Showing papers on "Atmospheric methane published in 2006"
TL;DR: It is found that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and it is estimated that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in the study region by 58 per cent.
Abstract: Large uncertainties in the budget of atmospheric methane, an important greenhouse gas, limit the accuracy of climate change projections Thaw lakes in North Siberia are known to emit methane, but the magnitude of these emissions remains uncertain because most methane is released through ebullition (bubbling), which is spatially and temporally variable Here we report a new method of measuring ebullition and use it to quantify methane emissions from two thaw lakes in North Siberia We show that ebullition accounts for 95 per cent of methane emissions from these lakes, and that methane flux from thaw lakes in our study region may be five times higher than previously estimated Extrapolation of these fluxes indicates that thaw lakes in North Siberia emit 38 teragrams of methane per year, which increases present estimates of methane emissions from northern wetlands (< 6-40 teragrams per year; refs 1, 2, 4-6) by between 10 and 63 per cent We find that thawing permafrost along lake margins accounts for most of the methane released from the lakes, and estimate that an expansion of thaw lakes between 1974 and 2000, which was concurrent with regional warming, increased methane emissions in our study region by 58 per cent Furthermore, the Pleistocene age (35,260-42,900 years) of methane emitted from hotspots along thawing lake margins indicates that this positive feedback to climate warming has led to the release of old carbon stocks previously stored in permafrost
1,004 citations
TL;DR: It is demonstrated using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process, suggesting that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.
Abstract: Methane is an important greenhouse gas and its atmospheric concentration has almost tripled since pre-industrial times. It plays a central role in atmospheric oxidation chemistry and affects stratospheric ozone and water vapour levels. Most of the methane from natural sources in Earth's atmosphere is thought to originate from biological processes in anoxic environments. Here we demonstrate using stable carbon isotopes that methane is readily formed in situ in terrestrial plants under oxic conditions by a hitherto unrecognized process. Significant methane emissions from both intact plants and detached leaves were observed during incubation experiments in the laboratory and in the field. If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62-236 Tg yr(-1) for living plants and 1-7 Tg yr(-1) for plant litter (1 Tg = 10(12) g). We suggest that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.
919 citations
Versailles Saint-Quentin-en-Yvelines University1, Centre national de la recherche scientifique2, University of Colorado Boulder3, Earth System Research Laboratory4, VU University Amsterdam5, South African Weather Service6, CSIRO Marine and Atmospheric Research7, Columbia University8, University of California, Irvine9, Institute of Arctic and Alpine Research10
TL;DR: The results indicate that wetland emissions dominated the inter-annual variability of methane sources, whereas fire emissions played a smaller role, except during the 1997–1998 El Niño event.
Abstract: Methane is an important greenhouse gas, and its atmospheric concentration has nearly tripled since pre-industrial times(1). The growth rate of atmospheric methane is determined by the balance between surface emissions and photochemical destruction by the hydroxyl radical, the major atmospheric oxidant. Remarkably, this growth rate has decreased(2) markedly since the early 1990s, and the level of methane has remained relatively constant since 1999, leading to a downward revision of its projected influence on global temperatures. Large fluctuations in the growth rate of atmospheric methane are also observed from one year to the next(2), but their causes remain uncertain(2-13). Here we quantify the processes that controlled variations in methane emissions between 1984 and 2003 using an inversion model of atmospheric transport and chemistry. Our results indicate that wetland emissions dominated the inter-annual variability of methane sources, whereas fire emissions played a smaller role, except during the 1997 - 1998 El Nino event. These top-down estimates of changes in wetland and fire emissions are in good agreement with independent estimates based on remote sensing information and biogeochemical models. On longer timescales, our results show that the decrease in atmospheric methane growth during the 1990s was caused by a decline in anthropogenic emissions. Since 1999, however, they indicate that anthropogenic emissions of methane have risen again. The effect of this increase on the growth rate of atmospheric methane has been masked by a coincident decrease in wetland emissions, but atmospheric methane levels may increase in the near future if wetland emissions return to their mean 1990s levels.
902 citations
TL;DR: The overall methane mass balance in field cells with a variety of designs, cover materials, and gas management strategies was assessed, and defaults for percent recovery were used as the basis for guidelines by the French environment agency.
