially important today. Emission factors for rice are notoriously heterogeneous in space and time due to numerous factors such as soil types, climate, water management, organic input, among others. Proper accounting requires a spatially high resolution inventory to fully capture these complexities. Several inventories currently exist and are available to the modeling community. These inventories use a variety of techniques such as process models, satellite imagery, country-reported data, to determine and distribute emissions over the rice-producing areas. Not unexpectedly, these inventories differ significantly not only in their spatial distributions, but in their integrated emissions as well. To best utilize these datasets, these differences must be known. In this study we first identify the similarities and discrepancies between the existing datasets. We then rank and assess the respective techniques used in each inventory, and create a synthesized weighted distribution of emissions on a 1x1 degree grid.
B52C-02
Estimating Global Methane Emissions From Reservoirs
No major scientific body has quantified the impacts of methane release from man-made reservoirs, despite an emerging consensus on the scale of the problem. Published estimates, based on varied assumptions, range from 20 to 120 million tons per year. For comparison, natural and anthropogenic emissions are thought to total 500 million tons per year, excluding reservoirs. This paper will examine the two key uncertainties in global estimation: first, the methodological accounting of individual reservoir emissions, and second, the distribution and extent of reservoir types. On the first question, political and scientific debate has mired progress in the published literature, although the evidence in the field is moving towards resolution on which parts of reservoirs are responsible for the majority of emissions, and the techniques needed to measure them. On the second question, data availability is a limitation. Tropical reservoirs with fast-cycling organic carbon have the highest potential for large emissions profiles. Temperate reservoirs in arid climates are less likely to have sizeable emissions. At present, no coupled database of reservoirs and data on local ecosystems and climatic conditions exists. A coordinated effort to collect this information and validate regional emissions estimates with targeted field studies would improve the quality of global estimates.
B52C-03
Can we reconcile our understanding of the atmospheric methane budget over the past decades with atmospheric observations?
The balance of methane in the atmosphere is determined by surface emission, and losses due to uptake in soils and reaction with the hydroxyl radical. The atmospheric abundance of methane has risen by about a factor of three since pre-industrial times, but the growth rate has decreased substantially since the 1990's. Thus, global atmospheric methane appears to have equilibrated to around 1780 ppb subject to considerable interannual variability, the causes of which are not well-understood. Methane emissions are expected to increase in the future due to increases in fossil fuel use and possible changes in wetlands at high-latitudes, and it is therefore important to test our understanding of the methane budget over the last two decades against network observations of atmospheric methane. Issues of interest are whether we can match the rise in methane over the 1980's, whether we can explain the decrease in growth rate during the 1990's, and whether we are able to simulate the observed interannual variability in the observations. We will show results from a multi-decade model simulation using analyzed meteorology from the ERA-40 reanalysis over this period. New times series of methane sources for 1980 through the early 2000's are used in the simulation. Anthropogenic sources include fossil fuels with a total of 7 fuel-process emission combinations associated with mining, processing, transport and distribution of coal, natural gas and oil; ruminant animals and manure based on regionally-representative profiles of bovine populations ; landfills including the impact of on- site methane capture; and irrigated rice cultivation based on seasonal rice-cropping calendars. Natural sources we include are biomass burning from the GFED emission data base, oceans, termites, and natural wetlands using a multiple-regression model derived from a process-based model. If time permits, we will also show preliminary results of a methane data assimilation using the Cooperative Air-Sampling and GMD network observations, and our new estimates of methane sources.
B52C-04 INVITED
Coupling Peatland Ecohydrology and Methane Ebullition
As natural sources of methane (CH4), peatlands play an important role in the global carbon cycle. The position of the water table within a peatland can have a large effect on CH4 emissions. With climate models predicting enhanced evapotranspiration and lower water-table position, peatland CH4 emissions generally are expected to decrease in coming decades. However, these predictions tend to focus almost exclusively on diffusive CH4 fluxes. Over the last five years, our research has examined the importance of ebullition of biogenic gas bubbles as a mechanism for the transport of CH4 to the atmosphere. Here we present a new conceptual model that addresses how changes in temperature, atmospheric pressure and peat structure affect CH4 bubble storage and release. We suggest that many peatlands may experience a (much) higher CH4 flux under warmer and drier conditions due to increased CH4 ebullition despite a decrease in CH4 flux via diffusion. In addition to influencing atmospheric CH4 emissions, recent research suggests that biogenic gas bubbles beneath the water table also affect hydrological processes in peat soils. Our work shows that these bubbles reduce saturated volumetric water content (VWC) and ‘saturated' hydraulic conductivity, and create zones of overpressure (zones of elevated pore-water pressure). All of these factors have large hydrological and biogeochemical implications, in that they will affect directions and rates of water flow, solute transfer, and the transport of CH4 to the atmosphere.
