Showing papers on "Atmospheric methane published in 1993"

Synchronous changes in atmospheric CH4 and Greenland climate between 40 and 8 kyr BP

Abstract: ICE-CORE reconstructions of atmospheric methane concentrations for the past 220 kyr have revealed large variations associated with different climatic periods1–4. But the phase relationship between climate and methane has been uncertain because of dating uncertainties and the coarse sampling interval of available methane records. Here we present a high-resolution record of atmospheric methane from 40 to 8 kyr ago from the GRIP ice core in Green-land. Our improved resolution and dating allow us to conclude that the large changes in atmospheric methane concentration during the last deglaciation were in phase (±200 years) with the variations in Greenland climate. Our results confirm the previous observation3 that methane increased to Holocene levels when much of the Northern wetlands was still ice-covered, lending support to the suggestion3 that low-latitude wetlands were responsible for the observed changes. We observe oscillations in methane concentration associated with the warm periods (interstadials) that occurred throughout the glacial period5, suggesting that the interstadials were at least hemispheric in their extent. We propose that variations in the hydrological cycle at low latitudes may be responsible for the variations in both methane and Greenland temperature during the interstadials.

Methane Consumption in Temperate and Subarctic Forest Soils: Rates, Vertical Zonation, and Responses to Water and Nitrogen

Abstract: Rates of methane consumption were measured in subarctic coniferous and temperate mixed-hardwood forest soils, using static chambers and intact soil cores. Rates at both sites were generally between 1 and 3 mg of CH4 m-2 day-1 and decreased with increasing soil water contents above 20%. Addition of ammonium (1 μmol g of soil-1) strongly inhibited methane oxidation in the subarctic soils; a lesser inhibition was observed for temperate forest samples. The response to nitrogen additions occurred within a few hours and was probably due to physiological changes in the active methane-consuming populations. Methane consumption in soils from both sites was stratified vertically, with a pronounced subsurface maximum. This maximum was coincident with low levels of both nitrate and ammonium in the mixed-hardwood forest soil.

Atmospheric methane : sources, sinks, and role in global change

Abstract: Methane plays many important roles in the Earth's environment. It is a potent "greenhouse gas" that warms the Earth, controls the oxidizing capacity of the atmosphere (OH) indirectly affecting the cycles and abundances of many atmospheric trace gases, provides water vapour to the stratosphere, scavenges chlorine atoms from the stratosphere, produces ozone, CO, and CO2 in the troposphere, and is an index of life on Earth. By all measures, methane is second only to CO2 in causing future global warming. The book presents a comprehensive account of the current understanding of atmospheric methane, and summarizes more than a decade of intensive research on the global sources, sinks, concentrations and environmental role of methane.

Oxidation of atmospheric methane in soil: Measurements in the field, in soil cores and in soil samples

Abstract: Methane fluxes and vertical profiles of CH4 mixing ratios were measured in different German soils both in situ and in soil cores. Atmospheric CH4 was oxidized in the soil by microorganisms resulting in an average CH4 flux of −1.39±1.5 μmol-CH4 m−2 h−1. Methane deposition showed only a weak positive correlation (r2 = 0.38) with soil temperature but a relatively strong negative correlation (r2 = 0.61) with soil moisture indicating limitation of the CH4 flux by gas transport. Diffusion experiments in soil cores showed that gas transport between atmosphere and soil was faster than microbial CH4 oxidation. However, the diffusion from the gas-filled soil pores to the CH4 oxidizing microorganisms may have been limiting. The main CH4− oxidizing activity was located in a few centimeter thick subsurface soil layer at the top of the Ah horizon, whereas no activity was found in the overlying O horizons and in deep soil below about 20-cm depth. In contrast, the highest CO2 production was found in the topmost O horizon. The effective diffusion coefficient of CH4 in soil was determined using a method based on relaxation experiments with argon. The diffusion coefficient was used to model the CH4 oxidation in soil cores from the vertical profiles of CH4 mixing ratios. The thus calculated CH4 oxidation rates and their localization in the soil profile compared fairly well with those determined directly from incubated soil samples. Fluxes were similar within a factor of 2–4 whether derived from the model, calculated from the measured CH4 oxidation rates of soil samples, or measured directly.

