Showing papers on "Atmospheric methane published in 1994"

The growth rate and distribution of atmospheric methane

Abstract: Methane was measured in air samples collected approximately weekly from a globally distributed network of sites from 1983 to 1992. Sites range in latitude from 90°S to 82°N. All samples were analyzed by gas chromatography, with flame ionization detection at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, and the measurements were referenced against a single calibration scale. The estimated precision of the measurements is ±0.2%. Samples which had clear sampling or analytical errors, or which appeared to be contaminated by local CH4 sources, were identified and excluded from the data analysis. The data reveal a strong north-south gradient in methane with an annual mean difference of about 140 ppb between the northernmost and southernmost sampling sites. Methane time series from the high southern latitude sites have a relatively simple seasonal cycle with a minimum during late summer-early fall, almost certainly dominated by the seasonality in its photochemical destruction. Typical seasonal cycle amplitudes there are about 30 ppb. Seasonal cycles at sites in the northern hemisphere are complex when compared to sites in the southern hemisphere due to the interaction among CH4 sources and sinks, and atmospheric transport. Seasonal cycle amplitudes in the high north are about twice those observed in the high southern hemisphere. Annual mean methane mixing ratios were ∼1% lower at 3397 m than at sea level on the island of Hawaii. Trends were determined at each site in the network and globally. The average increase in the globally averaged methane mixing ratio over the period of these measurements is (11.1±0.2) ppb yr−1. Globally, the growth rate for methane decreased from approximately 13.5 ppb yr−1 in 1983 to about 9.3 ppb yr−1 in 1991. The growth rate of methane in the northern hemisphere during 1992 was near zero. Various possibilities for the long-term, slow decrease in the methane growth rate over the last decade and the rapid change in growth rate in the northern hemisphere in 1992 are given. The most likely explanation is a change in a methane source influenced directly by human activities, such as fossil fuel production.

Methane in the Baltic and North Seas and a Reassessment of the Marine Emissions of Methane

Abstract: During three measurement campaigns on the Baltic and North Seas, atmospheric and dissolved methane was determined with an automated gas chromatographic system. Area-weighted mean saturation values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, February and July 1992) and 126 ± 8% (south central North Sea, September 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept of regional gas transfer coefficients. Our estimates computed with two different air-sea exchange models lie in the range of 11-18 Tg CH4 yr-1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three-layer model to numerically simulate the time dependent variation of the oceanic flux to the atmosphere. The model calculations indicate that even with increasing tropospheric methane concentration, the ocean will remain a source of atmospheric methane.

Mechanistic analysis of ammonium inhibition of atmospheric methane consumption in forest soils.

Abstract: Methane consumption by forest soil was studied in situ and in vitro with respect to responses to nitrogen additions at atmospheric and elevated methane concentrations. Methane concentrations in intact soil decreased continuously from atmospheric levels at the surface to 0.5 ppm at a depth of 14 cm. The consumption rate of atmospheric methane in soils, however, was highest in the 4- to 8-cm depth interval (2.9 nmol per g of dry soil per day), with much lower activities below and above this zone. In contrast, extractable ammonium and nitrate concentrations were highest in the surface layer (0 to 2 cm; 22 and 1.6 μmol per g of dry soil, respectively), as was potential ammonium-oxidizing activity (19 nmol per g of dry soil per day). The difference in zonation between ammonium oxidation and methane consumption suggested that ammonia-oxidizing bacteria did not contribute significantly to atmospheric methane consumption. Exogenous ammonium inhibited methane consumption in situ and in vitro, but the pattern of inhibition did not conform to expectations based on simple competition between ammonia and methane for methane monooxygenase. The extent of ammonium inhibition increased with increasing methane concentration. Inhibition by a single ammonium addition remained constant over a period of 39 days. In addition, nitrite, the end product of methanotrophic ammonia oxidation, was a more effective inhibitor of methane consumption than ammonium. Factors that stimulated ammonium oxidation in soil, e.g., elevated methane concentrations and the availability of cosubstrates such as formate, methanol, or β-hydroxybutyrate, enhanced ammonium inhibition of methane oxidation, probably as a result of enhanced nitrite production.

A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992

Abstract: Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr−1, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr−1, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities.

