Unexpected Changes to the Global Methane Budget over the Past 2000 Years (original) (raw)
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Methane and the CH4 related greenhouse effect over the past 400 million years
American Journal of Science, 2009
Natural variations in the tropospheric CH 4 concentration, excluding short bursts from geospheric reservoirs, have been estimated for the past 400 Ma by scaling a wetland CH 4 emission estimate for the middle Pliocene (ca. 3.6-2.6 Ma) by the relative rate of coal basin deposition at any given time in the past. Wetland CH 4 fluxes were used as inputs into the Cambridge 2-D chemistry-transport model to determine the equilibrium atmospheric response. The approach suggests tropospheric CH 4 concentrations reached exceptionally high values of ϳ12,000 ppb during the Permo-Carboniferous, when tropical swamplands were widespread, fell to minimum levels (ϳ100 ppb) during the Triassic 'coal gap', averaged around 2000 to 4000 ppb during the Mesozoic and < 1000 ppb in the Cenozoic. Peak Permo-Carboniferous CH 4 levels could have contributed additional radiative forcing of ϳ3 to 4 W m ؊2 , after accounting for the indirect effects of increased stratospheric H 2 O and tropospheric ozone. Assuming co-variance of N 2 O with CO 2 and CH 4 , we predict a combined additional forcing by these two trace greenhouse gases of up to 4 W m ؊2 during the warm Mesozoic. Although variations in Earth's Phanerozoic CH 4 history probably played a secondary role to atmospheric CO 2 and the evolution of the Sun in driving climate change, the combined effects CH 4 and N 2 O appear to be sufficiently large to warrant incorporation into global modeling studies of past warm climates. introduction Anthropogenic perturbation of the natural methane cycle over the past century (Cicerone and Oremland, 1988; IPCC, 2001, 2007) has contributed to a rise in the concentration of atmospheric CH 4. Ice-cores and geochemical evidence indicate natural variations in the atmospheric CH 4 concentration have also occurred in the geological past. Studies of air bubbles enclosed in glacial ice indicate that atmospheric CH 4 has at least doubled in concentration from the last glacial maximum to preindustrial time (Raynaud and others, 1988; Chappellaz and others, 1990; Spahni and others, 2005). In addition, natural CH 4 emissions from the breakdown of geospheric reservoirs (for example, methane hydrates and thermal decomposition of organic matter) have been called upon to explain relatively sudden and large changes in the carbon isotopic composition of the ancient oceans and atmosphere linked to climatic change events (
Independent variations of CH4 emissions and isotopic composition over the past 160,000 years
Nature Geoscience, 2013
During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition δ 13 C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the δ 13 C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO 2 concentrations. We interpret this to reflect a climatic and CO 2 -related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable δ 13 C values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources-most notably tropical wetlands-must have responded simultaneously to climate changes across these periods.
2014
Abstract. Little is known about how the methane source in-ventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition (12CH4, 13CH4 and 14CH4) from analyses of air trapped in polar ice and firn can enhance this knowl-edge. 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 exam-ine the constraints imposed by isotope information on those budget evolutions. Reconciling both 12CH4 and 13CH4 bud-gets with EDGAR-based source histories requires a combi-nation of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions
Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?
Atmospheric Chemistry and Physics, 2007
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.
