Showing papers on "Atmospheric methane published in 1991"

Three‐dimensional model synthesis of the global methane cycle

Abstract: The geographic and seasonal emission distributions of the major sources and sinks of atmospheric methane were compiled using methane flux measurements and energy and agricultural statistics in conjunction with global digital data bases of land surface characteristics and anthropogenic activities. Chemical destruction of methane in the atmosphere was calculated using three-dimensional OH fields every 5 days taken from Spivakovsky et al. (1990a, b). The signatures of each of the sources and sinks in the atmosphere were simulated using a global three-dimensional tracer transport model. Candidate methane budget scenarios were constructed according to mass balance of methane and its carbon isotopes. The verisimilitude of the scenarios was tested by their ability to reproduce the meridional gradient and seasonal variations of methane observed in the atmosphere. Constraints imposed by all the atmospheric observations are satisfied simultaneously by several budget scenarios. A preferred budget comprises annual destruction rates of 450 Tg by OH oxidation and 10 Tg by soil absorption and annual emissions of 80 Tg from fossil sources, 80 Tg from domestic animals, and 35 Tg from wetlands and tundra poleward of 50°N. Emissions from landfills, tropical swamps, rice fields, biomass burning, and termites total 295 Tg; however, the individual contributions of these terms cannot be determined uniquely because of the lack of measurements of direct fluxes and of atmospheric methane variations in regions where these sources are concentrated.

Is the extent of glaciation limited by marine gas-hydrates

Abstract: Methane may have been released to the atmosphere during the Quaternary from Arctic shelf gas-hydrates as a result of thermal decomposition caused by climatic warming and rising sea-level; this release of methane (a greenhouse gas) may represent a positive feedback on global warming. The authors consider the response to sea-level changes by the immense amount of gas-hydrate that exists in continental rise sediments, and suggest that the reverse situation may apply - that release of methane trapped in the deep-sea sediments as gas-hydrates may provide a negative feedback to advancing glaciation. Methane is likely to be released from deep-sea gas-hydrates as sea-level falls because methane gas-hydrates decompose with pressure decrease. Methane would be released to sediment pore space at shallow sub-bottom depths (100's of meters beneath the seafloor, commonly at water depths of 500 to 4,000 m) producing zones of markedly decreased sediment strength, leading to slumping and abrupt release of the gas. Methane is likely to be released to the atmosphere in spikes that become larger and more frequent as glaciation progresses. Because addition of methane to the atmosphere warms the planet, this process provides a negative feedback to glaciation, and could trigger deglaciation.

Carbon isotopic composition of atmospheric CH4: Fossil and biomass burning source strengths

Abstract: The 13C/12C of atmospheric methane (CH4) was measured at Point Barrow (71°N, 156°W), Olympic Peninsula (48°N, 126°W), Mauna Loa (19°N, 155°W), and Cape Grim (41°S, 144°E) between 1987 and 1989. The global average δ13CPDB from these measurements (n = 208) was −47.20 ± 0.13%o. The lowest mean annual δ13C value of-47.61 ± 0.14‰ was measured at Point Barrow with values increasing to -47.03 ± 0.14‰ at Cape Grim. The seasonal cycle in the δ13C of CH4 was greatest at Point Barrow, with an amplitude of 0.5‰, and varied inversely with concentration. The isotopic fractionation during CH4 oxidation is calculated to be 0.993 ± 0.002 based on the measured CH4 concentration and δ13C values. The 14C content of atmospheric CH4, measured at monthly intervals at the Olympic Peninsula site between 1987 and 1989, is increasing at 1.4 ± 0.5 pM yr−1, primarily owing to 14CH4 release from nuclear reactors. The global average 14C content of 122 pM for CH4 implies a fossil methane source strength that is 16% of the total source. The global mean δ13C of −47.2‰, when coupled with the 14C results, implies that ∼11% of the total CH4 release rate is derived from biomass burning. These results indicate for a total CH4 source of ∼550 Tg yr−1 that natural gas release accounts for ∼90 Tg yr−1 and biomass burning yields ∼60 Tg yr−1. Preliminary analyses of the δ13C data using a three-dimensional chemical tracer model indicate that the observed meridional gradients in the annual average δ13C and concentration of CH4 are most closely matched with a CH4 source scenario in which 11% of the CH4 is derived from biomass burning.

New Measurement of the rate coefficient for the reaction of OH with methane.