Potential distribution of methane hydrates in the world's oceans

Abstract: Estimates of the magnitudes and spatial distribution of potential oceanic methane hydrate reservoirs have been made from pressure-temperature phase relations and a plausible range of thermal gradients, sediment porosities, and pore fillings taken from published sources, based on two major theories of gas hydrate formation (1) in situ bacterial production and (2) pore fluid expulsion models The implications of these two models on eventual atmospheric methane release, due to global warming, are briefly examined The calculated range of methane volumes in oceanic gas hydrates is 264 to 1391 x 10{sup 15} m{sup 3}, with the most likely value on the lower end of this range The results for the bacterial model show a preferential distribution of hydrates at mid- to high latitudes, with an equatorial enhancement in the case of the fluid migration model The latter model also generates a deeper and thicker hydrate stability zone at most latitudes than does the former Preliminary results suggest that the hydrate distribution predicted by the fluid migration model may be more consistent with observations However, this preliminary finding is based on a very limited sample size, and there are high uncertainties in the assumptions The volume of methane hydrate within the uppermostmore » 1 m of the hydrate stability zone and within 1{degrees}-2{degrees}C of the equilibrium curve, assuming in situ bacterial generation, is 093-632 x 10{sup 12} m{sup 3}, or 00035-0012% of the maximal estimated hydrate reservoir Nevertheless this volume, if released uniformly over the next 100 years, is comparable to current CH{sub 4} release rates for several important CH{sub 4} sources Corresponding CH{sub 4} volumes calculated using the fluid migration model are nearly 2 orders of magnitude lower 52 refs, 2 figs, 5 tabs« less

Effect of increasing atmospheric methane concentration on ammonium inhibition of soil methane consumption

Abstract: SOILS currently consume about 30–40 Tg methane per year1,2, which is comparable to the net annual increase in atmospheric methane concentration from 1980 to 19903. Most soils consume methane2,4–9, but the extent varies with soil water content, land use and ammonium inputs5,10–13. Ammonium concentrations in many soils have increased in recent years as a result of land-use changes and increases in ammonium concentration in precipitation14,15. Ammonium strongly inhibits soil methane consumption, but the mechanism is uncertain. Even if enhanced ammonium concentrations are subsequently reduced, inhibition can still persist for months to years12,13. Here we show, from field and laboratory experiments, that the extent of ammonium inhibition increases with increasing methane concentration. We propose that nitrite formation from methanotrophic ammonium oxidation accounts for much of the observed inhibition, and that the persistence of inhibition with reduced ammonium concentrations is due to the limited capacity of methanotrophs to grow or recover in present concentrations of atmospheric methane. We suggest that past increases in atmospheric methane concentration may have increased the inhibitory effect of ammonium, thereby decreasing soil methane uptake capacity, and that this mechanism could also provide a positive feedback on future atmospheric methane concentrations.

An ecological perspective on methane emissions from northern wetlands

Abstract: Wetlands are significant sources of atmospheric methane, an important radiatively active ‘greenhouse' gas that accounts for an estimated 12% of total greenhouse warming. Since global climate models predict the greatest temperature and precipitation changes at high latitudes, and as the largest areas of wetland (346 × 10 6 ha) are in the boreal and subarctic regions (40–70°N), recent research has focused on Identifying the factors that control methane emission from northern wetlands. Over the past few years, the database has expanded tremendously, and much progress has been made in understanding the environmental controls on methane emission at small spatial and temporal scales. However, we now need to broaden our understanding of regional differences in methane emission, ecological responses of northern wetlands to climate change, and the effect of other perturbations such as drainage and flooding.

