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2.2 METHANE GENERATION AND CONSUMPTION

Methane is the third-most abundant greenhouse gas in Earth’s atmosphere, after water vapour and carbon diox- ide. Although the concentration of carbon dioxide in the atmosphere is more than 200 times that of methane (Blas- ing 2013), the impact of methane is magnified because it is about 23 times more potent than carbon dioxide as a greenhouse gas. This potency is related to methane’s radia- tive forcing capacity, which refers to the ability of a gas to absorb and trap heat radiating off Earth’s surface (Lacis et al. 1981; Hansen et al. 1988). Methane has a relatively short lifetime in the atmosphere (Boucher et al. 2009), because within about a decade, a combination of sunlight and chem- ical processes cause methane molecules in the atmosphere to break down to water and carbon dioxide, the two most abundant greenhouse gases. As reviewed by the IPCC (2007), the total flux of meth- ane carbon to the atmosphere from all sources is currently around 0.45 GtC/year, or 450 TgC/year (Tg = 10 12 grammes) (See Fig. 2.2). This flux is more than double the pre-indus- trial flux, and about 70 per cent of the emissions are due to human activity (Reeburgh 2007; Colwell and Ussler III 2010). Gas hydrates are estimated to account for only about 1 per cent of the annual methane emissions to the atmos- phere (Forster et al. 2007), but as discussed below, hydrates’ true methane contribution is not precisely known. Since most methane hydrates occur in marine sediments at water depths greater than ~500 metres, a key factor affect- ing how much methane released from dissociating gas hy- drates reaches the atmosphere is the efficiency of transfer- ring methane through the water column. For most bubbles released from the sea floor at water depths greater than 100 meters, the methane will be replaced by other gases during bubble ascent, and the methane will dissolve in the surround- ing waters (McGinnis et al. 2006) and can be consumed by microbes (see Section 2.2.2). Direct transfer of methane to

the atmosphere via bubbles is most relevant in shallow lakes, estuaries, and river deltas, and on continental shelves. Only in the case of Arctic Ocean continental shelves (Shakhova et al. 2010; Biastoch et al. 2011) could these methane release processes at shallow water depths be related to gas hydrates. In special cases, gas hydrate could play a role in enhancing transport of methane to the ocean-atmosphere interface. For example, bubbles released from the sea floor within the gas hydrate stability zone (greater than 500 metres) could form gas hydrate shells in the water column. With such ar- mouring, the bubbles may retain methane to shallower wa- ter depths during bubble ascent. Another enhanced transfer mechanism involves chunks of gas hydrate, which can occa- sionally break off from sea floor gas hydrate mounds (Mac- Donald et al. 2005). Because gas hydrate is buoyant, these chunks may reach the sea surface relatively intact before releasing their methane. Sea floor gas hydrate mounds are not widespread, and this process is not an important factor in transfer of methane to the atmosphere. (For additional discussion of bubble transfer, see Volume 1 Chapter 3). 2.2.1 Marine methane sources: How is methane produced? Most methane that reaches the Earth’s surface is produced by microbial activity in sediments, where a special group of archaea called methanogens produce methane via anaero- bic (without oxygen) decomposition of organic material. Intense heating of organic carbon also produces hydrocar- bon liquids (e.g., petroleum) and gases, including methane. These formation mechanisms are summarized here. Microbial methane production Methanogens generate methane in organic-rich sediments in many settings (e.g., marshes, rice paddies, estuaries, landfills, river deltas, and continental margins). We focus

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