Abstract: METHANE is an important greenhouse gas, whose concentration in the troposphere is steadily increasing. To estimate the flux of methane into the atmosphere and its atmospheric lifetime, its rate of removal needs to be accurately determined. The main loss process for atmospheric methane is the reaction with the hydroxyl radical OH. We have measured the rate coefficient for this reaction in carefully controlled experiments and found it to be smaller than currently accepted values. Our results indicate a longer CH4 lifetime (by ∼25%) and a correspondingly smaller flux (by ∼100 Tg CH4 yr−1) than previously calculated.

Methane flux to the atmosphere from the Arabian Sea

Abstract: ATMOSPHERIC concentrations of methane, an important greenhouse gas, have increased significantly over the past few decades1,2. Although attention has been focused on anthropogenic sources, data from ice cores show that large changes in atmospheric methane concentrations have occurred over glacial–interglacial time scales, indicating that there is significant variability in natural methane fluxes3,4. The surface waters of the oceans are often supersaturated with methane, which implies that the oceans are a net source, although their contribution to the global methane budget is small relative to other sources4. Here we report high concentrations of methane in the Arabian Sea, and calculate that the flux of methane to the atmosphere is up to five times greater than the previously reported average ocean flux. Methane production is associated with high phytoplankton biomass, which is closely coupled with the monsoon-driven upwelling of nutrient-rich water. We calculate that the Arabian Sea (representing 0.43% of the total surface area of the world's oceans) could account for between 1.3 and 133% of the current estimates of the open-ocean source of methane. Our results do not alter the view that the oceans are a relatively minor source of atmospheric methane, but the magnitude of the methane fluxes from the Arabian Sea and the link with the monsoon suggest that this region may be particularly sensitive to climate change, with a greater potential for feedback responses than its surface area might suggest.

A record of the atmospheric methane sink from formaldehyde in polar ice cores

Abstract: MEASUREMENTS of methane from ice cores show that the atmospheric concentration of methane has more than doubled since industrialization, and was only half of the pre-industrial value during the last ice age1–9. Natural sources of atmospheric methane are mainly biogenic, with the main sink for methane being its reaction with OH radicals. This reaction initiates a chain of reactions involving other trace gases and radicals, one of which is formaldehyde. In the remote troposphere, oxidation of methane followed by other reactions is the main source for formaldehyde. By reconstructing records of atmospheric methane and formaldehyde from ice cores, we can examine changes in sources of methane and in the oxidation capacity of the atmosphere.

Agronomic aspects of wetland rice cultivation and associated methane emissions

Abstract: Wetland rice cultivation is one of the major sources of atmospheric methane (CH4). Global rice production may increase by 65% between 1990 and 2025, causing an increase of methane emissions from a 92 Tg CH4 y−1 now to 131 Tg in 2025. Methane production depends strongly on the ratio oxidizing: reducing capacity of the soil. It can be influenced by e.g. addition of sulphate, which inhibits methanogenesis. The type and application mode of mineral fertilizers may also affect methane emissions. Addition of organic matter in the form of compost or straw causes an increase of methane emissions, but methane production is lower for materials with a low C/N ratio. High percolation rates in wetland rice soils and occasional drying up of the soil during the cultivation period depresses methane release. Water management practices aimed at reducing emissions are only feasible during specific periods in the rice growing season in flat lowland irrigated areas with high security of water availability and good control of the water supply. Intermittent drying of soils may not be possible on terraced rice lands. Assuming a 10 to 30% reduction in emission rates per unit harvested area, the global emission may amount to 93 Tg CH4 y− in 2025. A reduction of global emissions seems very difficult. To develop techniques for reducing CH4 emissions from wetland rice fields, research is required concerning interactions between soil chemical and physical properties, and soil, water and crop management and methanogenesis. Such techniques should not adversely affect rice yields.

Methane on the greenhouse agenda

Abstract: Options for reducing methane emissions, which could have a significant effect on global warming, are addressed. Emissions from landfills, coal mining, oil and natural gas systems, ruminants, animal wastes and wastewater, rice cultivation, and biomass burning are considered. Methods for implementing these emission reductions are discussed.

Determination of the isotopic composition of atmospheric methane and its application in the Antarctic

Abstract: A procedure for establishing the C-13/C-12 ratio and the C-14 abundance in the atmospheric methane is discussed. The method involves air sample collection, measurement of the methane mixing ratio by gas chromotography followed by quantitative conversion of the methane in the air samples to CO2 and H2O, and analysis of the resulting CO2 for the C-13/C-12 ratio by stable isotope ratio mass spectrometry and measurement of C-14 content by accelerator mass spectrometry. The carbon isotropic composition of methane in air collected at Baring Head, New Zealand, and in air collected on aircraft flights between New Zealand and Antarctica is determined by the method, and no gradient in the composition between Baring Head and the South Pole station is found. As the technique is refined, and more data is gathered, small seasonal and long-term variations in C-13 are expected to be resolved.