Role of the Hudson Bay lowland as a source of atmospheric methane

Abstract: Based on point measurements of methane flux from wetlands in the boreal and subarctic regions, northern wetlands are a major source of atmospheric methane. However, measurements have not been carried out in large continuous peatlands such as the Hudson Bay Lowland (HBL) (320,000 sq km) and the Western Siberian lowland (540,000 sq km), which together account for over 30% of the wetlands north of 40 deg N. To determine the role the Hudson Bay Lowland as a source of atmospheric methane, fluxes were measured by enclosures throughout the 1990 snow-free period in all the major wetland types and also by an aircraft in July. Two detailed survey areas were investigated: one (approximately 900 sq km) was in the high subarctic region of the northern lowland and the second area (approximately 4,800 sq km) straddled the Low Subarctic and High Boreal regions of the southern lowland. The fluxes were integrated over the study period to produce annual methane emissions for each wetland type. The fluxes were then weighted by the area of 16 different habitats for the southern area and 5 habitats for the northern area, as determined from Landsat thematic mapper to yield an annual habitat-weighted emission. On a per unit area basis, 1.31 +/- 0.11 and 2.79 +/- 0.39 g CH4 m(exp -2)/yr were emitted from the southern and northern survey areas, respectively. The extrapolated enclosure estimates for a 3-week period in July were compared to within 10% of the flux derived by airborne eddy correlation measurements made during the same period. The aircraft mean flux of 10 +/- 9 mg CH4 m(exp -2)/d was not statistically different from the extrapolated mean flux of 20 +/- 16 mg CH4 m(exp -2)/d. The annual habitat-weighted emission for the entire HBL using six wetland classes is estimated as 0.538 +/- 0.187 Tg CH4/yr (range of extreme cases is 0.057 to 2.112 Tg CH4/yr). This value is much lower than expected, based on previous emission estimates from northern wetlands.

Effect of ozone depletion on atmospheric CH 4 and CO concentrations

Abstract: GLOBAL surface-based measurements of atmospheric methane and carbon monoxide concentrations revealed a marked and unex-pected decrease in their growth rates in 1991 and 1992, particularly in the Northern Hemisphere1,2. Changes in emissions are unlikely to be the sole reason for the sudden reduction in the concentrations of these source gases2,3. The unprecedentedly large depletion of stratospheric ozone observed in 1991 and 1992 (ref. 4) may have contributed to the sharp decrease in the growth rates of both CH4 and CO by exposing the troposphere to more ultraviolet radiation. This would have resulted in increased concentrations of the hydroxyl radical, OH·, which is the major atmospheric sink for both CH4 and CO. Here we present two-dimensional model simulations which allow us to assess the significance of the link between stra-tospheric ozone depletion and the observed trends of CH4 and CO. We find that the low values in stratospheric ozone concentration can account for almost half of the 1992 decrease in the CH4 and CO growth rates.

Concentration and 13C records of atmospheric methane in New Zealand and Antarctica: Evidence for changes in methane sources

Abstract: Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41°S), over the 4-year period, 1989–1993, display a persistent but highly variable seasonal cycle. Values for δ13C peak in summer at about −46.9‰ and drop to around −47.5‰ in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78°S. While the phase of the δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its inter-annual variability to be due to episodic southward transport of isotopically heavy methane from large-scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head 13C data show no significant secular trend from 1989 to mid-1991, followed by a rapid trend toward methane less enriched in 13C. This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid-1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr−1 thereafter. A 2-box model of atmospheric methane (one box per hemispheric reservoir) suggests that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.

Rice Paddies as a Methane Source

Abstract: Rice fields are considered to be among the highest sources of atmospheric methane, an important source of global warming. In order to meet the projected rice needs of the increasing world population, it is estimated that the annual world’s rough rice production must increase to 760 million tons (a 65% increase) in the next 30 years. This will increase methane emissions from rice-fields if current technologies are kept. Methane emissions from ricefields are affected by climate, water regime, soil properties, and various cultural practices like irrigation and drainage, organic amendments, fertilization, and rice cultivars. Irrigated rice comprises 50% of the world-harvested rice area and contributes 70% to total rice production. Because of assured flooding during the growing period it is the primary source of methane. Rainfed rice emits less methane due to periods of droughts. Upland rice, being never flooded for a significant period of time, is not a significant source of methane. There is great potential to develop ‘no regret’ mitigation options that are in accordance with increasing rice production.

Trace Gas Emissions from Rice Fields

Abstract: Wetland rice cultivation is considered to be one of the larger sources of atmospheric methane, a gas which is an important potential driver of global warming. The atmospheric methane concentration is increasing at about 1% per year and it is an unanswered question as to how much of this increase is due to increased emissions from wetland ricefields. Objectives in current research are to reduce uncertainties concerning how much methane and other climatically active trace gases are annually emitted from irrigated, rainfed, and flood prone rice ecosystems at present, to predict future emissions for given management scenarios, and to develop feasible rice technologies that will reduce emissions and yet will meet the required increase in rice production.

