Showing papers on "Atmospheric methane published in 1998"

Changing concentration, lifetime and climate forcing of atmospheric methane

Abstract: Previous studies on ice core analyses and recent in situ measurements have shown that CH 4 has increased from about 075–173 μmol/mol during the past 150 years Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH 4 distribution are well represented The model has been used to derive the total CH 4 emission source, being about 600 Tg yr -1 Based on published results of isotope measurements the total contribution of fossil fuel related CH 4 emissions has been estimated to be about 110 Tg yr -1 However, the individual coal, natural gas and oil associated CH 4 emissions can not be accurately quantified In particular natural gas and oil associated emissions remain speculative Since the total anthropogenic CH 4 source is about 410 Tg yr -1 (∼70% of the total source) and the mean recent atmospheric CH 4 increase is ∼20 Tg yr -1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH 4 level We have calculated the indirect chemical effects of increasing CH 4 on climate forcing on the basis of global 3D chemistry-transport and radiative transfer calculations These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work The contribution of CH 4 (direct and indirect effects) to climate forcing during the past 150 years is 057W m −2 (direct 044W m −2 , indirect 013 W m −2 ) This is about 35% of the climate forcing by CO 2 (16W m −2 ) and about 22% of the forcing by all long-lived greenhouse gases (26 W m −2 ) Scenario calculations (IPCC-IS92a) indicate that the CH 4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 79 years Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O 3 (→OH) in the tropics The global mean concentration of CH 4 may increase to about 255 μmol/mol, its lifetime is expected to increase to 84 years in the year 2050 Further, we have calculated a CH 4 global warming potential (GWP) of 21 (kgCH 4 /kgCO 2 ) over a time horizon of 100 years, in agreement with IPCC (1996) Scenario calculations indicate that the importance of the climate forcing by CH 4 (including indirect effects) relative to that of CO 2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050 DOI: 101034/j1600-08891998t01-1-00002x

Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability

Abstract: Atmospheric methane mixing ratios from 1000 A.D. to present are measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resolution and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately determined. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N-S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr−1 in 1981 and decreasing significantly since. We calculate an average total methane source of 250 Tg yr−1 for 1000–1800 A.D., reaching near stabilization at about 560 Tg yr−1 in the 1980s and 1990s. The isotopic ratio, δ13CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid-1980s. The combined CH4 and δ13CH4 trends support the stabilization of the total CH4 source.

Continuing decline in the growth rate of the atmospheric methane burden

Abstract: The global atmospheric methane burden has more than doubled since pre-industrial times1,2, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. Research into future climate change and the development of remedial environmental policies therefore require a reliable assessment of the long-term growth rate in the atmospheric methane load. Measurements have revealed that although the global atmospheric methane burden continues to increase2 with significant interannual variability3,4, the overall rate of increase has slowed2,5. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984–96, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane's atmospheric lifetime. If the global methane sources and OH concentration continue to remain constant, we expect average methane mixing ratios to increase slowly from today's 1,730 nmol mol−1 to ∼1,800 nmol mol−1, with little change in the contribution of methane to the greenhouse effect.

Mitigating Agricultural Emissions of Methane

Abstract: Agricultural crop and animal production systems are important sources and sinks for atmospheric methane (CH4). The major CH4 sources from this sector are ruminant animals, flooded rice fields, animal waste and biomass burning which total about one third of all global emissions. This paper discusses the factors that influence CH4 production and emission from these sources and the aerobic soil sink for atmospheric CH4 and assesses the magnitude of each source. Potential methods of mitigating CH4 emissions from the major sources could lead to improved crop and animal productivity. The global impact of using the mitigation options suggested could potentially decrease agricultural CH4 emissions by about 30%.

Acidophilic methanotrophic communities from Sphagnum peat bogs.