373 citations
TL;DR: This work shows that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane, and predicts that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
Abstract: The 5% methane present in the nitrogen-rich atmosphere of Titan, Saturn's largest moon, would disappear in tens of millions of years in the absence of a methane source to replenish it. Until the arrival of the Cassini-Huygens mission at Saturn the favoured candidate source had been liquid hydrocarbon seas hundreds of metres thick, but no trace of such seas was found. Now numerical modelling suggests that episodic outgassing of methane stored as clathrate hydrates, associated with an ammonia-rich water ocean is the most likely source, which ties in with a Cassini fly-by image showing a possible cryovolcano. Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years1. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time2. Titan fly-by observations3,4,5 and ground-based observations6 rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
368 citations
TL;DR: In this paper, a 1-D numerical model was used to study the atmospheric photochemistry of oxygen, methane, and sulfur after the advent of oxygenic photosynthesis, and it was shown that the disappearance of a strong MIF sulfur signature at the beginning of the Proterozoic is better explained by the collapse of atmospheric methane, rather than by a failure of volcanism or the rise of oxygen.
Abstract: We use a 1-D numerical model to study the atmospheric photochemistry of oxygen, methane, and sulfur after the advent of oxygenic photosynthesis. We assume that mass-independent fractionation (MIF) of sulfur isotopes – characteristic of the Archean – was best preserved in sediments when insoluble elemental sulfur (S8) was an important product of atmospheric photochemistry. Efficient S8 production requires three things: (i) very low levels of tropospheric O2; (ii) a source of sulfur gases to the atmosphere at least as large as the volcanic SO2 source today; and (iii) a sufficiently high abundance of methane or other reduced gas. All three requirements must be met. We suggest that the disappearance of a strong MIF sulfur signature at the beginning of the Proterozoic is better explained by the collapse of atmospheric methane, rather than by a failure of volcanism or the rise of oxygen. The photochemical models are consistent in demanding that methane decline before O2 can rise (although they are silent as to how quickly), and the collapse of a methane greenhouse effect is consistent with the onset of major ice ages immediately following the disappearance of MIF sulfur. We attribute the decline of methane to the growth of the oceanic sulfate pool as indicated by the widening envelope of mass-dependent sulfur fractionation through the Archean. We find that a given level of biological forcing can support either oxic or anoxic atmospheres, and that the transition between the anoxic state and the oxic state is inhibited by high levels of atmospheric methane. Transition from an oxygen-poor to an oxygen-rich atmosphere occurs most easily when methane levels are low, which suggests that the collapse of methane not only caused the end of MIF S and major ice ages, but it may also have enabled the rise of O2. In this story the early Proterozoic ice ages were ended by the establishment of a stable oxic atmosphere, which protected a renewed methane greenhouse with an ozone shield.
281 citations
TL;DR: In this article, the authors used an atmospheric inversion approach to estimate methane surface emissions for different methane regional sources between 1996 and 2001, using data from 13 high-frequency and 79 low-frequency CH4 observing sites.
Abstract: [1] Using an atmospheric inversion approach, we estimate methane surface emissions for different methane regional sources between 1996 and 2001. Data from 13 high-frequency and 79 low-frequency CH4 observing sites have been averaged into monthly mean values with associated errors arising from instrumental precision, mismatch error, and sampling frequency. Simulated methane mole fractions are generated using the 3-D global chemical transport model (MATCH), driven by NCEP analyzed observed meteorology (T62 resolution), which accounts for the impact of synoptic and interannually varying transport on methane observations. We adapted the Kalman filter to optimally estimate methane flux magnitudes and uncertainties from seven seasonally varying (monthly varying flux) and two aseasonal sources (constant flux). We further tested the sensitivity of the inversion to different observing sites, filtered versus unfiltered observations, different model sampling strategies, and alternative emitting regions. Over the 1996–2001 period the inversion reduces energy emissions and increases rice and biomass burning emissions relative to the a priori emissions. The global seasonal emission peak is shifted from August to July because of increased rice and wetland emissions from southeast Asia. The inversion also attributes the large 1998 increase in atmospheric CH4 to global wetland emissions. The current CH4 observational network can significantly constrain northern emitting regions but not tropical emitting regions. Better estimates of global OH fluctuations are also necessary to fully describe interannual methane observations. This is evident in the inability of the optimized emissions to fully reproduce the observations at Samoa.