B52C-05
Environmental Controls on Methane Emission From Siberian wet Polygonal Tundra on Samoylov Island, Lena River Delta
The carbon budgets of the atmosphere and terrestrial ecosystems are closely coupled by vertical exchange fluxes of carbon dioxide and methane. Arctic tundra ecosystems have been major carbon sinks throughout the Holocene, resulting in a globally significant but highly sensitive carbon reservoir. Large uncertainties about the current and future contribution of these environments to the global carbon cycle remain especially with regard to methane emissions. In order to address this uncertainty and analyze the complex network of coupled processes and interconnected controls of tundra carbon exchange, we combined intensive field studies on multiple spatial scales with process-based and statistical model approaches. Methane emissions on both ecosystem-scale and plot-scale were measured in northern Siberia covering the entire growing season from end of May until end of September 2006 by the eddy covariance method, and from July through September 2006 by closed chambers, respectively. Our study site was located in the southern part of the Lena River Delta, which is characterized by arctic continental climate and cold continuous permafrost. Closed chamber measurements of methane fluxes were conducted daily on 15 plots in four differently developed polygon centers and on a polygon rim. This study adds significant findings on methane emission at different scales in an area that is seriously underrepresented in current efforts to quantify carbon emissions from high latitude environments. Controls on methane emission were identified by applying models of differing complexity ranging from more deterministic to more empirical approaches. We found relatively low fluxes on the landscape scale, which were mainly controlled by soil temperature, near-surface turbulence, and air pressure. On the micro-site scale, fluxes varied strongly and were controlled by different sets of environmental parameters. The results may have implications for the widely used closed chamber method.
B52C-06
Geophysical monitoring of biogenic gas dynamics in a northern peatland: seasonal variability and implications for production rates
In-situ biogenic gas dynamics and seasonal variability within a northern peatland in Maine (Caribou Bog) were investigated using a set of high resolution surface ground penetrating radar (GPR) surveys. Measurements were combined with elevation rod (to monitor surface deformation) and gas flux measurements. Spatial variability in gas production was also investigated by comparing two sites with different geological and ecological attributes, and showed a striking dependence on seasonal variability. One site characterized by thick highly humified peat deposits (5-6 m), wooded heath vegetation and open pools showed large ebullition events during the summer season, emitting up to 156 g CH4 m-2 during a single event. The other site characterized by thinner less humified peat deposits (2-3 m) and shrub vegetation showed much smaller ebullition events during the same season (accounting for up to 21 g CH4 m-2). A period of biogenic gas accumulation during the fall and winter (as enhanced by the frozen surficial peat acting as a confining layer) was followed by a large FPG release after the snow/ice melt that released approximately 180 g CH4 m-2 from both sites. Gas production rates were directly estimated from the time series of GPR measurements (ranging between 0.35-1.80 g CH4 m-3 d-1 during periods of biogenic gas accumulation) and reflected strong seasonal and spatial variability. Periods of decreased atmospheric pressure showed certain correspondence with short-period increases in biogenic gas flux (including a very rapid decrease in FPG content associated with an ebullition event that released an estimated 61 g CH4 m-2 in less than 3.5 hours). These results provide insights into the spatial and seasonal variability in production and emission of biogenic gases from northern peatlands
B52C-07
Late-Holocene reconstruction of methane fluxes for the last 3000 years based on testate amoebae assemblages: application to a Canadian boreal peat bog.
Boreal peatlands cover approximately 350 Mha from which about 50 % is found in North America. These ecosystems are substantial carbon sinks and constitute one of the most important natural source of methane (CH4) to the atmosphere. Annual release of CH4 to the atmosphere has been estimated to range between 20 and 50 Tg yr-1. A strong relationship between water table depth (WTD), plant community and CH4 fluxes is well documented, fluxes being more important as the WTD is closer to the peatland surface. Given their sensibility to peatland surface moisture changes, testate amoebae and plant macrofossils assemblages constitute valuable paleohydrological proxy indicators from which robust WTD reconstruction can be obtained. When coupled to modern CH4 fluxes, WTD fluctuation can potentially yield to holocene CH4 estimates. Based on these assumptions, late-holocene CH4 fluxes from boreal peat bog in the Eastmain-1 region (James Bay, Québec, Canada) were reconstructed. Modern CH4 fluxes were measured using static chambers over different vegetation types following a hydrologic gradient between 2005 and 2006 growing seasons. Two 1m-long peat cores were sampled at 2-cm intervals for macrofossil and testate amoebae analyses, and transfer functions were applied to the latter assemblages in order to estimate past WTD changes. Peat chronologies were controlled by AMS radiocarbon dates and 210Pb dates. Based on modern CH4 emissions, past fluxes were derived paleohydrological reconstruction and transfer to CH4 values throughout the Late-Holocene period. This research highlights the need for a better understanding of the role played by past CH4 emissions from peatlands in atmospheric CH4 concentration.
B52C-08
A 30 year study of carbon, groundwater, and climate coupling in a large boreal peat basin
Scaling biogeochemical processes across complex regional landscapes remains one of the most important challenges for deciphering the global methane cycle. For the past 30 years we have investigated the coupling of climate, groundwater, and methane cycling in the Glacial Lake Agassiz peatlands in northern Minnesota. Periodic droughts perturb the local and regional groundwater flow systems in this region altering the transport of inorganic solutes, organic acids and labile carbon substrates within the thick peat deposits. Two instrument stations at the bog crest and fen water track in the Red Lake peatland showed that large volumes of free-phase gas are trapped under confining layers in the deeper peat that episodically rupture to release large masses of methane bubbles to the atmosphere. These ebullition events are marked by abrupt depressuring cycles at depth and also by significant vertical and horizontal displacements of the peat surface. In the most recent phase of our investigations an integrated set of GPS stations and instrumented piezometers were installed to continuously pinpoint the location and calculate the magnitude of methane ebullition across a 160 square kilometer bog complex. The similarity of the vegetation patterns in this large bog complex to those found in other large peat basins in North America facilitates the transfer of these regional-scale ebullition fluxes to a broad swath of boreal America.