Methane fluxes in the southern North Sea: the role of European rivers

Abstract: Methane distributions were measured in the Southern Bight of the North Sea during March 1989 as part of the North Sea Program of the United Kingdom Natural Environment Research Council. Methane concentrations in the open North Sea were at or near solubility equilibrium with the atmosphere, but near the coast of the Netherlands, in the plume of the Rhine, extremely high methane values (as high as 120 times saturation) were observed. Both the Rhine and Scheldt rivers, and possibly coastal sediments, seemed to be significant regional sources of methane. Methane oxidation represented a relatively small sink for methane in the North Sea compared to gas loss to the atmosphere. This was due, in part, to the fact that methane oxidizing bacteria within the river plume appeared to be oxidizing the gas at close to their maximal rate, although offshore bacteria were able to increase oxidation rates as methane concentrations increased. Thus our studies suggested that bacterial methane oxidation kinetics were saturated at much lower methane concentrations in plume or estuarine waters than in open ocean situations. If the Rhine and Scheldt are the sources of the methane found in the Southern Bight, river concentrations of methane must be high (order of 50–5000 nM). Although rivers and coastal zones may not represent a major atmospheric source of methane, they can be very important as methane sources to the coastal ocean.

Beaver population fluctuations and tropospheric methane emissions in boreal wetlands

Abstract: Measurements of net methane flux were made during the 1988 ice-free season (May–October) at a beaver-meadow complex in northern Minnesota, USA. The site included upland boreal forest, sedge meadow, submerged aquatic plants, and the open water of a beaver pond. Annual fluxes were 8–11 g C/m2 in the permanently wetted zones and 0.2–0.4 g C/m2 at the occasionally inundated meadow and forest sites. These data, when coupled with long-term (46 yr) data on beaver (Castor canadensis) population size and habitat alteration, suggest that about 1% of the recent rise in atmospheric methane may be attributable to pond creation by beaver in North America.

Sources and sinks of methane in the African savanna. CH4 emissions from biomass burning

Abstract: Sources and sinks of atmospheric methane are studied in savanna regions of west and central Africa. Flux measured over dry savanna soils, using static chambers, is always negative the average uptake rate being 2×1010 molecules/cm2/s. In these regions, sources are linked to biomass burning. Methane and CO2 emission from combustion of savanna plants and wood is studied by both field experiments and laboratory experiments using a combustion chamber. For savanna plants most of the carbon (85%) contained in the biomaterial is volatilized as CO2 and 0.1 to 0.25% as methane. For graminaceous plants like loudetia simplex the ratio C-CH4/C-CO2 is 0.11%; it is 0.28% for hyparrhenia the other main type of savanna plants and it attains 1.4% for the combustion of wood. In natural fire plumes this ratio is around 0.26% for savanna fires and 0.56 to 2.22% for forest fires. These results show that methane release is highly dependent on the type of combustion. Methane to CO2 ratios are also studied in vertical profiles in the troposphere taken during the TROPOZ I campaign, an aerial research expedition carried out over west Africa during the bushfire period. Within polluted layers, the average ratio of CH4 to CO2 excess over ambient air concentration is 0.34%. These results show that biomass burning in tropical Africa constitutes an important source of atmospheric methane estimated to about 9.2×106 T(CH4)/yr.

Atmospheric methane: A global three‐dimensional model study

Abstract: We have developed a three-dimensional model to study global methane distributions and the transport of methane. The model includes transports, chemical reactions, and global methane sources. The calculations provide a fairly good simulation of the observed latitudinal gradient of methane and its global mean distribution in the stratosphere. The model results also suggest that rice paddies in Southeast Asia and wetlands in the high latitudes of North America can contribute to required sources for maximum of methane. The simulation also models the positive vertical gradients of methane in the troposphere of the southern hemisphere, which was observed by Fraser et al. (1984). The model analyses show that this positive gradient is the result of the interhemispheric transport of methane at upper levels of the troposphere.