254 citations
TL;DR: In this paper, the GENESIS climate model underpredicts tropical Atlantic temperatures inferred from ODP Leg 207 foraminiferal d 18 O and Mg/Ca when we specify approximate CO2 concentrations estimated from the biomarker isotopes in the same samples.
Abstract: foraminiferal d 18 O and Mg/Ca suggests that the ratio of magnesium to calcium in the Turonian-Coniacian ocean may have been lower than in the Albian-Cenomanian ocean, perhaps coincident with an ocean 87 Sr/ 86 Sr minimum. The carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples. The d 13 C values of phytane, combined with foraminiferal d 13 C and inferred temperatures, were used to estimate atmospheric carbon dioxide concentrations through this interval. Estimates of atmospheric CO2 concentrations range between 600 and 2400 ppmv. Within the uncertainty in the various proxies, there is only a weak overall correspondence between higher (lower) tropical temperatures and more (less) atmospheric CO2. The GENESIS climate model underpredicts tropical Atlantic temperatures inferred from ODP Leg 207 foraminiferal d 18 O and Mg/Ca when we specify approximate CO2 concentrations estimated from the biomarker isotopes in the same samples. Possible errors in the temperature and CO2 estimates and possible deficiencies in the model are discussed. The potential for and effects of substantially higher atmospheric methane during Cretaceous anoxic events, perhaps derived from high fluxes from the oxygen minimum zone, are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric CH4 concentrations. With 50 ppm CH4, GENESIS sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when CO2 concentrations specified to the model are near those inferred using the phytane d 13 C proxy. However, atmospheric CO2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures.
232 citations
TL;DR: In this paper, the vertical column density of methane is converted to column averaged mixing ratio by using carbon dioxide retrievals as proxy for the probed atmospheric column, and the global measurements with global coverage over continents are compared with model results from the chemistry-transport model TM4.
Abstract: The UV/Vis/near infrared spectrometer SCIAMACHY on board the European ENVISAT satellite enables total column retrieval of atmospheric methane with high sensitivity to the lower troposphere. The vertical column density of methane is converted to column averaged mixing ratio by using carbon dioxide retrievals as proxy for the probed atmospheric column. For this purpose, we apply concurrent total column measurements of CO_2 in combination with modeled column-averaged CO_2 mixing ratios. Possible systematic errors are discussed in detail while the precision error is 1.8% on average. This paper focuses on methane retrievals from January 2003 through December 2004. The measurements with global coverage over continents are compared with model results from the chemistry–transport model TM4. In the retrievals, the north-south gradient as well as regions with enhanced methane levels can be clearly identified. The highest abundances are found in the Red Basin of China, followed by northern South America, the Gangetic plains of India and central parts of Africa. Especially the abundances in northern South America and the Red Basin are generally higher than modeled. Further, we present the seasonal variations within the investigated time period. Peak values in Asia due to rice emissions are observed from August through October. We expand earlier investigations that suggest underestimated emissions in the tropics. It is shown that these underestimations show a seasonal behavior that peaks from August through December. The global measurements may be used for inverse modeling and are thus an important step towards better quantification of the methane budget.
209 citations
TL;DR: The activity of the methanogenic flora of healthy adults remained remarkably stable over the past 35 years despite widespread antibiotic use and dietary changes.
162 citations
TL;DR: In this paper, the authors simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA.
Abstract: Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH4 uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH4 uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm K3 may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH4 consumption, with maximum rate at 0.23 g H2 Og K1 soil. On average, about 74% of atmospheric CH4 was consumed in the top 4‐5 cm of the mineral soil. By contrast, little or no CH4 consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH4 uptake by about 700‐1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH4 uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH4 oxidation and that summer droughts may increase the soil CH4 sink of temperate forest soils. q 2005 Elsevier Ltd. All rights reserved.
TL;DR: In this article, a series of linked biosphere-atmosphere chemistry-climate modeling experiments were conducted to simulate the methane and BVOC emissions from the terrestrial biosphere from the Last Glacial Maximum (LGM) to the present.