Abstract: Northern wetlands have attracted considerable attention in the last decade as a possible significant source of atmospheric methane (12). Acidic ombrotrophic peat bogs are the most extensive type of wetland, occupying about 3% of total land area and being one of the dominant terrestrial ecosystems in the boreal forest zone of North America and Eurasia. There is a great body of evidence that peat bogs are inhabited by active methanotrophic bacteria that reduce the emission of methane to the atmosphere to 10 to 90% of that generated in the anaerobic layers of the bog profile (9, 17, 19, 21, 22). Although intensive methanotrophic activity in this habitat was recognized many years ago, the microorganisms responsible for this process have eluded isolation. All described methanotrophs are incapable of growth at pH values below 5.0 (7) and thus apparently are unable to oxidize methane in these Sphagnum peat bogs, which have a pH of 3.5 to 5. Other characteristic features of the Sphagnum bog potentially important to the microbial community are the low content of mineral elements in peat water (5 to 50 mg/liter), the presence of inhibitory products from mosses, and a broad annual temperature range from −30 to +30°C. Methane consumption, in particular, is very sensitive to temperature variation, especially during cold seasons. Routine enrichment techniques have failed to yield isolates of methanotrophs from this hostile environment. The only exception is a report on the isolation of a bacterium ascribed to the genus Methylosinus from an acidic peat lake (9), but no experimental confirmation of its activity at low pH was provided. Furthermore, no evidence was provided that the isolated bacterium exhibited any activity in situ, an especially important point since Methylosinus has the ability to form exospores and survive for a long time under unfavorable conditions (6). The ecological application of molecular techniques has opened up a new opportunity for direct detection of methane-oxidizing bacteria in environmental samples. Indeed, primers designed for amplification of the soluble methane monooxygenase (sMMO) gene cluster have shown the predicted PCR products from DNA from acidic peat, suggesting that these habitats contain numerous methanotrophs (13). Other evidence for the existence of acidophilic methanotrophs was obtained by screening 16S rDNA libraries from several peat samples by means of hybridization with specific probes (14). A few of these clones were found to be representatives of a potentially novel group of methanotrophs related to the Methylosinus-Methylocystis cluster. Our recent studies (4, 5) dealt with measurements of methanotrophic activity in samples of native peat from four different bogs under various environmental conditions. We found that indigenous methanotrophic populations, as reflected by their activity in peat, have temperature optima of 15 to 20°C and pH optima of 4.5 to 5.5 and are extremely sensitive to salt stress. The aim of the present study was to undertake the next step in the characterization of acidophilic methanotrophs and obtain a highly enriched methanotrophic population able to grow in acidic peat. We also report on the kinetic and physiological features of the organisms adapted to this unique habitat.

Nitrous oxide and methane fluxes from organic soils under agriculture

Abstract: Trace gas fluxes of N2O and CH4 were measured weekly over 12 months on cultivated peaty soils in southern Germany using a closed chamber technique. The aim was to quantify the effects of management intensity and of soil and climatic factors on the seasonal variation and the total annual exchange rates of these gases between the soil and the atmosphere. The four experimental sites had been drained for many decades and used as meadows (fertilized and unfertilized) and arable land (fertilized and unfertilized), respectively. Total annual N2O-N losses amounted to 4.2, 15.6, 19.8 and 56.4 kg ha–1 year–1 for the fertilized meadow, the fertilized field, the unfertilized meadow and the unfertilized field, respectively. Emission of N2O occurred mainly in the winter when the groundwater level was high. At all sites maximum emission rates were induced by frost. The largest annual N2O emission by far occurred from the unfertilized field where the soil pH was low (4.0). At this site 71% of the seasonal variation of N2O emission rates could be explained by changes in the groundwater level and soil nitrate content. A significant relationship between N2O emission rates and these factors was also obtained for the other sites, which had a soil pH between 5.1 and 5.8, though the relation was weak (R2 = 15–27%). All sites were net sinks for atmospheric methane. Up to 78% of the seasonal variation in CH4 flux rates could be explained by changes in the groundwater level. The total annual CH4-C uptake was significantly affected by agricultural land use with greater CH4 consumption occurring on the meadows (1043 and 833 g ha–1) and less on the cultivated fields (209 and 213 g ha–1).