Atmospheric methane concentrations---the NOAA/CMDL global cooperative flask sampling network, 1983--1988

Abstract: This document presents monthly averages and sampling statistics for atmospheric methane (CH{sub 4}) concentrations (mixing ratios) from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) global cooperative flask sampling network from 1983 to 1988. The data are derived from a network of 31 stations (27 of which were still active at the end of 1988), which collected air samples in flasks approximately once per week for measurement of both methane and carbon dioxide. The samples were analyzed for atmospheric methane on a gas chromatograph (fitted with a flame ionization detector) at the NOAA/CMDL Laboratory in Boulder, Colorado. These data represent the most spatially comprehensive atmospheric methane record available in the world. They have been used to study the temporal and spatial variations in atmospheric methane concentration and have applications in modeling, projections, and global mass balance calculations. The data consist of station identification code; year; and, for each month, mean monthly methane concentration, standard deviation, and number of samples contributing to the mean. This document provides sample listings of the NOAA/CMDL atmospheric methane data as they appear on magnetic tape, as well as a complete listing of the methane data (statistics excluded) in tabular form. This more » document also offers retrieval program listings, furnishes information on sampling and analysis methods and on data selection, defines limitations and restrictions of the data, and provides reprints of pertinent literature. 1 fig., 4 tabs. « less

Introduction: Facts and Uncertainties

Abstract: The unusually hot summer and drought in 1988 in parts of North America stimulated wide discussions about the cause of these events. While most scientists now studying climate believe that the 1988 events were shortrun phenomenon, some scientists and many policy makers in the U.S. Congress and Administration suggest that this weather was linked to global warming caused by a build-up of the so-called greenhouse gases: carbon dioxide, nitrogen oxides, methane and chlorofluorocarbons.

Climatic responses to increasing greenhouse gases

Abstract: Projections of greenhouse gas (GHG)-induced future climate change are based on climate models whose complexity ranges from the simple energy-balance, climate/upwelling-diffusion ocean model we used to make our Intergovernmental Panel on Climate Change (IPCC) projections [Houghton et al., 1990] and the revised projections of our Nature paper [Schlesinger and Jiang, 1991b], to the coupled atmosphere-ocean model which Cubasch et al. [1991] have now used to repeat our earlier projections. It is likely that even the most comprehensive model may not include some physical processes, and such absent physical processes, along with those included by potentially inadequate parameterizations, will render uncertain the validity of the results. Risbey et al. [1991b] are correct that the Cubasch et al. [1991] model does not include melting of the polar ice caps or the potential release of methane from methane clathrates. However, based on our simulations of glacial onset from 115,000–105,000 years BP with our atmospheric general circulation/mixed-layer ocean-ice sheet/asthenosphere model (M. E. Schlesinger and M. Ya. Verbitsky, Simulation of glacial onset with an atmospheric general circulation/mixed-layer ocean/ice sheet/ asthenosphere model, in preparation, 1991), it appears that the time scale of changes in the polar ice sheets is too long to be important for the greenhouse issue during the next century. Furthermore, Cicerone and Oremland [1988] conclude that: “In principle, there is a clear possibility of future atmospheric methane increases due to methane hydrate destabilization, but the size of the effect needs better quantification.” Consequently, the only way to assess the significance of not including these and other processes in our most comprehensive, contemporary climate models is to include them and then repeat the simulations, in the vein of those recently performed by Cubasch et al. [1991].

Rule-based expert system for evaluating the quality of long-term, in-situ, gas chromatographic measurements of atmospheric methane. Technical memo

Abstract: Methane is an important trace constituent of the earth's atmosphere because it is active both chemically and radiatively. The absorption of infrared radiation by atmospheric methane, and the rapid increase in the global atmospheric burden of methane over the past century combine to raise concerns that continued increases may contribute to global warming and climate change within the next century. The use of a rule-based expert system to assess the integrity of in situ gas chromatographic methane measurements made at the NOAA/CMDL Point Barrow, Alaska and Mauna Loa, Hawaii observatories is presented. The expert system flags ambient samples analyzed during chromatograph system instability and excludes them from further scientific analysis. The development and implementation of the expert system are described in detail. A comparison between data sets flagged by a human expert and by the expert system shows that the expert system can successfully reproduce the efforts of a human when evaluating gas chromatograph system stability. Advantages and limitations of the use of an expert system for the task are also discussed.

Atmospheric Methane: Estimates of Its Past, Present and Future and Its Role in Effecting Changes in Atmospheric Chemistry

Abstract: Land use and industrialization in the past have led to local or regional environmental degradation; now our activities have reached a level at which we are facing serious global consequences. This is particularly true for the atmosphere, in which anthropogenically released gases have led to global changes in the chemical composition of the air and changes in the Earth’s radiation budget, with likely consequences for the climate in the near future. Expected temperature rises due to increasing concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and chlorofluorocarbons (CFCs), have the potential to shift the climate regions within the coming decades (Bolin et al., 1986). This may be beneficial for some regions, but catastrophic for others. The regional climate shifts predicted are still subject to a high degree of uncertainty since the response of the climate system to perturbations is not fully understood. Model calculations agree to the extent that radiative forcing of the atmosphere due to increased concentrations of greenhouse gases globally leads to a warming of the Earth’s surface, but differ considerably in their predictions on a regional level.