Abstract: Recent analyses of ice core methane concentrations suggested that methane emissions from wetlands were the primary driver for prehistoric changes in atmospheric methane. However, these interpretations conflict as to the location of wetlands, magnitude of emissions, and the environmental controls on methane oxidation. The flux of other reactive trace gases to the atmosphere also controls apparent atmospheric methane concentrations because these compounds compete for the hydroxyl radical (OH), which is the primary atmospheric sink for methane. In a series of linked biosphere-atmosphere chemistry-climate modeling experiments, we simulate the methane and biogenic volatile organic compound emissions from the terrestrial biosphere from the Last Glacial Maximum (LGM) to the present. Using a state-of-the-art chemistry-climate model, we simulate the atmospheric concentrations of methane, OH, and other reactive trace gas species. Over the past 21,000 years, methane emissions from wetlands increased slightly to the end of the Pleistocene but then decreased again, reaching levels at the preindustrial Holocene that were similar to the LGM. Global wetland area decreased by 14% from LGM to the preindustrial time. Emissions of biogenic volatile organic compounds (BVOCs), however, nearly doubled over the same period of time. Atmospheric OH burdens and methane concentrations were affected by this major change in BVOC emissions, with methane lifetimes increasing by more than 2 years from LGM to the present. We simulate a change in methane concentration of ∼385 ppb, accounting for 88% of the ∼440 ppb increase in methane concentrations observed in ice cores. Thus glacial-interglacial changes in atmospheric methane concentrations would have been modulated by BVOC emissions. In addition, the increase in atmospheric methane concentrations since the mid-Holocene is partly caused in our results by the increases in anthropogenic methane emissions over this period. While the interplay between BVOC and wetland methane emissions since the LGM cannot explain the entire record of ice core methane concentrations, consideration of BVOC source dynamics is central to understanding ice core methane. Rapid changes in atmospheric methane concentrations, also observed in ice cores, require further study.
TL;DR: In this article, a blowout from a shallow (22 m) hydrocarbon seep was determined from atmospheric plume measurements, which indicated a 1.1% gas loss to dissolution compared to ∼ 10% loss for a typical low-flux bubble plume.
Abstract: The release of methane sequestered within deep-sea methane hydrates is postulated as a mechanism for abrupt climate change; however, whether emitted seabed methane reaches the atmosphere is debatable. We observed methane emissions for a blowout from a shallow (22 m) hydrocarbon seep. The emission from the blowout was determined from atmospheric plume measurements. Simulations suggest a 1.1% gas loss to dissolution compared to ∼ 10% loss for a typical low-flux bubble plume. Transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge. This mechanism could allow methane suddenly released from deeper (>250 m) waters to contribute significantly to atmospheric methane budgets. Copyright 2006 by the American Geophysical Union.
TL;DR: Type II methanotrophs are most likely responsible for atmospheric methane uptake in these soils, while type I methanolipid fatty acids become active when methane is produced in the soil.
Abstract: Methanotrophic communities were studied in several periodically water-saturated gleyic soils. When sampled, each soil had an oxic upper layer and consumed methane from the atmosphere (at 1.75 ppmv). In most gleyic soils the K(m(app)) values for methane were between 70 and 800 ppmv. These are higher than most values observed in dry upland soils, but lower than those measured in wetlands. Based on cultivation-independent retrieval of the pmoA-gene and quantification of partial pmoA gene sequences, type II (Alphaproteobacteria) methanotrophs of the genus Methylocystis spp. were abundant (> 10(7) pmoA target molecules per gram of dry soil). Type I (Gammaproteobacteria) methanotrophs related to the genera Methylobacter and Methylocaldum/Methylococcus were detected in some soils. Six pmoA sequence types not closely related to sequences from cultivated methanotrophs were detected as well, indicating that diverse uncultivated methanotrophs were present. Three Gleysols were incubated under different mixing ratios of (13)C-labelled methane to examine (13)C incorporation into phospholipid fatty acids (PLFAs). Phospholipid fatty acids typical of type II methanotrophs, 16:0 and 18:1omega7c, were labelled with (13)C in all soils after incubation under an atmosphere containing 30 ppmv of methane. Incubation under 500 ppmv of methane resulted in labelling of additional PLFAs besides 16:0 and 18:1omega7c, suggesting that the composition of the active methanotrophic community changed in response to increased methane supply. In two soils, 16:1 PLFAs typical of type I methanotrophs were strongly labelled after incubation under the high methane mixing ratio only. Type II methanotrophs are most likely responsible for atmospheric methane uptake in these soils, while type I methanotrophs become active when methane is produced in the soil.