Methane efflux from high-latitude lakes during spring ice melt

Abstract: Ice cores removed from shallow ice-covered tundra lakes near Barrow, Alaska, and taiga lakes near Anchorage, Alaska, exhibit increasing concentrations of methane with depth. Methane concentrations in the ice cores increased from 0 μM in the top 15 cm sections to a maximum of 23 μM in the lowest 15 cm sections of tundra lake ice and to a maximum of 147 μM in taiga lake ice. Methane concentrations in the water beneath the ice reflect a similar pattern, with values near 5 μM early in the ice-covered season, increasing up to 42 μM in the tundra lakes, and up to 730 μM in the taiga lakes. Methane levels increase in the water beneath the ice during the course of the winter due to decreasing water volume, exclusion from growing ice, and continued methane production in thawed sediments. Since the ice layer prohibits gas exchange with the atmosphere, the methane is not oxidized, as it would be during the summer months, allowing the winter accumulation and storage of methane in the ice and lake waters. Efflux measurements, taken with floating chambers on the taiga lakes, indicated a large pulse of methane released during the period of ice melt and spring turnover. The efflux from one lake ranged from 2.07 g CH4 m−2 in 1995 to 1.49 g CH4 m−2 in 1996 for the 10 day period immediately after ice melt. Estimation of methane efflux using a boundary layer diffusion model and surface water concentrations during the entire ice-free period in 1996 predicted an efflux of 1.79 g CH4 m−2 during the same 10 day period, compared with 2.28 g CH4 m−2 for the remainder of the summer season. This observation suggests that almost as much methane efflux can occur during a brief period immediately after ice melt as occurs during the remainder of the ice-free season. Since measurements of methane efflux from high-latitude-lakes are generally made after this breakup period, the overall contribution to atmospheric methane from high-latitude lakes may be twice that of current estimates.

Polar stratospheric clouds: A high latitude warming mechanism in an ancient greenhouse world

Abstract: The presence of water vapor clouds in the stratosphere produces warming in excess of tropospheric greenhouse warming, via radiative warming in the lower stratosphere. The stratospheric clouds form only in regions of very low temperature and so the warming produced by the clouds is concentrated in polar winter regions. Results from a paleoclimate modeling study that includes idealized, prescribed polar stratospheric clouds (PSCs) show that the clouds cause up to 20°C of warming at high latitude surfaces of the winter hemisphere, with greatest impact in oceanic regions where sea ice is reduced. The modeled temperature response suggests that PSCs may have been a significant climate forcing factor for past time intervals associated with high concentrations of atmospheric methane. The clouds and associated warming may help to explain long-standing discrepancies between model-produced paleotemperatures and geologic proxy temperature interpretations at high latitudes, a persistent problem in studies of ancient greenhouse climates.

Methane oxidation in landfill cover soils, as revealed by potential oxidation measurements and phospholipid fatty acid analyses

Abstract: Landfills account for ca. 10% of the annual global burden of atmospheric methane. Part of the efflux is mitigated by means of biological methane oxidation in the landfill covers. In this study, two types of landfill cover soils (mineral soil and sewage sludge) were compared with respect to methane emissions as well as potential methane oxidation capacity and the PLFA (phospholipid fatty acid) content of soil samples. Methane fluxes were lowest at a landfill site where wastes were covered with old sewage sludge. This site consumed atmospheric methane on most occasions. In incubated soil samples from the landfill cover composed of mineral soil, potential methane oxidation was most strongly correlated with the concentration of PLFA 18:1ω8, which is typical for type-II methanotrophic bacteria. In contrast, in samples from a landfill cover composed of fresh sewage sludge, methane oxidation was most strongly correlated with 16:1-PLFAs, indicating that type-I methanotrophs predominated, probably owing to nutritional conditions being more favourable in the sludge. The results also indicate that it takes a long time, i.e. several years, for methanotrophs to get well established in landfill cover soils.

Inhibition of Methane Consumption in Forest Soils by Monoterpenes

Abstract: Selected monoterpenes were tested for their ability to inhibit atmospheric methane consumption by three forest soils from different vegetation types and by the cultured methanotrophic strain, Methylosinus trichosporium OB3b. Subsurface soil from coniferous (Pinus banksiana), deciduous (Populus tremuloides), and mixed hardwood (Tsuga canadensis and Prunus pensylvanica) stands was used under field-moist (bulk and intact cores) and slurry conditions. Most of the hydrocarbon monoterpenes tested significantly inhibited (40–100%) methane consumption by soils at environmentally relevant levels, with (–)-α-pinene being the most effective. With the exception of β-myrcene, monoterpenes also strongly inhibited methane oxidation by Methylosinus trichosporium OB3b. Carbon dioxide production was stimulated in all of the soils by the monoterpenes tested. In one case, methane production was stimulated by (–)-α-pinene in an intact, aerobic core. Oxide and alcohol monoterpenoids stimulated methane production. Thus, monoterpenes appear to be potentially important regulators of methane consumption and carbon metabolism in forest soils.