Atmospheric methane isotope records covering the Holocene period
Quaternary Science Reviews, 2010
Records of the 13 C/ 12 C (d 13 CH 4) and the D/H (dD CH4) ratio of atmospheric methane were recovered from the GISP II ice core covering the last 11,000 years. All totaled, 76 samples were analyzed for d 13 CH 4 and 65 adjacent samples for dD CH4 between 86 and 1696 m below surface (mbs) providing a temporal resolution that is better than one pair of isotope samples every 200 years. The d 13 CH 4 record exhibits a decreasing trend throughout the Holocene beginning at À46.4& at 11,000 years BP (BP defined as 1950 AD ¼ 11 ka), and decreasing to À48.4& at 1 ka. The 2& d 13 CH 4 drop is likely to be a combination of increased CH 4 emissions from Arctic lake ecosystems and an increase in the ratio of C 3 /C 4 plants in wetlands where CH 4 is emitted. The C 3 /C 4 ratio increase is the result of increasing CO 2 values throughout the Holocene combined with the activation of high NH ecosystems that are predominantly C 3 type. The dD CH4 record over the early-mid Holocene shows a slightly decreasing trend that would be predicted by increased CH 4 emissions from Arctic lakes. Between 4 ka and 1 ka, dD CH4 values increase by w20& while d 13 CH 4 values remain effectively constant. There are at least two plausible explanations for this 20& dD CH4 shift. First, a dramatic shift in CH 4 emissions from higher latitudes to the tropics could account for the observed shift though the lack of a corresponding d 13 CH 4 shift is difficult to reconcile. Secondly, a gradual release of marine clathrates with enriched dD CH4 values explains both the dD CH4 and d 13 CH 4 records over this period.
A 21st-century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4
Science, 2016
Anthropogenic CH4-emissions have almost tripled [CH4] since preindustrial times (1-3). This contributes strongly to anthropogenic climate change through radiative forcing and impacts on atmospheric chemistry, particularly hydroxyl consumption, tropospheric ozone generation and water vapor formation in the stratosphere (4). In a positive feedback to climate change, natural sources like CH4-hydrates, tundra, and permafrost may increase (5). We must therefore understand how the CH4-budget responds to human activities and environmental change. The onset and end of the 1999-2006 [CH4]-plateau (3, 6, 7) (Fig. 1) have been studied with inverse models (top-down) (8-14), as well as process modelling (6, 8, 15-20) and emission estimates (21-23) (bottom-up). These approaches are either not emission-specific or uncertain in scaling and process representation (8). In contrast, the 13 C/ 12 C-ratio in atmospheric CH4 (δ 13 C(Atm); expressed in δ-notation relative to VPDB-standard) is controlled by the relative contributions from source types with distinctive isotope signatures δ 13 C(So) (biogenic ~-60‰, e.g., wetlands, agriculture, waste; thermogenic ~-37‰, e.g., fossil-fuels; pyrogenic ~-22‰, e.g., biomass burning) (3, 24). Large and overlapping ranges for δ 13 C(So) in field studies of the main source types and even individual sources (e.g., wetlands) (24) average out at the global scale so that δ 13 C(So) is suitable to characterize emissions. Sink processes with characteristic isotopic fractionation ε (25) (e.g., hydroxyl (OH) ε =-3.9‰, chlorine in the marine boundary layer (Cl-MBL) ε =-60‰; stratospheric loss ε =-3‰; oxidation by soils ε =-20‰) (26, 27) (table S1) also influence δ 13 C(Atm). Therefore, δ 13 C(Atm)-variations indicate changes in CH4budgets, where pertinent sources are industrial (thermogenic); agricultural, i.e., ruminants, rice (biogenic); and climate dependent, i.e., biomass burning (pyrogenic) and natural wetlands incl. freshwater and permafrost (biogenic). Other sources lack magnitude (termites, wild animals, ocean, hydrates (8)) or known processes (geologic sources) to force abrupt and sustained changes (Supplement). Changes in the dominating OH-sink may affect [CH4] and δ 13 C(Atm)-trends, while substantial changes in other sinks are unlikely or uncertain (Supplement). We reconstructed [CH4] and δ 13 C(Atm) time-series by splicing measurements from ice cores, firn air, archived air (1, 2) and global networks (3) (Fig. 1, fig. S1 and tables S2 and S3) (25). 13 C-enrichment followed by stable δ 13 C(Atm) parallels [CH4]-trends until the end of the 1999-2006 plateau. Afterwards, [CH4] increases while δ 13 C(Atm) becomes more 13 Cdepleted. This suggests that the increasing emissions before and after the plateau differ in δ 13 C(So). We use a one-box model (25, 27) to quantify changes in the CH4-budget. An inversion run derives the history of