TL;DR: In this paper, the authors report soil methane (CH 4 ) and nitrous oxide (N 2 O) fluxes of adjacent pure and mixed stands of beech and spruce at Soiling, Germany.
Abstract: Tree species can affect the sink and source strength of soils for atmospheric methane and nitrous oxide. Here we report soil methane (CH 4 ) and nitrous oxide (N 2 O) fluxes of adjacent pure and mixed stands of beech and spruce at Soiling, Germany. Mean CH 4 uptake rates ranged between 18 and 48 μg C m -2 hour -1 during 2.5 years and were about twice as great in both mixed and the pure beech stand as in the pure spruce stand. CH 4 uptake was negatively correlated with the dry mass of the O horizon, suggesting that this diminishes the transport of atmospheric CH 4 into the mineral soil. Mean N 2 O emission was rather small, ranging between 6 and 16 μg N m -2 hour -1 in all stands. Forest type had a significant effect on N 2 O emission only in one mixed stand during the growing season. We removed the O horizon in additional plots to study its effect on gas fluxes over 1.5 years, but N 2 O emissions were not altered by this treatment. Surprisingly, CH 4 uptake decreased in both mixed and the pure beech stands following the removal of the O horizon. The decrease in CH 4 uptake coincided with an increase in the soil moisture content of the mineral soil. Hence, O horizons may maintain the gas diffusivity within the mineral soil by storing water which cannot penetrate into the mineral soil after rainfall. Our results indicate that conversion of beech forests to beech-spruce and pure spruce forests could decrease soil CH 4 uptake, while the long-term effect on N 2 O emissions is expected to be rather small.
TL;DR: Using free-air CO2 enrichment (FACE) techniques, the authors carried out a 3-year monofactorial experiment in temperate paddy rice fields of Japan (1998-2000) and investigated the methane (CH4) emissions in response to an elevated atmospheric CO2 concentration (200 +/- 40 mmol mol(-1) higher than that in the ambient atmosphere).
Abstract: Using the free-air CO2 enrichment (FACE) techniques, we carried out a 3-year monofactorial experiment in temperate paddy rice fields of Japan (1998-2000) and a 3-year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001-2003), to investigate the methane (CH4) emissions in response to an elevated atmospheric CO2 concentration (200 +/- 40 mmol mol(-1) higher than that in the ambient atmosphere). No significant effect of the elevated CO2 upon seasonal accumulative CH4 emissions was observed in the first rice season, but significant stimulatory effects (CH4 increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO2 upon seasonal accumulative CH4 emissions were negatively correlated with the addition rates of decomposable organic carbon (P < 0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (P < 0.05) or the whole year (P < 0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO2 and CH4 emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO2 enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO2 enrichment upon methane emissions from nitrogen-poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen-rich paddy rice ecosystems.
TL;DR: Constant δ13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario.
Abstract: We report atmospheric methane carbon isotope ratios (δ13CH4) from the Western Greenland ice margin spanning the Younger Dryas–to–Preboreal (YD-PB) transition. Over the recorded ∼800 years, δ13CH4 was around –46 per mil (‰); that is, ∼1‰ higher than in the modern atmosphere and ∼5.5‰ higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant δ13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.
TL;DR: In this paper, the authors developed a methodology of coupling a CH 4 emission model with regional databases by which CH 4 emissions from Chinese rice paddies can then be estimated, which is of great potential in terms of upscaling as it provided a realistic estimate of the observed results from various soils, climates and agricultural practices.
TL;DR: In this article, the authors use existing bottom-up constructions of the source history, including "EDGAR"-based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century.
Abstract: Little is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition ( 12 CH 4 , 13 CH 4 and 14 CH 4 ) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including "EDGAR"-based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both 12 CH 4 and 13 CH 4 budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for 14 CH 4 exposes uncertainties in inferring the fossil-methane source from atmospheric 14 CH 4 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb 14 C" (originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of 14 CH 4 from nuclear-power facilities is influential but poorly quantified. Atmospheric 14 CH 4 determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source.
TL;DR: The MOZART-2 global tropospheric chemistry model qualitatively captures the observed CH4 trend (increasing in the early 1990s and then leveling off) with constant emissions as mentioned in this paper.
