Showing papers on "Atmospheric methane published in 2004"

Detection of Methane in the Atmosphere of Mars

Abstract: We report a detection of methane in the martian atmosphere by the Planetary Fourier Spectrometer onboard the Mars Express spacecraft. The global average methane mixing ratio is found to be 10 ± 5 parts per billion by volume (ppbv). However, the mixing ratio varies between 0 and 30 ppbv over the planet. The source of methane could be either biogenic or nonbiogenic, including past or present subsurface microorganisms, hydrothermal activity, or cometary impacts.

Siberian Peatlands a Net Carbon Sink and Global Methane Source Since the Early Holocene

Abstract: Interpolar methane gradient (IPG) data from ice cores suggest the “switching on” of a major Northern Hemisphere methane source in the early Holocene. Extensive data from Russia9s West Siberian Lowland show (i) explosive, widespread peatland establishment between 11.5 and 9 thousand years ago, predating comparable development in North America and synchronous with increased atmospheric methane concentrations and IPGs, (ii) larger carbon stocks than previously thought (70.2 Petagrams, up to ∼26% of all terrestrial carbon accumulated since the Last Glacial Maximum), and (iii) little evidence for catastrophic oxidation, suggesting the region represents a long-term carbon dioxide sink and global methane source since the early Holocene.

A humid climate state during the Palaeocene/Eocene thermal maximum

Abstract: An abrupt climate warming of 5 to 10 degrees C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of approximately 1,050-2,100 Gt of carbon from sea-floor methane hydrate reservoirs. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine and terrestrial carbon isotope records spanning the PETM. We use models of key carbon cycle processes to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.

Regionalization of methane emissions in the Amazon Basin with microwave remote sensing

Abstract: Wetlands of the Amazon River basin are globally significant sources of atmospheric methane. Satellite remote sensing (passive and active microwave) of the temporally varying extent of inundation and vegetation was combined with field measurements to calculate regional rates of methane emission for Amazonian wetlands. Monthly inundation areas for the fringing floodplains of the mainstem Solimoes/Amazon River were derived from analysis of the 37GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer from 1979 to 1987. L-band synthetic aperture radar data (Japanese Earth Resources Satellite-1) were used to determine inundation and wetland vegetation for the Amazon basin (o500m elevation) at high (May-June 1996) and low water (October 1995). An extensive set of measurements of methane emission is available from the literature for the fringing floodplains of the central Amazon, segregated into open water, flooded forest and floating macrophyte habitats. Uncertainties in the regional emission rates were determined by Monte Carlo error analyses that combined error estimates for the measurements of emission and for calculations of inundation and habitat areas. The mainstem Solimoes/Amazon flood- plain (54-701W) emitted methane at a mean annual rate of 1.3TgCyr � 1 , with a standard deviation (SD) of the mean of 0.3TgCyr � 1 ; 67% of this range in uncertainty is owed to the range in rates of methane emission and 33% is owed to uncertainty in the areal estimates of inundation and vegetative cover. Methane emission from a 1.77 million square kilometers area in the central basin had a mean of 6.8TgCyr � 1 with a SD of 1.3TgCyr � 1 . If extrapolated to the whole basin below the 500m contour, approximately 22TgCyr � 1 is emitted; this mean flux has a greenhouse warming potential of about 0.5PgC as CO2. Improvement of these regional estimates will require many more field measurements of methane emission, further examination of remotely sensed data for types of wetlands not represented in the central basin, and process-based models of methane production and emission.

Atmospheric methane and carbon dioxide from SCIAMACHY satellite data: initial comparison with chemistry and transport models

Abstract: . The remote sensing of the atmospheric greenhouse gases methane (CH4) and carbon dioxide (CO2) in the troposphere from instrumentation aboard satellites is a new area of research. In this manuscript, results obtained from observations of the up-welling radiation in the near-infrared by SCIAMACHY on board ENVISAT are presented. Vertical columns of CH4, CO2 and oxygen (O2) have been retrieved and the (air or) O2-normalised CH4 and CO2 column amounts, the dry air column averaged mixing ratios XCH4 and XCO2 derived. In this manuscript the first results, obtained by using the version 0.4 of the Weighting Function Modified (WFM) DOAS retrieval algorithm applied to SCIAMACHY data, are described and compared with global models. For the set of individual cloud free measurements over land the standard deviation of the difference with respect to the models is in the range ~100–200 ppbv (5–10%) for XCH4 and ~14–32 ppmv (4–9%) for XCO2. The inter-hemispheric difference of the methane mixing ratio, as determined from single day data, is in the range 30–110 ppbv and in reasonable agreement with the corresponding model data (48–71 ppbv). The weak inter-hemispheric difference of the CO2 mixing ratio can also be detected with single day data. The spatiotemporal pattern of the measured and the modelled XCO2 are in reasonable agreement. However, the amplitude of the difference between the maximum and the minimum for SCIAMACHY XCO2 is about ±20 ppmv which is about a factor of four larger than the variability of the model data which is about ±5 ppmv. More studies are needed to explain the observed differences. The XCO2 model field shows low CO2 concentrations beginning of January 2003 over a spatially extended CO2 sink region located in southern tropical/sub-tropical Africa. The SCIAMACHY data also show low CO2 mixing ratios over this area. According to the model the sink region becomes a source region about six months later and exhibits higher mixing ratios. The SCIAMACHY and the model data over this region show a similar time dependence over the period from January to October 2003. These results indicate that for the first time a regional CO2 surface source/sink region has been detected by measurements from space. The interpretation of the SCIAMACHY CO2 and CH4 measurements is difficult, e.g., because the error analysis of the currently implemented retrieval algorithm indicates that the retrieval errors are on the same order as the small greenhouse gas mixing ratio changes that are to be detected.

Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis

Abstract: It has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice-core atmospheric methane record during the Pleistocene. Because submarine sediment failures can involve as much as 5000 Gt of sediment and have the capacity to release vast quantities of methane hydrates, one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental-slope failure and atmospheric methane correlate. To test the clathrate gun hypothesis, we have collated published dates for submarine sediment failures in the North Atlantic sector and correlated them with climatic change for the past 45 k.y. More than 70% by volume of continental-slope failures during the past 45 k.y. was displaced in two periods, between 15 and 13 ka and between 11 and 8 ka. Both these intervals correlate with rising sea level and peaks in the methane record during the Bolling-Allerod and Preboreal periods. These data support the clathrate gun hypothesis for glacial-interglacial transitions. The data do not, however, support the clathrate gun hypothesis for glacial millennial-scale climate cycles, because the occurrence of sediment failures correlates with Heinrich events, i.e., lows in sea level and atmospheric methane. A secondary use of this data set is the insight into the possible cause of continental-slope failures. Glacial-period slope failures occur mainly in the low latitudes and are associated with lowering sea level. This finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause. The Bolling-Allerod sediment failures were predominantly low latitude, suggesting an early tropical response to deglaciation, e.g., enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters. In contrast, sediment failures during the Preboreal period and the majority of the Holocene occurred in the high latitudes, suggesting either isostatic rebound–related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound, causing destabilization of gas hydrates.

CH4 sources estimated from atmospheric observations of CH4 and its 13C/12C isotopic ratios : 1. Inverse modeling of source processes

Abstract: A time-dependent inverse modeling approach that estimates the global magnitude of atmospheric methane sources from the observed spatiotemporal distribution of atmospheric CH4, C-13/C-12 isotopic ratios, and a priori estimates of the source strengths is presented. Relative to the a priori source estimates, the inverse model calls for increased CH4 flux from sources with strong spatial footprints in the tropics and Southern Hemisphere and decreases in sources in the Northern Hemisphere. The CH4 and C-13/C-12 isotopic ratio observations suggest an unusually high CH4 flux from swamps (similar to200 +/- 44 Tg CH4/yr) and biomass burning (88 +/- 18 Tg CH4/yr) with relatively low estimates of emissions from bogs (similar to20 +/- 14 Tg CH4/yr), and landfills (35 +/- 14 Tg CH4/yr). The model results support the hypothesis that the 1998 CH4 growth rate anomaly was caused in part by a large increase in CH4 production from wetlands, and indicate that wetland sources were about 40 Tg CH4/yr higher in 1998 than 1999.

A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

Abstract: A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km2) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y−1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y−1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.

Methane and Nitrogen Oxide Fluxes in Tropical Agricultural Soils: Sources, Sinks and Mechanisms

Abstract: Tropical soils are important sources and sinks of atmospheric methane (CH4) and major sources of oxides of nitrogen gases, nitrous oxide (N2O) and NOx (NO+NO2). These gases are present in the atmosphere in trace amounts and are important to atmospheric chemistry and earth’s radiative balance. Although nitric oxide (NO) does not directly contribute to the greenhouse effect by absorbing infrared radiation, it contributes to climate forcing through its role in photochemistry of hydroxyl radicals and ozone (O3) and plays a key role in air quality issues. Agricultural soils are a primary source of anthropogenic trace gas emissions, and the tropics and subtropics contribute greatly, particularly since 51% of world soils are in these climate zones.