Methane emissions from rice fields: Effect of soil properties

Abstract: Flooded rice fields emit methane and are important contributors to the increasing atmospheric methane concentration. Various estimates of global release rates of methane from rice paddies range from a low of 20 Tg per year to a high of 200 Tg per year. Global estimates of methane emissions from rice fields depend upon obtaining reliable data from a variety of soil types. We have compared a variety of methane emission data sets obtained over a four-year period from three different soil types found at the Texas Agricultural Experiment Station near Beaumont, Texas, with several physical and chemical properties of the soils. We find that seasonal methane emissions directly correlate with the percent sand in the soils. Along a transect with soil sand content ranging from 18.8% to 32.5%, seasonal methane emissions ranged from 15.1 g m −2 to 36.3 g m−2.

Methane Flux From Mangrove Sediments Along the Southwestern Coast of Puerto Rico

Abstract: Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH4 m−2 d−1, 42 mg CH4 m−2 d−1, and 82 mg CH4 m−2 d−1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH4 flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.

The Greenhouse Gas Methane (CH4): Sources and Sinks, the Impact of Population Growth, Possible Interventions

Abstract: Methane (CH4) is one of the trace gases in the atmosphere that is considered to play a major role in what is called the “greenhouse effect.” There are six major sources of atmospheric methane: emission from anaerobic decomposition in (1) natural wetlands; (2) paddy rice fields; (3) emission from livestock production systems (including intrinsic fermentation and animal waste); (4) biomass burning (including forest fires, charcoal combustion, and firewood burning); (5) anaerobic decomposition of organic waste in landfills; and (6) fossil methane emission during the exploration and transport of fossil fuels. Obviously, human activities play a major role in increasing methane emissions from most of these sources. Especially the worldwide expansion of paddy rice cultivation, livestock production and fossil fuel exploration have increased the methane concentration in the atmosphere. Several data sets help estimate atmospheric methane concentration up to 160,000 years back. Major sources and sinks of present-day methane emission and their relative contribution to the global methane balance demonstrate great uncertainties in the identification and quantification of individual sources and sinks. Most recent methane projections of the Intergovernmental Panel on Climate Change (IPCC) for 2025 and 2100 are discussed and used to estimate the contribution of population growth to future methane emission. Finally the paper discusses options and restrictions of reducing anthropogenic methane emissions to the atmosphere.

Methane and nitrous oxide emissions: an introduction

Abstract: Methane and nitrous oxide are important greenhouse gases. They contribute to global warming. To a large extent, emissions of methane and nitrous oxide are connected with the intensification of food production. Therefore, feeding a growing world population and at the same time controlling these emissions is a great challenge. Important anthropogenic sources of biogenic methane are wet rice fields, cattle, animal waste, landfills and biomass burning. Important anthropogenic sources of biogenic nitrous oxide are land-use change, fertilizer production and use and manure application. The ultimate objective of the Framework Convention on Climate Change implies a stabilization of greenhouse gas concentrations in the atmosphere. As a small first step towards achieving this objective, the Convention requires the industrialized countries to bring their anthropogenic emissions of greenhouse gases by 2000 back to 1990 levels. It was also agreed that all parties would make national inventories of anthropogenic greenhouse gas emissions and programmes for control (UN, 1992). In this context, in February 1993 an international workshop was held in Amersfoort in the Netherlands to discuss methods in national emission inventories for methane and nitrous oxide, and options for control (Van Amstel, 1993). A selection of the papers presented in Amersfoort that focus on agricultural sources is published in this volume. This introductory chapter gives background information on biogenic sources and sinks of methane and nitrous oxide and options for their control. The goal of the Climate Convention is described as well as the IPCC effort to develop an internationally accepted methodology for the monitoring of greenhouse gas emissions and sinks. Finally, some preliminary results from country inventories are given. It is concluded that a common reporting framework and transparency of the inventories are important to obtain comparable results that can be used for complying with the requirements of the Climate Convention and for facilitating the international debate about appropriate response strategies.

Comment on ‘A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992' by E. J. Dlugokencky et al.