Abstract: [1] Over the past century, atmospheric methane (CH4) rose dramatically before leveling off in the late 1990s. The processes controlling this trend are poorly understood, limiting confidence in projections of future CH4. The MOZART-2 global tropospheric chemistry model qualitatively captures the observed CH4 trend (increasing in the early 1990s and then leveling off) with constant emissions. From 1991–1995 to 2000–2004, the CH4 lifetime versus tropospheric OH decreases by 1.6%, reflecting increases in OH and temperature. The rise in OH stems from an increase in lightning NOx as parameterized in the model. A simulation including annually varying anthropogenic and wetland CH4 emissions, as well as the changes in meteorology, best reproduces the observed CH4 distribution, trend, and seasonal cycles. Projections of future CH4 abundances should consider climate-driven changes in CH4 sources and sinks.
TL;DR: In this paper, a top-down approach determined that global plant emissions must be much lower than proposed by Keppler et al. during the last 2000 years and are likely to lie in the range 0 −46 Tg yr −1 and 0 −176 Tg yr −1 during the pre-industrial and modern eras, respectively.
Abstract: Recently Keppler et al. (2006) discovered a surprising new source of methane – terrestrial plants under aerobic conditions, with an estimated global production of 62–236 Tg yr −1 by an unknown mechanism. This is ~10–40% of the annual total of methane entering the modern atmosphere and ~30–100% of annual methane entering the pre-industrial (0 to 1700 AD) atmosphere. Here we test this reported global production of methane from plants against ice core records of atmospheric methane concentration (CH 4 ) and stable carbon isotope ratios (δ 13 CH 4 ) over the last 2000 years. Our top-down approach determines that global plant emissions must be much lower than proposed by Keppler et al. (2006) during the last 2000 years and are likely to lie in the range 0–46 Tg yr −1 and 0–176 Tg yr −1 during the pre-industrial and modern eras, respectively.
TL;DR: In this article, the authors show that methane may be released by the destabilization of methane clathrate hydrates, triggered by the interglacial climate change starting 0.4 Ma.
Abstract: The presence of methane has been recently detected in the martian atmosphere, suggesting a contemporary source such as volcanism or microbial activity. Here we show that methane may be released by the destabilization of methane clathrate hydrates, triggered by the interglacial climate change starting 0.4 Ma. Clathrate hydrates are nonstoichiometric crystalline compounds in which a water ice lattice forms cages that contain apolar gas molecules, such as methane [CH4· n H2O] and carbon dioxide [CO2· n H2O]. The loss of shallow ground ice eliminates confining pressure, initiating the destabilization of clathrate hydrates and the release of methane to the atmosphere. This alternative process does not restrict the methane's age to 430 yr (maximum residence time of methane gas in martian atmosphere), because clathrate hydrates can preserve (encage) methane of ancient origin for long time periods.
TL;DR: In this paper, the mean methane oxidation rate was found to be 1.90mg CH4m−m−2−day−1 and the main variables controlling this rate were soil water content and inorganic nitrogen in the soils.
Abstract: Methane is one of the strongest of the greenhouse gases, being 30-fold more radiatively active than carbon dioxide on a molar basis. In addition, its atmospheric concentrations have increased by 1% per year since the Industrial Revolution. As such, the dynamics of methane is of great importance for the prediction of global climatic changes caused by increasing concentrations of greenhouse gases in the atmosphere. One of the most important biological sinks for methane is forest soils, where methanotrophic bacteria oxidize methane to carbon dioxide. Based on data mined from a review of the literature, we determined that the mean methane oxidation rate was 1.90 mg CH4 m−2 day−1 and that the main variables controlling this rate were soil water content and inorganic nitrogen in the soils. In contrast, the effects of temperature and pH are minimal. In addition to reviewing the literature, we monitored methane oxidation rates in a temperate forest soil in Korea on a monthly basis for a year, using a static chamber method. The mean oxidation rate was 1.96 mg CH4 m−2 day−1 and was positively correlated with nitrate concentration in the soil.
TL;DR: In this article, the authors demonstrate that the methane destruction rate increases by a factor of 1012 as the dust storm E-fields, E, increase from 5 to 25 kV/m, resulting in an apparent decrease in methane stability from ∼ 1010 sec to a value of ∼1000 seconds.