Effects of Ammonium and Non-Ammonium Salt Additions on Methane Oxidation by Methylosinus trichosporium OB3b and Maine Forest Soils

Abstract: Additions of ammonium and non-ammonium salts inhibit atmospheric methane consumption by soil at salt concentrations that do not significantly affect the soil water potential. The response of soils to non-ammonium salts has previously raised questions about the mechanism of ammonium inhibition. Results presented here show that inhibition of methane consumption by non-ammonium salts can be explained in part by ion-exchange reactions: cations desorb ammonium, with the level of desorption varying as a function of both the cation and anion added; differential desorption results in differential inhibition levels. Differences in the extent of inhibition among ammonium salts can also be explained in part by the effects of anions on ammonium exchange. In contrast, only minimal effects of cations and anions are observed in liquid cultures of Methylosinus trichosporium OB3b. The comparable level of inhibition by equinormal concentrations of NH4Cl and (NH4)2SO4 and the insensitivity of salt inhibition to increasing methane concentrations (from 10 to 100 ppm) are of particular interest, since both of these patterns are in contrast to results for soils. The greater inhibition of methane consumption for NH4Cl than (NH4)2SO4 in soils can be attributed to increased ammonium adsorption by sulfate; increasing inhibition by non-ammonium salts with increasing methane concentrations can be attributed to desorbed ammonium and a physiological mechanism proposed previously for pure cultures.

Groundwater seepage in eckernforde bay (western baltic sea) : effect on methane and salinity distribution of the water column

Abstract: The effluent activity from a well-known pockmark structure in Eckernforde Bay was monitored for methane, salinity, and temperature signals in the water column intermittently over three years between 1991, 1993 and 1994. Groundwater discharge from an aquifer into the brackish waters of the western Baltic, dilutes bottom water salinities to values as low as 2.9‰. Seasurface height and the amount of precipitation preceding sampling periods by 5 days correlated significantly with the rate of groundwater discharge. Concentrations of methane in bottom water at the pockmark site were strongly influenced by seepage intensity. At two sampling sites (control and pockmark site) distinctly lower methane concentrations were observed towards the sea surface, although the entire water body of Eckernforde Bay appears to be affected by methane seeping from the sediments. This is supported by high methane concentrations above equilibrium with atmospheric methane throughout most of the year. Maximum concentration above the equilibrium value in surface waters was 2800‰. Methane flux from surface waters into the atmosphere follows strong seasonal variations, with maximum values in the winter (200–400 μmol m-2 d-1). The study reveals the important role of coastal oceans in the global methane cycle, as an intense but variable source of methane of largely unknown magnitude.

Methanol promotes atmospheric methane oxidation by methanotrophic cultures and soils

Abstract: Two methanotrophic bacteria, Methylobacter albus BG8 and Methylosinus trichosporium OB3b, oxidized atmospheric methane during batch growth on methanol. Methane consumption was rapidly and substantially diminished (95% over 9 days) when washed cell suspensions were incubated without methanol in the presence of atmospheric methane (1.7 ppm). Methanotrophic activity was stimulated after methanol (10 mM) but not methane (1,000 ppm) addition. M. albus BG8 grown in continuous culture for 80 days with methanol retained the ability to oxidize atmospheric methane and oxidized methane in a chemostat air supply. Methane oxidation during growth on methanol was not affected by methane deprivation. Differences in the kinetics of methane uptake (apparent K(m) and V(max)) were observed between batch- and chemostat-grown cultures. The V(max) and apparent K(m) values (means +/- standard errors) for methanol-limited chemostat cultures were 133 +/- 46 nmol of methane 10 cells h and 916 +/- 235 ppm of methane (1.2 muM), respectively. These values were significantly lower than those determined with batch-grown cultures (V(max) of 648 +/- 195 nmol of methane 10 cells h and apparent K(m) of 5,025 +/- 1,234 ppm of methane [6.3 muM]). Methane consumption by soils was stimulated by the addition of methanol. These results suggest that methanol or other nonmethane substrates may promote atmospheric methane oxidation in situ.

