Showing papers on "Atmospheric methane published in 1996"

Rapid Variations in Atmospheric Methane Concentration During the Past 110,000 Years

Abstract: A methane record from the GISP2 ice core reveals that millennial-scale variations in atmospheric methane concentration characterized much of the past 110,00 years. As previously observed in a shorter record from central Greenland, abrupt concentration shifts of about 50 to 300 parts per billion by volume were coeval with most of the interstadial warming events (better known as Dansgaard-Oeschger events) recorded in the GISP2 ice core throughout the last glacial period. The magnitude of the rapid concentration shifts varied on a longer time scale in a manner consistent with variations in Northern Hemisphere summer insolation, which suggests that insolation may have modulated the effects of interstadial climate change on the terrestrial biosphere.

Global carbon exchange and methane emissions from natural wetlands : Application of a process-based model

Abstract: Wetlands are one of the most important sources of atmospheric methane (CH 4 ), but the strength of this source is still highly uncertain. To improve estimates of CH 4 emission at the regional and global scales and predict future variation requires a process-based model integrating the controls of climatic and edaphic factors and complex biological processes over CH 4 flux rates. This study used a methane emission model based on the hypothesis that plant primary production and soil organic matter decomposition act to control the supply of substrate needed by methanogens ; the rate of substrate supply and environmental factors, in turn, control the rate of CH 4 production, and the balance between CH 4 production and methanotrophic oxidation determines the rate of CH 4 emission into the atmosphere. Coupled to data sets for climate, vegetation, soil, and wetland distribution, the model was used to calculate spatial and seasonal distributions of CH 4 emissions at a resolution of 1° latitude x 1° longitude. The calculated net primary production (NPP) of wetlands ranged from 45 g C m -2 yr -1 for northern bogs to 820 g C m -2 yr -1 for tropical swamps. CH 4 emission rates from individual gridcells ranged from 0.0 to 661 mg CH 4 m -2 d -1 , with a mean of 40 mg CH 4 m -2 d -1 for northern wetland, 150 mg CH 4 m -2 d -1 for temperate wetland, and 199 mg CH 4 m -2 d -1 for tropical wetland. Total CH 4 emission was 92 Tg yr -1 . Sensitivity analysis showed that the response of CH 4 emission to climate change depends upon the combined effects of soil carbon storage, rate of decomposition, soil moisture and activity of methanogens.

A process-based model to derive methane emissions from natural wetlands

Abstract: A process-based model has been developed in order to calculate methane emissions from natural wetlands as a function of the hydrologic and thermal conditions in the soil. The considered processes in the model are methane production, methane consumption and transport of methane by diffusion, ebullition and through plants. The model has been tested against data from a three-year field study from a Michigan peatland. The interannual and seasonal variations of the modelled methane emissions and methane concentration profiles are in good agreement with the observations. During the growing season the main emission pathway proceeds through plants. Ebullition occurs whenever the water table is above the soil surface, while diffusion is only significant in the first 15 days after a drop of the water table below the peat surface.

A reevaluation of the open ocean source of methane to the atmosphere

Abstract: Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from −0.1 to 0.4 μmol m−2 d−1. The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr−1 (0.4 Tg CH4 yr−1), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.

A reevaluation of the open ocean source of methane to the atmosphere

Abstract: Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from −0.1 to 0.4 μmol m−2 d−1. The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr−1 (0.4 Tg CH4 yr−1), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.

Responses of methanotrophic activity in soils and cultures to water stress

Abstract: Diffusive gas transport at high water contents and physiological water stress at low water contents limited atmospheric methane consumption rates during experimental manipulations of soil water content and water potential. Maximum rates of atmospheric methane consumption occurred at a soil water content of 25% (grams per gram [dry weight]) and a water potential of about -0.2 MPa. In contrast, uptake rates were highest at a water content of 38% and a water potential of -0.03 MPa when methane was initially present at 200 ppm. Uptake rates of atmospheric and elevated methane decreased when water potentials were reduced by adding either ionic or nonionic solutes to soils with a fixed water content. Uptake rates during these manipulations were lower when sodium chloride or potassium chloride was used to adjust water potential rather than sucrose. The response of methane consumption by soils to water potential was somewhat less pronounced than the response of methanotrophic cultures (e.g., Methylosinus trichosporium OB3b, Methylomonas rubra [= M. methanica], an isolate from a freshwater peat, and an isolate from an intertidal marine mudflat). However, unlike soils, methanotrophic cultures exhibited a stronger adverse response to nonionic solutes than to sodium chloride.