Abstract: [1] Recent laboratory tests, analog studies and numerical simulations all suggest that Martian dust devils and larger dusty convective storms generate and maintain large-scale electric fields. Such expected E-fields will have the capability to create significant electron drift motion in the collisional gas and to form an extended high energy (u ≫ kT) electron tail in the distribution. We demonstrate herein that these energetic electrons are capable of dissociating any trace CH4 in the ambient atmosphere thereby acting as an atmospheric sink of this important gas. We demonstrate that the methane destruction rate increases by a factor of 1012 as the dust storm E-fields, E, increase from 5 to 25 kV/m, resulting in an apparent decrease in methane stability from ∼ 1010 sec to a value of ∼1000 seconds. While destruction in dust storms is severe, the overall methane lifetime is expected to decrease only moderately due to recycling of products, heterogeneous effects from localized sinks, etc. We show further evidence that the electrical activity anticipated in Martian dust storms creates a new harsh electro-chemical environment.
TL;DR: In this paper, a study was conducted to investigate methanotrophic activity under practical storage conditions Surface crusts from slurry storages at two pig farms and four dairy farms were sampled in late autumn, and mixed samples were used to determine changes in CH4, O2 and CO2 during incubation.
Abstract: Storages with liquid manure (slurry) may develop a surface crust of particulate organic matter, or an artificial crust can be established Slurry storages are net sources of atmospheric methane (CH4), but a potential for bacterial oxidation of CH4 in surface crusts was recently suggested in a study of experimental storages The present study was conducted to investigate methanotrophic activity under practical storage conditions Surface crusts from slurry storages at two pig farms and four dairy farms were sampled in late autumn Mixed samples (0–4 cm depth) were used to determine changes in CH4, O2 and CO2 during incubation, while intact subsamples were used to characterize CH4 oxidation as a function of CH4 availability and moisture content Methane oxidation was observed in all materials except for an expanded clay product (Leca) sampled from a pig slurry storage Despite significant variation between replicate subsamples, there was a significant increase in methanotrophic activity when CH4 concentrations increased from 500 to 50,000 ppmv Maximum fluxes ranged from −1 to −45 g CH4 m−2 d−1 Surface crust samples were partly dried and then re-wetted in four steps to the original moisture content, each time followed by determination of CH4 fluxes Only one surface crust material showed a relationship between CH4 fluxes and moisture content that would implicate gas diffusivity in the regulation of CH4 oxidation The occurrence of inducible CH4 oxidation activity in slurry storage surface crusts indicates that there is a potential for stimulating the process by manipulation of gas phase composition above the stored slurry
TL;DR: In this article, the authors examined CH4 dynamics in temperate sedge-fen peatlands to determine controls on the spatial and temporal variability in CH4 fluxes and, therefore, how the biogeochemistry of CH4 in sedge fens may respond to predicted changes in climate.
Abstract: Minerotrophic sedge fens are common in sub-arctic regions and are a significant source of atmospheric methane (CH4), yet they have received less attention than other peatlands, such as boreal ombrotrophic bogs, which are smaller sources of CH4. At the process level, CH4 fluxes in sub-arctic systems are limited primarily by cold temperatures, and thus are sensitive to potential climate change. This study examined CH4 dynamics in a temperate sedge-fen to determine controls on the spatial and temporal variability in CH4 fluxes and, therefore, how the biogeochemistry of CH4 in sedge-fen peatlands may respond to predicted changes in climate. We used flux chambers and laboratory peat incubations over a six to seven-year period (1994–2000) to study fluxes, pools, and potential production of CH4 in a peat-forming wetland in central New York State, USA. Results showed that precipitation (i.e., dry years and depth to water table) exerted an important control on annual and seasonal patterns of CH4 fluxes. Mean summer flux rates ranged from 2258 nmol m−2 s−1 in the wettest year to −934 nmol m−2 s−1 (net consumption) in the driest year. CH4 concentrations in the surface peat were as low as 0.01 μatm and as high as 10 matm in the summer months depending on precipitation patterns. In contrast, CH4 concentrations were consistently two to three times greater in sub-surface than in surface peat, and pools persisted during dry years and were temporally less variable. Fluxes were only weakly associated with potential CH4 production rates, which showed little seasonal variation. In-vitro measurements of potential CH4 production did not sufficiently explain fluxes, suggesting a need for improved in-situ methods for measuring CH4 production. Site differences associated with different dominant vegetation had a significant effect on CH4 cycling in all years except the driest, suggesting sensitivity to vegetation changes. These results indicate that predicting responses of fen peatlands to environmental requires an improved understanding of the underlying microbial processes and mechanisms that control CH4 cycling.