Natural seabed gas seeps as sources of atmospheric methane

Abstract: Microbial and thermogenic methane migrates towards the seabed where some is utilised during microbially-mediated anaerobic oxidation. Excess methane escapes as gas seeps, which occur in a variety of geological contexts in every sea and ocean, from inter-tidal zones to deep ocean trenches. Some seeps are localised, gentle emanations; others are vigorous covering areas of >1 km2; the most prolific seeps reported (offshore Georgia) produce ~40 t CH4 per year. Gas bubbles lose methane to the water as they rise, so deep water seeps are unlikely to contribute to the atmosphere. However, bubbles break the surface above some shallow water seeps. Estimates of the total methane contribution to the atmosphere are poorly constrained, largely because the data set is so small. 20 Tg yr−1 is considered a realistic first approximation. This is a significant contribution to the global budget, particularly as methane from seeps is 14C-depleted. A seep measurement programme is urgently required.

Plant-mediated methane emission from an Indian mangrove

Abstract: Mangroves have been considered for a long time to be a minor methane source, but recent reports have shown that polluted mangroves may emit substantial amounts of methane. In an unpolluted Indian mangrove, we measured annual methane emission rates of 10gCH4yr � 1 from the stands of Avicennia marina. This rate is of the same order of magnitude as rates from Northern wetlands. Methane emission from a freshwaterinfluenced area was higher, but was lower from a stunted mangrove growing on a hypersaline soil. Methane emission was mediated by the pneumatophores of Avicennia. This was consistent with the methane concentration in the aerenchyma, which decreased on average from 350ppmv in the cable roots to 10ppmv in the emergent part of the pneumatophores. However, the number of pneumatophores varied seasonally. The minimum number occurred during the monsoon season, which reduced methane emissions largely. Ebullition from unvegetated areas may also be important, at least during monsoon season when measured bubble fluxes were occasionally about five times as high as pneumatophore-mediated emissions.

Inventory of methane and nitrous oxide emissions from agricultural soils of India and their global warming potential

Abstract: Agricultural soils contribute towards the emission of methane and nitrous oxide, the two important greenhouse gases causing global warming. Due to the diverse soil, land-use types and climatic conditions, there are uncertainties in quantificatio n of greenhouse gas emission from agricultural soils in India. An inve ntory of the emission of methane and nitrous oxide from different states in India was prepared using the methodology given by the Inter-Governmental Panel on Climate Change. For methane emission, state-specific emission coefficients have been used for all major rice ecosystems. In case of nitrous oxide, both direct and indirect emissions from agricultural soils in different states have been calculated using the emission coeff icients derived from the experiments conducted in India. For the base year 1994–95, methane and nitrous oxide emissions from Indian agricultural fields were est imated to be 2.9 Tg (61 Tg CO2 equivalent) and 0.08 Tg (39 Tg CO2 equivalent) respectively.

Hemispheric average Cl atom concentration from 13 C/ 12 C ratios in atmospheric methane

Abstract: . Methane is a significant atmospheric trace gas in the context of greenhouse warming and climate change. The dominant sink of atmospheric methane is the hydroxyl radical (OH). Recently, a mechanism for production of chlorine radicals (Cl) in the marine boundary layer (MBL) via bromine autocatalysis has been proposed. The importance of this mechanism in producing a methane sink is not clear at present because of the difficulty of in-situ direct measurement of Cl. However, the large kinetic isotope effect of Cl compared with OH produces a large fractionation of 13C compared with 12C in atmospheric methane. This property can be used to estimate the likely minimum size of the methane sink attributable to MBL Cl. By taking account of the mixing of MBL air into the free troposphere, we estimate that the global methane sink due to reaction with Cl atoms in the MBL could be as large as 19Tgyr-1, or about 3.3% of the total CH4 sink. However, its impact on the methane stable carbon isotope budget is large and warrants further attention.

Globally significant changes in biological processes of the Amazon Basin: results of the Large‐scale Biosphere–Atmosphere Experiment

Abstract: The Amazon River, its huge basin, and the changes in biological processes that are rapidly occurring in this region are unquestionably of global significance. Hence, Global Change Biology is delighted to host a special thematic issue devoted to the Large-scale Biosphere–Atmosphere Experiment in Amazonia (LBA), which is a multinational, interdisciplinary research program led by Brazil. The goal of LBA is no less modest than its subject: to understand how Amazonia functions as a regional entity in the Earth system and how these functions are changing as a result of ongoing changes in land use. This compilation of 26 papers resulting from LBA-related research covers a broad range of topics: forest stocks of carbon (C) and nitrogen (N); fluxes of greenhouse gases and volatile organic compounds from vegetation, soils and wetlands; mapping and modeling land-use change, fire risk, and soil properties; measuring changes caused by logging, pasturing and cultivating; and new research approaches in meteorology to estimate nocturnal fluxes of C from forests and pastures. Some important new synthesis can be derived from these and other studies. The aboveground biomass of intact Amazonian forests appears to be a sink for atmospheric carbon dioxide (CO2), while the wetlands and soils are a net source of atmospheric methane (CH4) and nitrous oxide (N2O), respectively. Land-use change has, so far, had only a minor effect on basin-wide emissions of CH4 and N2O, but the net effect of deforestation and reforestation appears to be a significant net release of CO2 to the atmosphere. The sum of the 100-year global warming potentials (GWP) of these annual sources and sinks of CH4, N2O, and CO2 indicate that the Amazonian forest–river system currently may be nearly balanced in terms of the net GWP of these biogenic atmospheric gases. Of course, large uncertainties remain for these estimates, but the papers published here demonstrate tremendous progress, and also large remaining hurdles, in narrowing these uncertainties in our understanding of how Amazonia functions as a regional entity in the Earth system.

