Atmospheric methane

About: Atmospheric methane is a research topic. Over the lifetime, 2034 publications have been published within this topic receiving 119616 citations.

Soil, plant, and transport influences on methane in a subalpine forest under high ultraviolet irradiance

Abstract: . Recent studies have demonstrated direct methane emission from plant foliage under aerobic conditions, particularly under high ultraviolet (UV) irradiance. We examined the potential importance of this phenomenon in a high-elevation conifer forest using micrometeorological techniques. Vertical profiles of methane and carbon dioxide in forest air were monitored every 2 h for 6 weeks in summer 2007. Day to day variability in above-canopy CH4 was high, with observed values in the range 1790 to 1910 nmol mol−1. High CH4 was correlated with high carbon monoxide and related to wind direction, consistent with pollutant transport from an urban area by a well-studied mountain-plain wind system. Soils were moderately dry during the study. Vertical gradients of CH4 were small but detectable day and night, both near the ground and within the vegetation canopy. Gradients near the ground were consistent with the forest soil being a net CH4 sink. Using scalar similarity with CO2, the magnitude of the summer soil CH4 sink was estimated at ~1.7 mg CH4 m−2 h−1, which is similar to other temperate forest upland soils. The high-elevation forest was naturally exposed to high UV irradiance under clear sky conditions, with observed peak UVB irradiance >2 W m−2. Gradients and means of CO2 within the canopy under daytime conditions showed net uptake of CO2 due to photosynthetic drawdown as expected. No evidence was found for a significant foliar CH4 source in the vegetation canopy, even under high UV conditions. While the possibility of a weak foliar source cannot be excluded given the observed soil sink, overall this subalpine forest was a net sink for atmospheric methane during the growing season.

Observationally derived rise in methane surface forcing mediated by water vapour trends

Abstract: Atmospheric methane (CH4) mixing ratios exhibited a plateau between 1995 and 2006 and have subsequently been increasing. While there are a number of competing explanations for the temporal evolution of this greenhouse gas, these prominent features in the temporal trajectory of atmospheric CH4 are expected to perturb the surface energy balance through radiative forcing, largely due to the infrared radiative absorption features of CH4. However, to date this has been determined strictly through radiative transfer calculations. Here, we present a quantified observation of the time series of clear-sky radiative forcing by CH4 at the surface from 2002 to 2012 at a single site derived from spectroscopic measurements along with line-by-line calculations using ancillary data. There was no significant trend in CH4 forcing between 2002 and 2006, but since then, the trend in forcing was 0.026 ± 0.006 (99.7% CI) W m2 yr−1. The seasonal-cycle amplitude and secular trends in observed forcing are influenced by a corresponding seasonal cycle and trend in atmospheric CH4. However, we find that we must account for the overlapping absorption effects of atmospheric water vapour (H2O) and CH4 to explain the observations fully. Thus, the determination of CH4 radiative forcing requires accurate observations of both the spatiotemporal distribution of CH4 and the vertically resolved trends in H2O. Observations of the radiative forcing from methane at the Earth’s surface are influenced by absorption effects from water vapour, according to spectroscopic measurements and line-by-line calculations.

Synthetic aperture radar (SAR) backscatter response from methane ebullition bubbles trapped by thermokarst lake ice

Abstract: Thermokarst lakes, formed by permafrost thaw, are an important source of atmospheric methane (CH4), a powerful greenhouse gas. Ebullition (bubbling) is often the dominant mode of lake CH4 emission. Because extrapolating spatially limited field measurements of CH4 ebullition induces large uncertainties in regional emission estimates, there is a need for remote sensing based approaches to detect and quantify CH4 ebullition at larger spatial scales in lakes. We examined the relationship between spaceborne synthetic aperture radar (SAR) pixel values of lake ice and biogeochemical field measurements of CH4 ebullition on ten lakes on the northern Seward Peninsula. Among lakes, ebullition ranged from low to high. We found that both the area of ice-bound ebullition-bubble clusters and the bubbling rates that generated the clusters were correlated with L-band single-polarized (HH) SAR (R2 = 0.70, p = 0.002, n = 10) and with the “roughness” component of a Pauli decomposition of L-band quad-polarized SAR (R2 = 0.77,...

Interannual variability of atmospheric methane: possible effects of the el nino--southern oscillation.

Abstract: Nearly continuous measurements at Cape Meares, Oregon, revealed that methane was increasing in the earth's atmosphere and that its concentration varied cyclically with the seasons. After 6 years of measurements, results show that the rate of increase in methane undergoes interannual variations; the most prominent of these coincided with the last major El Nino-Southern Oscillation, when methane concentrations fell far below expected levels. One of the consequences of the interannual variability is that the long-term rate of increase at Cape Meares is now about 16 parts per billion by volume per year, or about 1 percent annually, which is significantly less than that indicated by the earliest calculations.