Methanol Improves Methane Uptake in Starved Methanotrophic Microorganisms

Abstract: Methanotrophs in enrichment cultures grew and sustained atmospheric methane oxidation when supplied with methanol. If they were not supplied with methanol or formate, their atmospheric methane oxidation came to a halt, but it was restored within hours in response to methanol or formate. Indigenous forest soil methanotrophs were also dependent on a supply of methanol upon reduced methane access but only when exposed to a methane-free atmosphere. Their immediate response to each methanol addition, however, was to shut down the oxidation of atmospheric methane and to reactivate atmospheric methane oxidation as the methanol was depleted.

Measurements of methane emissions from rice fields in China

Abstract: Rice fields have always been regarded as one of the largest anthropogenic sources of atmospheric methane. Here we report the results of a 7-year study of methane emissions from rice fields in the Sichuan Province of China. In this region, there is one crop of rice per year, the fields are continuously flooded from transplanting to harvest, and there is heavy use of organic fertilizers. Emissions over the entire growing season were measured from each of up to 24 plots. Environmental variables were measured and relevant supporting data on the agricultural practices were recorded. The fields were studied under prevailing agricultural practices of the local farmers. The results represent emissions under standard agricultural practices and the year to year variability of climate, fertilizers, available irrigation water, and cultivars. Based on some 5000 flux measurements, the average emission rates between 1988 and 1994 were 30 mg/m(2)/h for a growing season of between 100 and 120 days. This emission rate is comparable to other published data from similar rice fields but somewhat on the high side of the range. There were no systematic trends of emissions during the 7 years of our experiment, but there was substantial year to year variability. The data have been subjected to exhaustive analyses for validity, accuracy, and reliability. From this, a high-quality, spatially averaged data set has been constructed representing average emissions from the rice fields for each day when measurements were taken. We describe here the main observational results and document the spatial and temporal variability observed on timescales ranging from a day to several years and on spatial scales ranging from 0.5 m(2) to 16 m(2).

Seven years of continuous methane observations at a remote boreal site in Ontario, Canada

Abstract: A 7-year record (1990–1996) of continuous atmospheric methane (CH4) measurements is presented from a remote midcontinental monitoring station at Fraserdale, Ontario (49°53′N, 81°34′W). Ninety-six air samples per day were measured with a fully automated gas chromatograph with flame ionization detection. Five-day Lagrangian back trajectories over the 7-year period were used to establish a climatology in the region of the station. The site is predominantly influenced by air flow from northern and high-latitude regions and therefore uniquely positioned to monitor wetland emissions. During winter, CH4 concentration time series from Fraserdale often match the short-term variability observed at the high Arctic monitoring station at Alert, Northwest Territories (82°27′N, 61°31′W). During summer, due to diurnal changes of vertical mixing in the boundary layer, large diurnal cycles in CH4 mixing ratio up to 150 ppb are observed. The data selected for the afternoon, when the boundary layer is well-mixed, are representative of a larger spatial scale. The mean annual cycle of CH4 at Fraserdale determined using these selected data is significantly different from annual cycles at other mid- and high-northern latitude sites thus providing key information for global atmospheric CH4 models. In late summer the annual cycle at Fraserdale shows a distinct secondary maximum in CH4. This is the result of advection of air with enhanced CH4 due to emissions from the extensive wetland areas to the north and northwest. The average growth rate (using selected data) for the period was 5.6 ppb yr−1 with a growth rate pattern that is slightly different and out of phase with growth rate changes observed at other high-latitude observing sites by 2 to 6 months.