The Global Methane Cycle

Abstract: Methane (CH4) is the most abundant organic species in the Earth's atmo­ sphere. It is a greenhouse gas, as are water vapor (H20), carbon dioxide (C02), nitrous oxide (NzO), ozone (03), and the chlorofluorocarbon com­ pounds. It absorbs long wave radiation emitted from the Earth's surface in the 4 100 /lm atmospheric window and therefore affects atmospheric temperature directly (Lacis et al 1981, Ramanathan 1 988, Hansen et a1 1988). It is chemically reactive, and influences the abundance of ozone in the troposphere and in the stratosphere (Johnston 1984), and it is a major source of stratospheric water (Ehhalt 1979, Pollock et al 1980). Methane thus affects temperature indirectly through its chemical interactions. Systematic measurements of the global tropospheric CH4 mixing ratios since 1978 reveal a steady increase with time by about I % per year, due to anthropogenic activities (Blake & Rowland 1988, Steele et al 1987). The history of CH4 atmospheric mixing ratios has been reconstructed from measurements of air occluded in ice cores for glacial and interglacial times, as well as during the more recent 200 years through the industrial era (Chappellaz et a1 1990; Pearman et al 1986; Etheridge et al 1988; Stauffer et a1 1985; Craig & Chou 1982; Rasmussen & Khalil 1 98 1a, 1981c; Robbins et al 1973). Large natural and anthropogenically influenced variations are observed over different time scales. In the troposphere CH4 is oxidized to CO and ultimately to CO2 and H20. This oxidation reaction sequence is initiated by the hyroxyl (OH) radical. This constitutes the major sink for CH4• The atmospheric lifetime for methane is 8-12 yr. Methane emitted from inundated anoxic environ-

Methane emission and methane oxidation in land-fill cover soil

Abstract: The vertical profiles of methane and oxygen concentrations were measured in the cover soil at four sites in a restored and covered landfill. At sites 2 and 3 within the landfill area methane was detectable even to the soil surface and emission of methane occured at these two sites. Measured methane emission rates varied seasonally and appeared to be most influenced by soil water content. On an annual basis methane emissions at these two sites were 495 and 909 mol methane m−2 y−1, respectively. At sites 1 and 4 methane was detected in the cover soil but was not present in the immediate subsurface layer, and emission of methane did not occur. Oxidation of methane by bacteria within the soil profile at these two sites appeared to prevent methane emission from the surface. A methane-oxidising microflora had been enriched in the soils of all four landfill sites, as shown by counts of methanotrophs and methylotrophs garden soil not subjected to elevated methane. Counts of methanotrophs and methylotrophs were generally higher in those soil strata where methane concentrations were greatest. Methane oxidation rates were maximum at soil depths where gradients of methane and oxygen overlapped, usually 10–30 cm depth. The depth integrated rates of methane oxidation were very high at sites 2 and 3, the sites also where methane was emitted from the soil surface. A maximum oxidation rate of 450 mmol CH4 m−2 d−1 was measured at site 3. The data suggested that the microflora in the soil above landfill adapted to the presence of elevated methane concentrations by selection of a more methanotrophic community which was able to rapidly oxidise methane. Optimisation of microbial oxidation of methane by bacteria in landfill cover soil may provide a cheap management strategy to minimise the emissions of methane to the atmosphere from landfill.