TL;DR: In inverse (top-down) modelling to quantify variations in methane emissions from different sources between 1983 and 2004, Bousquet et al. find that a decrease in the growth rate of atmospheric methane during the 1990s was caused by a decreases in Anthropogenic emissions, but that anthropogenic emissions have increased again since 1999.
Abstract: The Kyoto Protocol aims to reduce emissions of greenhouse gases such as methane. But it seems that the fall in human-induced methane emissions in the 1990s was only transitory, and atmospheric methane might rise again.
01 Sep 2006
TL;DR: In this paper, a numerical model for the Laurentide-Cordilleran ice sheet was used to assess the aerial extent, thickness, and thermal conditions at the base of the ice sheet as a function of time.
Abstract: Ice-age cycles are associated with large fluctuations in the concentration of atmospheric methane These fluctuations are commonly attributed to changes in wetlands, although clathrates have also been proposed as a potential source We examine the possibility that methane clathrates accumulate below continental ice sheets during an ice age The source of methane is due to microbial decomposition of organic material below the ice sheet Methane is stored in clathrate when the pressure and temperature conditions permit thermodynamic stability Deglaciation releases methane from clathrate into the atmosphere We use a numerical model for the Laurentide–Cordilleran ice sheet [Marshall, SJ, Tarasov, L, Clarke, GKC, Peltier, WR, 2000 Glaciological reconstruction of the Laurentide ice sheet: physical processes and modeling challenges, Can J Earth Sci 37, 769–793] to assess the aerial extent, thickness, and the thermal conditions at the base of the ice sheet as a function of time Both low and high inventories of the organic carbon below the ice sheet are considered, based on soil carbon estimates for tundra and for the present potential vegetation We model the spatial distribution of clathrate as the ice sheet grows and quantify the amplitude and timing of methane releases as the ice sheet retreats The predicted fluctuations in atmospheric methane are 80–200 ppbv, which are comparable to fluctuations recorded in ice cores from Greenland and Antarctica However, clathrates cannot explain the entire atmospheric methane record because there is insufficient methane in clathrate to sustain the elevated atmospheric concentration for more than 1 kyr
TL;DR: In this paper, the authors used a weighted non-linear errors-in-variables regression technique to determine the isotope fractionation factor (α) of methane oxidation in landfill cover soils.
TL;DR: In this article, the effects of CH4 and oxygen (O2) concentrations on C isotope fractionation and CH4 oxidation in tropical rain forest soils were investigated using soil incubation experiments.
Abstract: [1] Humid tropical forests have the potential to be significant sources or sinks of atmospheric methane (CH4), a radiatively important trace gas. Methane oxidation can consume a large fraction of the CH4 produced in tropical soils, although controls on this process are poorly understood. Using soil incubation experiments, we investigated the effects of CH4 and oxygen (O2) concentrations on C isotope fractionation and CH4 oxidation in tropical rain forest soils. We also explored the effects of these environmental variables on the isotope fractionation factor for CH4 oxidation (α), which is widely used to evaluate the relative contributions of CH4 production and oxidation to the atmospheric CH4 pool. Methane oxidation was sensitive to CH4 at lower CH4 concentrations (<850 ppmv) and insensitive to O2 concentrations between 3 and 21%. Maximum rates of CH4 oxidation were between 8.2 ± 1.2 and 11.3 ± 1.5 nmol CH4 hour−1 g dry soil−1. Measured values for α were sensitive to both CH4 oxidation rate and CH4 concentration. Alpha was inversely proportional to CH4 oxidation rate (r2 = 0.86, P < 0.001) and positively correlated with CH4 concentration (r2 = 0.52, P < 0.01). A multiple regression model that included CH4 oxidation rate, CH4 concentration, and the interaction of the two terms explained a high proportion of the variability in α (r2 = 0.94, P < 0.0001). These data suggest that it is possible to accurately determine α, allowing for more precise estimates of CH4 oxidation by isotope mass balance.