A new method to study simultaneous methane oxidation and methane production in soils

Abstract: Results of laboratory experiments show that 14C-labeled methane added to soil was consumed faster than atmospheric 12C methane. This implies a source of methane, presumably through methanogenesis, in a soil that is a net consumer of atmospheric methane. The soil was well-drained forest soil from Ispra, Italy. An undisturbed sample was taken with a steel corer and incubated under oxic conditions in a jar. Headspace samples were taken at time intervals and analyzed for total methane by gas chromatography and analyzed for 14C methane by liquid scintillation counting. Fluxes calculated from the decreasing headspace mixing ratios were, for example, −6.5 and −7.1 μmol m−2 hr−1 for 12C methane and 14C methane, respectively. A simple model is considered which reproduces reasonably well the observed mixing ratios as function of time.

Natural microseepage of methane to the atmosphere from the Denver‐Julesburg basin, Colorado

Abstract: Methane fluxes were measured at locations across the Denver-Julesburg (DJ) basin of northeastern Colorado, which is in a semiarid climate. There is a potential for microseepage of thermogenically derived methane and light hydrocarbons to the atmosphere, which during drier or colder seasons may exceed the capacity for methanotrophic oxidation in the shallow soils. Triplicate measurements of methane flux in three seasons across the basin found areas of positive and negative fluxes in a patchy pattern which averaged +0.57 mg CH4 m−2 d−1. The possibility of drier areas of the continents, which are underlain by thick sedimentary units seasonally producing positive methane fluxes to the atmosphere, represents a second-order effect in the estimation of the terrestrial sources/sinks for atmospheric methane.

Atmospheric methane consumption in adjacent arable and forest soil systems

Abstract: During a 24 h incubation at 15°C atmospheric methane uptake was measured in closed bottles containing moistened (30% v/v) structurally different soils from five sites in southern Norway. Each site had a natural sub-site and an adjacent disturbed counterpart within 50 m. Highest methane uptake was found in the uppermost mineral horizon in undisturbed forest soils, with a maximum of 1.154±0.002 ng CH4 g−1 dw soil h−1. In contrast, the adjacent disturbed arable counterpart of this site had a 130-fold reduction in CH4 uptake (9±3 pg CH4 g−1 dw h−1). The highest uptake rate in arable soils was 129±8 pg CH4 g−1 dw h−1. This site was a former forest soil, cultivated for only 2 years. Similar CH4 uptake rates occurred at a forested spruce site which had earlier been cultivated. In 1994, 30 years after forestation, this soil had a subsurface methane uptake of 134±8 pg g−1 dw h−1, the lowest uptake found for top mineral forest soil. Conditioning the fresh soil samples at 15°C for 3 weeks in a 20% CH4 atmosphere changed the soil's capacity to consume atmospheric methane. Generally the methane uptake rates at ambient CH4 concentrations increased in arable soils whereas the uptake rates decreased in forest soils. Blocking methane oxidation with dimethyl ether resulted in a considerable methane accumulation. Methane production was highest in the top organic soil horizons, with a maximum of 7.31 ng CH4 g−1 dry soil 24 h−1.

Effects of production and oxidation processes on methane emissions from rice fields

Abstract: The emission of methane from rice fields is the difference between the amount produced in the anaerobic zone below the soil and the amount oxidized in the root zone. Plants can also contribute to methane production by exuding organic compounds that may be utilized by methanogenic bacteria. We measured methane emissions from rice fields at Tu Zu in China between 1988 and 1994, which gave average emissions of about 30 mg m−2 h−1. We estimate that 45–60% of the methane produced was oxidized before reaching the atmosphere; and root exudates may have contributed of the order of 10% of the methane that was produced. The fraction of methane oxidized is low compared to experimental studies at other locations (60–85%). At Tu Zu, methane production is enhanced by continuously flooded fields and the use of large amounts of organic fertilizers; in addition, the lower oxidation rate may also contribute to the higher methane emissions observed compared to other locations. In the past, most of the attention has been devoted to the factors that affect methane production and transport, but it seems that the factors that affect methane oxidation are equally important in determining the flux, if not more so. The comparison of methane fluxes observed at different locations and the extrapolation of field measurements to accurately estimate global emissions will require a better understanding of the rate of methane oxidation in the soils and the factors that control it.