Abstract: The carefully measured decrease in the growth rate of atmospheric methane (CH4) in 1992 reported by Dlugokencky et al. (1994) is an impressive accomplishment, and testimony for the importance of maintaining high-quality, long-term monitoring of atmospheric composition. The changing growth rate of atmospheric CH4 has important implications for assessing and understanding the potential magnitude and rates of a future greenhouse gas-induced climate change. Furthermore, the CH4 data from the current Climate Monitoring and Diagnostics Laboratory (CMDL) globally-distributed network of cooperative air sampling sites are clearly the best record of global CH4 trends and distribution currently available. However, we argue briefly here that the speculation by Dlugokencky et al. (1994) on possible mechanisms for the decreased growth rate in 1992 is only one scenario of many that could possibly fit with the constraints imposed by the reported data. Our comments are to (1) illustrate the difficulties of deducing small changes in complex, poorly understood, geographically diverse natural and anthropogenic sources of CH4 from measurements at the remotely-located CMDL sampling sites and (2) emphasize that detailed bottoms-up analyses are necessary to really advance the understanding of changes in source strengths; we are not promoting alternative mechanisms to explain the 1992 decrease in atmospheric CH4.

Methane in ocean waters: Concentration and carbon isotope variability at East Pacific Rise and in the Arabian Sea

Abstract: Methane concentrations and stable carbon isotope ratios of water samples from the East Pacific Rise (EPR) at 21°S and the Arabian Sea (24°N, 65°E) have been determined. EPR surface water is in equilibrium (ca. 50 nl/L and −50%o l00nl/L and −30%o < δ13CH4 < −22%o) is detectable only close to the seep site. There is no input of hydrothermal methane into the atmosphere. EPR water is considered to be rather a sink than a source of atmospheric methane. Surface waters of the Arabian Sea are enriched in methane relative to the atmosphere (source for atmospheric methane). Carbon isotope ratios point to a bacterial origin of methane (δ13CH4 < −55%o) that is generated in the surface waters. Concentration changes and variations of carbon isotope ratios also suggest that methane seeping from the sea floor sediments of the Arabian Sea is oxidized by bacterial activity and does not reach the atmosphere.

Marine sediments as a source of atmospheric methane

Abstract: More than 99% of the diffusive methane flux from marine sediments appears to be consumed by microbial oxidation when dissolved sulfate is present. At 3 sites, methane bubble vents were found. Two of the vents, located in the Okhotsk Sea, appear to result from gas being released along faults. The third plume site was located in a shallow harbour where a large amount of organic carbon is deposited in shallow water. The global diffusive and advective (bubbles) release of methane to the atmosphere from marine sediments is estimated to fall into the range of 1 - 10 TgC a"1 (1 Tg = 10 12 g). Researchers have speculated that marine gas hydrate deposits hold vast quantitites of methane that may melt and be released as a result of global warming. A warmer atmosphere could directly heat the ocean and/or change ocean current systems, which could bring warmer water to some areas. Hydrate samples were recovered from two sites in the Okhotsk Sea. These deposits, in 700 to 800 m of water, would require water temperatures to increase by 8°C in order to melt the hydrate. Quantitative estimates of hydrate reservoirs near the minimum pressure stability zone are needed to remove the uncertainty whether this will be a significant positive feedback loop for global warming.

Assessment of uncertainties in the projected concentrations of methane in the atmosphere (Technical Report)

Abstract: Sinks and sources of methane have been examined in the papers presented at the IUPAC sponsored Workshop held in Moscow in July 1992. The conclusion reached was that very large uncertainties exist in the assessment of sources. The emissions from wetlands and rice paddies could be much lower than formerly assumed. The emissions from other sources, e.g. from landfills, could be larger. The most important sink, oxidation by OH-radicals in the atmosphere, has an uncertainty of 40% or more and cannot be used to evaluate the quality of the emission data. As a consequence, it is very difficult to predict future atmospheric concentrations, as function of changes in land use and economic activities. The prediction of the radiative balance of the earth is very difficult not only because of the uncertainty in future greenhouse gas concentrations but also because important factors, like the influence of aerosols, are insufficiently characterized.

Measurements of the 14C content of atmospheric methane in The Netherlands to determine the regional emissions of 14CH4

Abstract: We give a brief description of a method to determine the 14 CH 4 emissions in NW Europe. We report the first measurements of atmospheric 14 CH 4 in The Netherlands, ranging from 124.3 to 426.5 pM. Preliminary analysis suggests a contribution of 14 CH 4 from nuclear installations other than pressurised/boiling water reactors.