Modeling Diffusion and Reaction in Soils: II. Atmospheric Methane Diffusion and Consumption in a Forest Soil

Abstract: A mixed hardwood forest with a maximum potential atmospheric methane consumption at 4 to 6 cm depth was investigated. Vertical variation of soil-water content, gas diffusivity and atmospheric methane uptake was measured with high spatial resolution in intact soil cores (2-5 cm depth intervals). Gas diffusivity increased rapidly with decreasing soil-water potential and a linear relationship between gas diffusivity, and the logarithm to the volumetric soil-water content was found (R 2 ≥ 0.98). Using this relationship in a simple, dynamic diffusion-reaction model, the vertical methane concentration profiles in intact soil cores were simulated. Only diffusion of methane in the soil air and variable methane consumption with depth was considered in the model. An excellent agreement between simulated and measured methane profiles indicated that a main control of methane consumption in non-waterlogged soils is methane diffusion in the soil air. Simulated methane uptake rates, calculated by summing up the methane oxidation at each 1-cm-depth interval, agreed well with measured methane fluxes into the soil cores. Model sensitivity analyses showed an accurate estimation of the effective gas diffusion coefficient at and above the zone of maximum methane consumption to be the most critical parameter for a realistic simulation of methane concentration profiles and total uptake rates.

A new tracer experiment to estimate the methane emissions from a dairy cow shed using sulfur hexafluoride (SF6)

Abstract: Methane emissions from livestock and agricultural wastes contribute globally more than 30% to the anthropogenic atmospheric methane source. Estimates of this number have been derived from respiration chamber experiments. We determined methane emission rates from a tracer experiment in a modern cow shed hosting 43 dairy cows in their accustomed environment. During a 24-hour period the concentrations of CH4, CO2, and SF6, a trace gas which has been released at a constant rate into the stable air, have been measured. The ratio between SF6 release rate and measured SF6 concentrations was then used to estimate the ventilation rate of the stable air during the course of the experiment. The respective tatio between CH4 or CO2 and SF6 concentration together with the known SF6 release rate allows us to calculate the CH4 (and CO2) emissions in the stable. From our experiment we derive a total daily mean CH4 emission of 441 L(STP) per cow (9 cows nonlactating), which is about 15% higher than previous estimates for German cows with comparable milk production obtained during respiration chamber experiments. The higher emission in our stable experiment is attributed to the contribution of CH4 release from about 50 m3 of liquid manure present in the cow shed in underground channels. Also, considering measurements we made directly on a liquid manure tank, we obtained an estimate of the total CH4 production from manure: The normalized contribution of methane from manure amounts to 12-30% of the direct methane release of a dairy cow during rumination. The total CH4 release per dairy cow, inncluding manure, is 521-530 L(STP) CH4 per day.

Control of Tundra Methane Emission by Microbial Oxidation

Abstract: The recent global increase of 1% per year in the concentration of atmospheric methane (CH4) is well documented (Rasmussen and Khalil 1984; Steele et al. 1987; Blake and Rowland 1989). This increase causes concern because CH4 is an important trace gas in the earth’s atmosphere. Greenhouse warming from CH4 is 25% of CO2-induced warming, and together these gases account for 75% of the radiative trapping from atmospheric gases (Rodhe 1990).

Methanogenesis in saltmarsh soils of the North Sea coast of Germany

Abstract: Temperate saltmarshes are a potential source of atmospheric methane. We have measured the concentration and emission of methane in typical saltmarsh soils (Salic Fluvisols) and humus-rich saltmarsh soils (Thionic Fluvisols) from the German North Sea coast. We also measured the methane production rates of the latter. The methane content of typical saltmarsh soils reached 12.0 mmol 1 -1 , although values of 1-4 μmol 1 -1 were usual. The sulphate concentrations of the pore-water were about 10 mM, which means sulphate reduction is not limited and methanogenesis would be suppressed. Methane concentrations were generally largest in summer. Independent of the redox potential and the degree of soil development, methane concentrations were smallest in those soils poorest in humus. Methane emission rates were almost zero. In the humus-rich saltmarsh soils, methane concentrations were roughly a thousand times larger than those in typical saltmarsh soils, reaching values of 23 mmol 1 -1 . The sulphate concentrations of the pore-water were often less than 1 mM, indicating limited sulphate reduction. Methane production was up to 80 μg cm -3 day -1 and was not inhibited when we added sulphate. Methane emission rates reached up to 190 μg m -2 day -1 in summer, with values up to 20 μg m -2 day -1 at other times. The two kinds of saltmarsh soil behave quite differently: the typical saltmarsh soils act as a sink for methane; the humus-rich saltmarsh soils are a source.