Rice Agriculture: Factors Controlling Emissions

Abstract: Recent atmospheric measurements indicate that concentrations of greenhouse gases are increasing. Atmospheric methane concentration has increased at about 1% annually to 1.7 ppmV during the last decades (Khalil and Rasmussen, 1987). The resulting effect on global temperature is highly significant because the warming efficiency of methane is up to 30 times that of carbon dioxide (Dickinson and Cicerone, 1986). Data from polar ice cores indicate that tropospheric methane concentrations have increased by a factor of 2–3 over the past 200–300 years (Khalil and Rasmussen, 1989). The increase of methane concentrations in the troposphere correlate closely with global population growth and increased rice production (Figure 1), suggesting a strong link to anthropogenic activities. The total annual global emission of methane is estimated to be 420–620 Tg/yr (Khalil and Rasmussen, 1990), 70–80% of which is of biogenic origin (Bouwman, 1990). Methane emissions from wetland rice agriculture have been estimated up to 170 Tg/yr, which account for approximately 26% of the global anthropogenic methane budget. Flooded ricefields are probably the largest agricultural source of methane, followed by ruminant enteric digestion, biomass burning, and animal wastes (summarized by Bouwman, 1990).

Atmospheric methane, record from a Greenland Ice Core over the last 1000 year

Abstract: The atmospheric methane concentration in ancient times can be reconstructed by analysing air entrapped in bubbles of polar ice sheets. We present results from an ice core from Central Greenland (Eurocore) covering the last 1000 years. We observe variations of about 70 ppbv around the mean pre-industrial level, which is confirmed at about 700 ppbv on a global average. According to our data, the beginning of the anthropogenic methane increase can be set between 1750 and 1800. Changes in the oxidizing capacity of the atmosphere may contribute significantly to the pre-industrial methane concentration variations, but changes in methane emissions probably play a dominant role. Since methane release depends on a host of influences it is difficult to specify clearly the reasons for these emission changes. Methane concentrations correlate only partially with proxy-data of climatic factors which influence the wetland release (the main source in pre-industrial times). A good correlation between our data and a population record from China suggests that man may already have influenced the CH4-cycle significantly before industrialisation.

The Beaufort Sea continental shelf as a seasonal source of atmospheric methane

Abstract: Methane concentrations in the Beaufort Sea under the winter ice canopy offshore from northern Alaska are 3 to 28 times greater than they are in late summer when the ice is absent in a similar region offshore from northern Canada where methane is in approximate equilibrium with the atmosphere. These observations suggest that methane concentrates in the water under the sea-ice cover during winter and ventilates rapidly in late summer as the ice melts and retreats. Conditions similar to those on the Beaufort Sea shelf likely exist on the much larger Siberian shelf, making the Arctic Ocean margin a possible seasonal, high-latitude, marine source of about 0.1 Tg yr[sup [minus]1] atmospheric methane. The small addition of methane likely contributes to the late-summer increase in atmospheric methane that is observed each year particularly in the northern hemisphere. 32 refs., 2 figs.

Factors affecting methane production under rice

Abstract: To understand why atmospheric methane is increasing worldwide, accurate estimates are needed of the global input from rice fields. We report greenhouse and laboratory studies over three growing seasons to isolate and control factors that might affect methane emission from rice paddies, including soil texture, added exogenous organic matter (OM), nitrogen and sulfate ion, and water management. Without added OM, methane production was relatively low, increasing during the growing season, and continuing after harvest, provided the soil remained water-logged. If ground rice straw was added to the soil prior to planting, methane production began shortly after flooding, with an initial burst of the gas after 3 to 5 weeks, and then a gradual increase to a second peak later in the season (and after harvest), with rates considerably higher than in treatments without added OM.

Sources of Methane: An Overview

Abstract: The sources of methane are the most complex and critical element in understanding the concentrations of atmospheric methane and their trends. For those who want to reduce methane in the atmosphere or prevent it from increasing, controlling the sources is perhaps the only practical approach. Accordingly,.a significant portion of this book is devoted to estimating the global and regional emission rates. The purpose of this chapter is to introduce the subsequent chapters on individual sources and to lay the foundation for the common elements of determining global emission rates from the many and varied sources of methane.

