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Atmospheric Methane news & views ATMOSPHERIC METHANE Shifting sources Atmospheric concentrations of methane — by changes in atmospheric reactants, a potent greenhouse gas — have been rising namely hydroxyls, whereas the rise steadily since the Industrial Revolution. in methane concentrations following This steady rise was, however, interrupted 2006 is best explained by an increase in by a plateau in methane concentrations biogenic emissions. between 1999 and 2006. The cause of this Unfortunately, carbon isotopes are plateau is the subject of some debate as not able to readily distinguish between the amount of methane in the atmosphere different biogenic sources of methane, is controlled by both the magnitude of for instance ruminant livestock, rice methane emissions and the availability of cultivation, tropical wetlands and Arctic reactants to break that methane down. permafrost. However, satellite methane A full account of the methane budget measurements rule out any substantial over this interval therefore requires an emissions increases in the high latitudes; understanding of how much methane was tropical climate changes over this time being produced by various sources, as well would be expected to increase wetland as the availability of atmospheric chemicals emissions, but not in the areas where that decompose methane. the greatest emissions were observed. In The carbon isotopic composition contrast, increases in livestock inventories of the methane can go some way to and rice cultivation have been reported identify the sources of methane to the for this interval, and methane inventories atmosphere; the largest sources — fossil record increasing methane emissions from fuel burning, biomass burning, and agriculture between 2000 and 2006. Thus, agriculture and wetland emissions — it perhaps isn’t surprising that agriculture each produce methane with a different is having such a large influence on © KUTTIG - TRAVEL / ALAMY STOCK PHOTO / ALAMY STOCK - TRAVEL © KUTTIG isotopic composition. Hinrich Schaefer and atmospheric methane concentrations. colleagues utilize measurements of the with a fossil fuel-like source. However, this It is, however, surprising that fossil isotopic composition of methane contained trend broke down during the plateau period fuels seem to have less of a contribution, in ice cores and archived air samples as and, after 2006, methane concentrations especially given the recent exploitation well as the composition as recorded by a started to rise again while the isotopic of both coal and unconventional gas network of monitoring stations to isolate composition fell. This indicates that the reserves. This finding suggests that the factors that contributed to the methane methane being emitted had a lower amount carbon budgeting — and mitigation plateau (Science 352, 80–84; 2016). of 13C, consistent with methane derived efforts — must closely consider the Prior to 1999, methane concentrations from biogenic sources. Box-modelling expansion of agriculture, particularly if and isotopic composition rose in tandem, efforts confirm that the observed trend the trend towards increased global meat indicating increasing emissions of methane is best explained by a reduction in fossil consumption continues. that had a relatively higher ratio of fuel-related emissions starting between 13C to 12C. The isotopic ratio is consistent 1992 and 1993, with possible contributions ALICIA NEWTON EARTHQUAKES Megathrusts and mountain building Coastlines above subduction zones slowly emerge from the sea despite repeated drowning by great, shallow earthquakes. Analysis of the Chilean coast suggests that moderate-to-large, deeper earthquakes may be responsible for the net uplift. Rich Briggs harles Darwin was awed and delighted hypothesis that the high elevation of could build mighty onshore mountains. Yet, by metres of sudden coastal uplift coastal islands, and by inference the Andes the largest subduction zone earthquakes — that accompanied the Concepción Mountains themselves, had been “effected by great earthquakes of magnitude 8 to 9 or C 1 earthquake in Chile in 1835. In typical successive small uprisings” . Indeed, it seems more — typically drown coastlines. Writing fashion, he posed a startling but clear intuitive that massive offshore earthquakes in Nature Geoscience, Daniel Melnick2 346 NATURE GEOSCIENCE | VOL 9 | MAY 2016 | www.nature.com/naturegeoscience ©2016 Mac millan Publishers Li mited. All ri ghts reserved. .
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  • Minimal Geological Methane Emissions During the Younger Dryas-Preboreal Abrupt Warming Event
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