Atmospheric methane measurements in central New England: An analysis of the long‐term trend and the seasonal and diurnal cycles

Abstract: We have compiled a unique high-resolution ambient-air methane data set consisting of approximately 125,000 independently measured data points for the years 1991-1995 that have been collected at a site in the northeastern United States. The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10-30% of the data from each month were defined as representative of background air and were compared with the global Climate Monitoring and Diagnostics Laboratory (CMDL) data set. The background data exhibit a variable upward trend of 5.5 + 2 ppbv/yr during the 4-year time period, with most of the increase observed during 1993 and 1994. The seasonal cycle for the back- ground data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differ- ences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmo- spheric stability and changes in emissions from local and regional sources throughout the year. Diurnal cycles exist in the data, witlthe magnitude and timing of maximum and minimum values being controlled by inputs from local sources and atmospheric stability. Nearby weftands contribute to an ovemight buildup of methane in tle late spring and summer. The magnitude of the daily cycle is largest in July and August (--23 ppbv), when inputs from weftands are large and wind speed is generally low. In April, the daily cycle is smallest (--6 ppbv), when inputs from local sources are low and more vigorous atmospheric mixing limits pollution buildup. In 1993 the United States released a climate action plan propos- ing to return U.S. greenhouse gas emissions to the 1990 level by the year 2000. The largest contributor to the potential of global warming is carbon dioxide (CO2), followed by methane (CH4). Stabilizing the atmospheric burden of CO 2 will require a reduction in emissions of 60-80%. This is unlikely in the near future given industrial societies' strong dependence on fuels coupled with the 200-year lifetime of CO 2. In comparison, CH 4 emissions need to be reduced only 10-15% to stabilize its global concentration, and the 11-year lifetin-re of CH 4 means the effects of mitigation strategies may be observed within several decades. A critical factor in determining if mitigation strategies are working is the paucity of quantitative measurements of CH 4 at local to regional scales in which changes in emissions over long time periods may be verified. To address this problem, automated high-frequency (8-11 min) CH 4 measurements have been made by Patrick M. Crill from the University of New Hampshire at the Harvard Forest (HF) research site since 1992. The proximity of

Changing concentration, lifetime and climate forcing of atmospheric methane

Abstract: Previous studies on ice core analyses and recent in situ measurements have shown that CH 4 has increased from about 0.75–1.73 μmol/mol during the past 150 years. Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH 4 distribution are well represented. The model has been used to derive the total CH 4 emission source, being about 600 Tg yr -1 . Based on published results of isotope measurements the total contribution of fossil fuel related CH 4 emissions has been estimated to be about 110 Tg yr -1 . However, the individual coal, natural gas and oil associated CH 4 emissions can not be accurately quantified. In particular natural gas and oil associated emissions remain speculative. Since the total anthropogenic CH 4 source is about 410 Tg yr -1 (∼70% of the total source) and the mean recent atmospheric CH 4 increase is ∼20 Tg yr -1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH 4 level. We have calculated the indirect chemical effects of increasing CH 4 on climate forcing on the basis of global 3D chemistry-transport and radiative transfer calculations. These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work. The contribution of CH 4 (direct and indirect effects) to climate forcing during the past 150 years is 0.57W m −2 (direct 0.44W m −2 , indirect 0.13 W m −2 ). This is about 35% of the climate forcing by CO 2 (1.6W m −2 ) and about 22% of the forcing by all long-lived greenhouse gases (2.6 W m −2 ). Scenario calculations (IPCC-IS92a) indicate that the CH 4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 7.9 years. Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O 3 (→OH) in the tropics. The global mean concentration of CH 4 may increase to about 2.55 μmol/mol, its lifetime is expected to increase to 8.4 years in the year 2050. Further, we have calculated a CH 4 global warming potential (GWP) of 21 (kgCH 4 /kgCO 2 ) over a time horizon of 100 years, in agreement with IPCC (1996). Scenario calculations indicate that the importance of the climate forcing by CH 4 (including indirect effects) relative to that of CO 2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050. DOI: 10.1034/j.1600-0889.1998.t01-1-00002.x