Atmospheric methane at Cape Meares: Analysis of a high‐resolution data base and its environmental implications

Abstract: Between 1979 and 1992 we took some 120,000 measurements of atmospheric methane at Cape Meares on the Oregon coast. The site is representative of methane concentrations in the northern latitudes (from 30 deg N to 90 deg N). The average concentration during the experiment was 1698 parts per billion by volume (ppbv). Methane concentration increased by 190 ppbv (or 11.9 percent) during the 13-year span of the experiment. The rate of increase was about 20 +/- 4 ppbv/yr in the first 2 yr and 10 +/- 2 ppbv/yr in the last 2 yr of the experiment, suggesting a substantial decline in the trend at northern middle and high latitudes. Prominent seasonal cycles were observed. During the year, the concentration stays more or less constant until May and then starts falling, reaching lowest levels in July and August, then rises rapidly to nearly maximum concentrations in October. Interannual variations with small amplitudes of 2-3 ppbv occur with periods of 1.4 and 6.5 yr.

Methane emissions from northern high latitude wetlands

Abstract: Methane emissions from northern high-latitude wetlands are an important consideration for understanding past, present, and future atmospheric concentrations of this important greenhouse gas. In this chapter we review progress on measuring methane emissions from northern wetlands and, through a model, estimate emission variability in relation to one component of climate variability. Our conclusions are as follows: (1) Methane emissions from northern wetlands are dependent on both soil moisture and temperature. The relative influence of these soil climate parameters is quite variable from one region to another, as is the magnitude of the net emission rate to the atmosphere. Some important wetland regions have not been surveyed for methane emissions (e.g., the Siberian Lowlands); further progress on defining global emissions from northern wetlands awaits field data from these areas. (2) Our preliminary modeling of the sensitivity of methane flux from northern wetlands to variability in temperature indicates that feedbacks from this source are unlikely to significantly influence rates of climate change during the initial stages of a global warming.

Stable Isotopes and Global Budgets

Abstract: Global climate change associated with the increasing atmospheric methane burden is an important societal concern. Today we can monitor with good precision the yearly 1% rise in lower tropospheric methane mixing ratios (e.g., Blake and Rowland, 1988), and we have adequate, basic global coverage of atmospheric methane latitudinal variation. Mesoscopically, we are able to roughly estimate the various source strengths, e.g., from wetlands, agriculture, fossil fuels, but there is considerable uncertainty in the actual magnitudes of the various individual fluxes of methane across the geosphere-biosphere-atmosphere interface. This knowledge deficit includes our understanding of both release and uptake process-groups. Control of methane emissions to the atmosphere requires that we reliably characterize these source-sink relationships.

Methane consumption and carbon dioxide emission in tallgrass prairie: Effects of biomass burning and conversion to agriculture

Abstract: Consumption of atmospheric methane and emission of carbon dioxide by soils were measured on unburned and annually burned tallgrass prairie and on adjacent wheat and sorghum agricultural plots in Kansas. Profiles of CH4 and CO2 concentration with soil depth were also measured. Overall patterns of CH4 consumption by soils varied temporally, with soil depth and land use. Mean CH4 consumption for the 200-day sampling period was −1.02 mg CH4 m−2 d−1 (SE=0.13, n=41) for burned prairie, −0.63 (SE=0.09, n=45) for unburned prairie, −0.85 (SE=0.20, n=36) for wheat, and −0.45 (SE=0.08, n=40) for sorghum. Less than 20 % of the variance in CH4 consumption was explained by soil temperature and/or moisture content. Overall patterns of CO2 emission from prairie and agricultural soils varied temporally, but not among land use. Mean CO2 emission for the 200-day sampling period was 15.7 g CO2 m−2 d−1 (SE=1.8, n=41) for burned prairie, 14.5 (SE=1.3, n=45) for unburned prairie, 13.9 (SE=2.1, n=36) for wheat, and 10.3 (SE=2.1, n=40) for sorghum. More than 70% of the variance in prairie CO2 emission rate was explained by soil temperature and moisture. Crop management practices influenced the timing of CO2 emission from agricultural plots but not the net annual rate of emission. Methane concentrations generally decreased and CO2 concentrations increased with soil depth, and the magnitude of CH4 and CO2 flux generally increased with increased magnitude of the soil gas concentration gradient. Fertilization of agricultural fields had no measured effect on CH4 or CO2 flux or on soil gas concentrations.