Determination of global scale emissions of atmospheric methane using an inverse modelling method

Abstract: The large-scale source distribution of methane is reflected in the spatial and temporal variation of the atmospheric methane mixing ratio. Furthermore, the stable isotope ratio of methane (13 CH 4/12 CH 4) contains additional information on the methane sources, because these differ in their isotopic composition. The constraints on the magnitude of the various sources of methane provided for by atmospheric measurements of the CH 4 mixing ratio and its 13 C/12 C ratio are investigated by means of a three-dimensional atmospheric transport model that includes a tropospheric background chemistry module which calculates the oxidation of CH 4 by chemical reaction with the hydroxyl radical (OH).

Annual Variation of Methane in Seawater in Funka Bay,Japan

Abstract: The concentration of methane in seawater was determined approximately once a month for one year from August 1990 to July 1991 at a station close to the center of Funka bay (92 m depth) and some supplementary observations were also carried out. The concentration of methane was usually increased with increasing depth, suggesting that methane was emitted from the bottom of the bay. While highly variable both spatially and temporally, the emission was intense in March and April, a period immediately after the spring bloom of phytoplankton. The maximum of methane found in the intermediate water suggests its source from the slope of the bay. The concentration of methane in the surface water changed seasonally and also interannually. The annually averaged flux of methane transferred to the atmosphere in the bay was estimated to be 6×10−3 gCH4m2/day. The coastal zone in the world may be a significant source of the atmospheric methane, although its source strength has yet to be accurately estimated from more data in different coastal seas.

The Recent State of Carbon Cycling through the Atmosphere

Abstract: A summary of our knowledge about the recent carbon cycle is presented from the perspective of atmospheric observations. Particular emphasis is given to carbon isotopes as a tool to determine global carbon sources and sinks as well as their recent anthropogenic changes. In the case of atmospheric carbon dioxide, the yearly anthropogenic emissions are small if compared to the gross exchange between the most important reservoirs: biosphere and ocean. In order to derive net fluxes caused by climatic or anthropogenic perturbations, the gross fluxes have to be known as accurately as possible. One isotopic tool is to trace the penetration of bomb 14C into the ocean surface water so as to derive the atmosphere/ocean gas-exchange rate. Moreover, the distribution of δ13C in atmospheric CO2 can help to constrain the amount of excess CO2 taken up (or released) by the global biosphere. As for atmospheric methane, anthropogenic emissions today exceed natural release rates by nearly a factor of two. Although the total yearly methane emissions to the atmosphere are believed to be known to within 10–20%, large uncertainties still remain in the estimates for individual sources, in particular for natural ecosystems like wetlands and the oceans. Isotope observations provide strong constraints also for the global atmospheric methane budget because different source types (biogenic vs. thermogenic) produce their own characteristic isotopic fractionations during methane production. These isotopic signatures can be used further to predict the future development of atmospheric methane concentrations and to understand better the drastic changes observed in the past under different conditions.

Permafrost Melting And Stability Of Offshore Methane Hydrates Subject To Global Warming

Abstract: Recently proposed global warming scenarios are employed to determine the effect of a wide range of anticipated global temperature rises on the stability of: (a) onshore permafrost; and (b) methane gas hydrates located below the ocean floor in deep water. Temperature profiles, the time required for the onset of hydrate decomposition and the rate of permafrost melting are computed. It is found that while permafrost decomposition due to global warming is feasible, decomposition of the suboceanic hydrates is not likely in the foreseeable future.

Consumption of methane by soils.

Abstract: Measurements of the methane flux and methane concentration profiles in soil air are presented. The flux of methane from the soil is calculated by two methods: a) Direct by placing a static open chamber at the soil surface, b) Indirect, using the 222Rn concentrations profile and the 222Rn flux in the soil surface in parallel with the methane concentration (222Rn calibrated fluxes). The methane flux has been determined in two kinds of soils (sandy and loamy) in the surroundings of Malaga (SPAIN). The directly measured methane fluxes at all investigated sites is higher than methane fluxes derived from “Rn calibrated fluxes”. Atmospheric methane is consumed by soils, mean direct flux to the atmosphere were - 0.33 g m-2yr-l. The direct methane flux is the same within the measuring error in sandy and loamy soils. The influence of the soil parameters on the methane flux indicates that microbial decomposition of methane is primarily controlled by the transport of methane.