Contrasting pattern of methane flux in rice agriculture

Abstract: We compared methane flux from irrigated and dryland rice fields, employing three rice varieties in each condition, and examined the possible relationships of CH4 flux to plant variables. The results suggest that the dryland rice fields are a methane sink, and several plant characteristics singly or in combination affected CH4 flux from both irrigated and dryland rice agriculture. Nitrogen fertilization sharpened the differences in flux between irrigated cultivars but masked the effect of varieties in dryland agriculture. Wetland rice agriculture is a major anthropogenic source of atmospheric methane (CH4), a greenhouse gas, and this source has increased in recent decades due to the expansion of rice cultivation (Singh and Singh 1995). The aerenchyma tissue of rice plant serves as a conduit to transport CH4 from the anoxic soils to the atmosphere (Mariko et al. 1991), such that more than 90% of CH4 emission from paddy soils is through the rice plant (Inubushi et al. 1989; Schutz et al. 1989). Therefore rice plant characteristics could potentially influence CH4 emission from the soils, but reports regarding the effect of rice cultivars are contradictory (Khalil et al. 1991; Watanabe et al. 1995; Neue et al. 1996). In addition to wetlands, 5–20% of the total harvested rice area in South Asia is under rainfed dryland condition (Matthews et al. 1991) Table 1. Grain yield, and seasonal (121 d for irrigated and 105 d for dryland) methane flux from irrigated and dryland rice fields

The IPCC future projections: are they plausible?

Abstract: IPCC (Intergovernmental Panel on Climate Change) future projections are based on a set of emission scenarios, IS92a to f, which are used to calculate future atmospheric concentrations of greenhouse gases. These, in turn, are used to calculate projections of radiative forcing, and then projections of future temperature and sea level change to the year 2100, using computer climate models. The assumptions of these 6 IPCC emission scenarios for the years 1995 and 2000 are compared with currently available information on greenhouse gas emissions, world population trends, and trends in world coal production. All of the scenarios exaggerate one or more of these quantities. Calculations of confidence limits on the net human-induced contribution of carbon dioxide to the atmosphere show a very high level of inaccuracy. When added to the even greater uncertainties connected with assumptions on the main greenhouse gas, water vapour, and also on clouds, plus the uncertainties of the computer models themselves, the current IPCC future projections of global temperature and sea level must be regarded as extremely unreliable. Fossil fuel emissions assumed by the IPCC scenarios for the year 2000 are plausible for scenarios IS92a, b, c and d, but not for e and f. The calculated rate of increase of atmospheric carbon dioxide concentration since 1990 assumed by the IPCC is exaggerated by 13% for all scenarios. The calculated rates of increase in atmospheric methane from 1990 to 2000 are exaggerated by 3 to 7 times, world population increases by up to 5.5%, and world coal production increases by 60 to 510%. The rate of increase of carbon dioxide in the atmosphere has been almost constant, at 0.4% a year, between 1971 and 1996, despite a 54% increase in emissions from the combustion of fossil fuels over that period. Currently suggested reductions from present emission levels are therefore unlikely to influence carbon dioxide concentrations, or global temperatures. Since all of the IS92 scenarios exaggerate one or more current climate and economic trends, the calculated future projections of atmospheric greenhouse gas concentrations are thus correspondingly exaggerated. A more realistic set of scenarios, which would include a mechanism for continuous updating, needs to be developed, thus scaling down the current values. Even if this is done, however, the accumulated inaccuracies inherent in the final calculations of climatic effects are so great as to render them unreliable as a guide to public policy.

Laser Long-Path Absorption Lidar Technique for Measuring Methane Using Gas Correlation Method

Abstract: We studied a new laser long-path absorption lidar technique using a gas correlation method for measuring methane in the atmosphere. The method employs a broad-band multimode infrared pulsed laser and a cell containing methane as the correlation filter. The correlation filter is used to analyze the spectra of the transmitted and received pulses. We obtained a simple equation for deriving the density of the target molecule from an analogy of the differential absorption method. The measurement method was evaluated by a simulation study to measure methane in the 3 µm region using a solid-state optical parametric oscillator. The result shows that atmospheric methane can be measured by a system using the reflection from a hard target, and that the methane density derived by a simple equation has good linearity in the practical density range in methane measurements. It also indicates that the measurement is not sensitive to fluctuations in the multimode laser spectrum. The measurement system is compact and useful for various field measurements of atmospheric methane.