Atmospheric methane

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

A reevaluation of the open ocean source of methane to the atmosphere

Abstract: Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from −0.1 to 0.4 μmol m−2 d−1. The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr−1 (0.4 Tg CH4 yr−1), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.

Methane flux from coastal salt marshes

Abstract: It is thought that biological methanogenesis in natural and agricultural wetlands and enteric fermentation in animals are the dominant sources of global tropospheric methane. It is pointed out that the anaerobic soils and sediments, where methanogenesis occurs, predominate in coastal marine wetlands. Coastal marine wetlands are generally believed to be approximately equal in area to freshwater wetlands. For this reason, coastal marine wetlands may be a globally significant source of atmospheric methane. The present investigation is concerned with the results of a study of direct measurements of methane fluxes to the atmosphere from salt marsh soils and of indirect determinations of fluxes from tidal creek waters. In addition, measurements of methane distributions in coastal marine wetland sediments and water are presented. The results of the investigation suggest that marine wetlands provide only a minor contribution to atmospheric methane on a global scale.

Global ocean methane emissions dominated by shallow coastal waters.

Abstract: Oceanic emissions represent a highly uncertain term in the natural atmospheric methane (CH4) budget, due to the sparse sampling of dissolved CH4 in the marine environment. Here we overcome this limitation by training machine-learning models to map the surface distribution of methane disequilibrium (∆CH4). Our approach yields a global diffusive CH4 flux of 2-6TgCH4yr-1 from the ocean to the atmosphere, after propagating uncertainties in ∆CH4 and gas transfer velocity. Combined with constraints on bubble-driven ebullitive fluxes, we place total oceanic CH4 emissions between 6-12TgCH4yr-1, narrowing the range adopted by recent atmospheric budgets (5-25TgCH4yr-1) by a factor of three. The global flux is dominated by shallow near-shore environments, where CH4 released from the seafloor can escape to the atmosphere before oxidation. In the open ocean, our models reveal a significant relationship between ∆CH4 and primary production that is consistent with hypothesized pathways of in situ methane production during organic matter cycling.

Productivity and depth regulate lake contributions to atmospheric methane

Abstract: Despite significant contributions of inland lakes to the global methane cycle, we lack a process-based understanding of what regulates inter-lake variation in methane emissions. Previous comparative work has identified a potential link between lake primary productivity and methane emissions; also, lab-scale experiments suggest that the addition of algal substrate to anoxic sediments rapidly enhances rates of methanogenesis. This existing work indicates that primary productivity could enhance lake contributions to the global methane cycle. However, a more systematic investigation of the links between lake primary production, methanogenesis, and methane emission to the atmosphere is required to quantify the implications of increased cultural eutrophication for methane evasion from lakes. Using paired measurements of methanogenesis and methane emissions on 16 north temperate lakes, we documented a positive relationship between lake productivity and sediment methanogenesis rates. However, increased methanogenesis rates did not result in an increase in diffusive methane emissions. Rather, they generated greater methane storage during summer stratification and enhanced methane emission to the atmosphere via bubbling (ebullition), dependent on site depth. Ebullition most frequently occurred at sites less than 6 m deep and where methanogenesis rates were high. The relationships between lake productivity, methanogenesis, and depth-dependent ebullition suggests it is likely that shallow, productive lakes contribute significantly more methane to the atmosphere than deep, clear lakes and will continue to do so in light of the growing prevalence of lake eutrophication.