Abatement of nitrous oxide, methane, and the other non-CO2 greenhouse gases: the need for a systems approach.

Abstract: Carbon dioxide currently accounts for about 49 percent of the radiative forcing of the atmosphere that is attributable to greenhouse gases (3.0 watts per square meter [W m-2]; Houghton et al. 2001). Tropospheric ozone and black carbon are responsible for another 18 percent, and the well-mixed greenhouse gases (GHGs)—principally methane, nitrous oxide, and various halocarbons—are responsible for the remaining 33 percent (Figure 29.1). Changes in well-mixed GHGs and in black carbon, tropospheric ozone, and ozone precursors (NOx, CO, and NMVOCs), whether engineered or unintentional, could thus have a substantial impact on the radiative forcing of future atmospheres.

Methane production by microbial mats under low sulphate concentrations

Abstract: Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulfate concentrations ([SO4]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm paleoclemates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulfate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [SO4=] (ca. 70 mM) and "Archean" [SO4] (less than 0.2 mM). At low [SO4], primary production in the mats was essentially unaffected, while rates of sulfate reduction decreased by a factor of three, and methane fluxes increased by up to ten-fold. However, remineralization by methanogenesis still amounted to less than 0.4 % of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats w-ere probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low- environmental levels oxygen and sulfate.

Emission of methane from rice fields: A review

Abstract: Methane (CH 4 ) with its current concentration of 1.72 ppmV in the atmosphere accounts for 15 per cent of the enhanced greenhouse effect. The atmospheric concentration of CH 4 is increasing at 0.3 per cent/y. Lowland rice soil is considered to be one of the major contributors of atmospheric methane. Various soil, climate, and management factors control methanogenesis, the geochemical process that occurs in all anaerobic environments in which organic matter undergoes decomposition, resulting in the formation of CH 4 . Methane formed in soil escapes to the atmosphere through vascular transport, ebullition or diffusion. Emission of CH 4 from rice fields can be reduced by: (i) Midseason drainage instead of continuous flooding, (ii) Use of cultivars with low emission potential, (iii) Use of low C:N organic manure, and (iv) Direct establishment of rice crop like dry direct seeded rice.

Terminal Electron Acceptors for Controlling Methane Emissions from Submerged Rice Soils

Abstract: Wetland rice fields are source of atmospheric methane. Methane is formed following reduction of carbon dioxide by hydrogen and through decarboxylation of acetate in anaerobic soils under reduced conditions. Methane production requires flow of carbon and electrons to microbial population of methanogens under reduced conditions in the strict absence of free oxygen. Application of or in-situ availability of terminal electron acceptors (oxidants) such as ferric iron or sulfate allows iron or sulfate reducers to successfully compete for substrates, hydrogen or acetate, with methanogens. This stops methane production. Electron acceptors also oxidize methane and reduce its emission. Since iron redox system plays a dominant role in tropical rice soils, which are rich in iron, the role of iron oxides and hydroxides as electron acceptor for controlling methane emission from wetland rice fields deserves more attention. For mitigating methane emission from wetland rice fields, ferric iron as a terminal electron-accepting agent, can either be added to the soil or regenerated in the soil by manipulating redox potential through soil and water management. Examples are given from recent literature illustrating the role of electron acceptors (ferric iron, sulfate, etc.) in reducing methane emission from submerged rice soils. The need for future research is also examined.

Seasonal dynamics of atmospheric methane oxidation in gray forest soils

Abstract: Seasonal fluctuations in the methane flow in the soil-atmosphere system were determined for gray forest soils of Central Russia. Consumption of atmospheric methane was found to exceed methane emission in gray forest soils under forest and in agrocenosis. The average annual rates of atmospheric methane consumption by the soil under forest and in agrocenosis were 0.026 and 0.008 mg CH4-C/(m2 h), respectively. The annual rate of atmospheric methane oxidation in the gray forest soils of Moscow oblast was estimated to be 0.68 kton. Seasonal fluctuations in the methane oxidation activity were due to changes in the hydrothermal conditions and in the reserves of readily decomposable organic matter and mineral nitrogen, as well as to changes in the activity of methane oxidizers.