Aircraft measurements of the stable carbon isotopic ratio of atmospheric methane over Siberia

Abstract: Air samples collected using aircraft during the Siberian Terrestrial Ecosystem-Atmosphere-Cryosphere Experiments (STEACE) in the summer of 1993 and 1994 were analyzed for the carbon isotopic ratio, δ13C, of atmospheric CH4 as well as for the CH4 concentration. The CH4 concentrations and δ13C values observed in the lower troposphere over wetlands in the West Siberian Lowland varied considerably, showing a clear negative correlation between the two components. From the relationships between measured values of the CH4 concentration and δ13C, values of δ13C of CH4 released from wetlands into the atmosphere were estimated to be −75 to −67‰. The results observed over oil wells and pipelines showed isotopic evidence for leakage of natural gas. Mean values of δ13C measured in the middle and upper troposphere over Siberia in the summer season were −47.9±0.3 and −47.8±0.2‰ for 1993 and 1994, respectively, which are quite similar to each other.

Ab initio study on carbon Kinetic Isotope Effect (KIE) in the reaction of CH4+Cl•

Abstract: Recent studies suggest that the destruction of methane by Cl• in the marine boundary layer could be accounted for as another major sink besides the methane destruction by OH•. High level ab initio molecular orbital calculations have been carried out to study the CH4+Cl• reaction, the carbon Kinetic Isotope Effect (KIE) is calculated using Conventional Transition-State Theory (CTST) plus Wigner and Eckart semiclassical tunneling corrections. The calculated KIE is around 1.026 at 300 K and has a small temperature variation. This is by far the largest KIE among different processes involving atmospheric methane destruction (e.g., OH•, soil). A calculated mass balance of atmospheric methane including the KIE for the CH4+Cl• reaction is found to favor those methane budgets with enhanced biological methane sources, which have relatively lighter carbon isotope composition.

Methane in the East China Sea Water

Abstract: Methane in the East China Sea water was determined four times at a fixed vertical section along “PN line” consisting of 11–14 stations, in February 1993, October 1993, June 1994 and August 1994. The mean concentration of methane in the surface water was not significantly higher than that in the open ocean. The methane concentration below the pycnocline increased during the stratified period in summer to autumn and reached to 15 nmoles/l at most in October. The concentration of methane was fairly well correlated with AOU in the layer below the pycnocline in the stratified season. This means that methane in the bottom water has only a single source, which is expected to be anoxic sediments near the coast, and that the oxidation rate of methane in the water is extremely slow in the oxic water. The high methane observed in October completely disappeared in February, indicating that the methane was escaped to the atmosphere or transported to the pelagic ocean by the Kuroshio current. The East China Sea, therefore, is not a large direct and stationary source for the atmospheric methane, but may have some role as a source by supplying it sporadically to the atmosphere in early winter or indirectly from the surface of the pelagic ocean.

Methane emission from wetland rice fields

Abstract: Methane (CH 4 ) is an important greenhouse gas and plays a key role in tropospheric and stratospheric chemistry. Wetland rice fields are an important source of methane, accounting for approximately 20% of the global anthropogenic methane emission. Methane fluxes from wetland rice fields in the Philippines were monitored with a closed chamber technique in close cooperation with the International Rice Research Institute (IRRI). The field studies were complemented by laboratory and greenhouse experiments. Up to 90 % of the methane emitted from a rice field may be transported from soil to atmosphere through the rice plant. It was shown that this plant-mediated transport is diffusion controlled. Methane emitted from a rice field is the net effect of methane production and methane oxidation. Methane oxidation in the rice rhizosphere depended on the growth stage of the rice plant and becomes of much less importance when the rice plant reaches the ripening stage. Maximum rhizospheric methane oxidation efficiency observed was about 30%, which is much lower than the 70-90% estimated from indirect measurements in previous studies. A higher percentage of oxygen in the air resulted in lower methane emission indicating that breeding rice cultivars that transport more oxygen to their rhizosphere may be a promising mitigation option. Field studies with several soil related factors that influence methane emission were conducted; salinity, sulfate availability, organic amendments and soil types. Organic amendments strongly stimulated methane emission. The impact of organic amendments on methane emission can be described by a dose-response curve. This approach proofed successful for data from various locations of the world. Salinity partly inhibited methane production but methane oxidation in the salt-amended plot was even more inhibited, indicating that a reduction in CH 4 production does not necessarily cause a proportional reduction in CH 4 emission. This illustrates the importance of both production and oxidation of methane when designing mitigation strategies to reduce methane emission. Different soil types had different methane emission levels. Wetland rice fields on saline, low-sulfate soils emit less methane than comparable non-saline rice fields. On soils high in sulfate or amended with large amounts of sulfate containing substances, methane emissions are reduced even more. Continuous monitoring of methane fluxes showed that upon soil drying considerable amounts of soil-entrapped methane may be released. In previous monitoring studies these periods were not included, which may cause an underestimation of, total seasonal emission by 10-15%.

Airborne measurements of atmospheric methane over oil fields in western Siberia

Abstract: Airborne measurements of atmospheric methane (CH4) over oil fields in western Siberia were carried out on August 1, 1994. Extremely sharp CH4 peaks were observed in the horizontal distribution of CH4 at an altitude of 150 m above the ground surface; the half widths of the peaks were 3–4 km and the concentration of the largest peak exceeded 2.9 ppmv. Since the CH4 distribution was considered to reflect the distribution of CH4 emission strength on the surface, there was strong CH4 emission at the peak positions. All of the observed CH4 peak positions were located at or near oil production sites and/or oil pipelines, suggesting that natural gas was emitted from the facilities. Leakage or venting of natural gas are the probable CH4 sources.

Flux estimates from soil methanogenesis and methanotrophy: Landfills, rice paddies, natural wetlands and aerobic soils.

Abstract: Present and future annual methane flux estimates out of landfills, rice paddies and natural wetlands, as well as the sorption capacity of aerobic soils for atmospheric methane, are assessed. The controlling factors and uncertainties with regard to soil methanogenesis and methanotrophy are also briefly discussed.

Atmospheric methane over the North Pacific from 1987 to 1993

Abstract: Atmospheric methane mixing ratios were measured over the North Pacific during the winter season from 1987 to 1993 to extend our methane record since 1978. The latitudinal distribution of methane mixing ratio showed a north-to-south gradient from mid-latitudes to the equator every year. A sharp mixing ratio gradient often appeared at the boundary between the winter monsoon and the trade wind regions around 20°N. No significant longitudinal gradient was found during the winter season, although methane levels along the equator showed a large difference between the western and eastern Pacific. The overall methane increase rate in the western Pacific was estimated to be 13 ppb/yr on the basis of the long-term record for 15 years from 1978 to 1993. This record indicates that the methane growth rate over this Pacific region was gradually slowing down until 1990, followed by no significant increase in the 1990's. The overall deceleration of the growth rate was more rapid in the middle latitudinal zone (20°N–30°N) than in the lower latitudinal zone (3°N–20°N). This latitudinal difference suggests a rapid reduction of methane emissions from the continental regions. The methane growth rate showed an interannual variation with an increasing trend around 1983 and 1987, which was roughly related to the El Nino events. It is suggested that the methane growth rate was affected by a change of interhemispheric transport due to the ENSO events.

Changing trends in tropospheric methane and carbon monoxide : A sensitivity analysis of the OH-radical

Abstract: A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.

Physiological Limitations of Methanotrophic Activity in situ

Abstract: Methanotrophic bacteria play an important role in the atmospheric methane budget that at present includes a biospheric flux of about 400 Tg yr-1 (e.g. Cicerone and Oremland, 1988). The role of methanotrophs is primarily a function of two processes: 1) consumption of atmospheric methane by soils, accounting for about 40 Tg yr-1 (see King, 1992; Reeburgh et a1., 1993), and 2) oxidation of methane that diffuses to oxic surfaces in wetlands, especially the sediment-water (or air) interface and the rhizosphere of rooted aquatic vegetation (e.g. King. 1990a; King et al. , 1990b; Chanton and Dacey, 1991; Schutz et al., 1991; King, 1994) . The magnitude of methane consumption by the latter process is uncertain. However, it may be approximated by making several assumptions: 1) 90% of the total wetland emission term results from methane transport through plants (Chanton and Dacey, 1991 and references therein); 2) 25% of the methane that diffuses into plants and 80% of the methane diffusing to the sediment-water (or air) interface is oxidized (King, 1990b; Epp and Chanton, 1993; King, in prep.).

Regulation of methane oxidation: contrasts between anoxic sediments and oxic soils

Abstract: Although the majority of organic matter mineralization occurs under oxic conditions, significant rates of mineralization occur in the anoxic guts of herbivores, and in the anoxic sediments and water columns of freshwater and marine ecosystems (Henrichs, Reeburgh 1987). Anaerobic metabolism in these systems is often coupled to methane production, with variations in the extent of methanogenesis among systems a function of organic matter, nitrate, ferric iron, and sulfate availability. Controls of anaerobic metabolism and methanogenesis have been reviewed extensively elsewhere (e.g., Henrichs, Reeburgh 1987; Oremland 1988), and will not be considered further here. Instead, I will illustrate the controls of methane oxidation by comparing two very different: anaerobic methane oxidation in marine systems and methane consumption in terrestrial systems.