Exploring Hydrocarbon Depletion
Research led by the University of Reading indicates that emissions of methane due to human activity have, to date, caused a warming effect which is about one-third of the warming effect due to carbon dioxide emissions – this methane contribution is 25% higher than previous estimates.
The new study, by Maryam Etminan and colleagues, is published in the American Geophysical Union's journal Geophysical Research Letters. The full report is open access and freely available.
The scientists calculated that, while carbon dioxide remains by far the most significant gas driving human-induced climate change, methane, while much less abundant, is even more potent than previously thought. They found that a one tonne emission of methane has the equivalent warming effect of 32 tonnes of carbon dioxide – up from the previous estimate of 28.
Gas hydrate, a frozen, naturally-occurring, and highly-concentrated form of methane,sequesters signiﬁcant carbon in the global system and is stable only over a range of low-temperature and moderate-pressure conditions. Gas hydrate is widespread in the sediments of marine continental margins and permafrost areas, locations where ocean and atmospheric warming may perturb the hydrate stabilityﬁeld and lead to release of the sequestered methane into the overlying sediments and soils. Methane and methane-derived carbon that escape from sediments and soils and reach the atmosphere could exacerbate greenhouse warming. The synergy between warming climate and gas hydrate dissociation feeds a popular perception that global warming could drive catastrophic methane releases from the contemporary gas hydrate reservoir. Appropriate evaluation of the two sides of the climate-methane hydrate synergy requires assessing direct and indirect observational data related to gas hydrate dissociation phenomena and numerical models that track the interaction of gas hydrates/methane with the ocean and/or atmosphere.Methane hydrate is likely undergoing dissociation now on global upper continental slopes and on continental shelves that ring the Arctic Ocean. Many factors—the depth of the gas hydrates in sediments,strong sediment and water column sinks, and the inability of bubbles emitted at the seaﬂoor to deliver methane to the sea-air interface in most cases—mitigate the impact of gas hydrate dissociation on atmospheric greenhouse gas concentrations though. There is no conclusive proof that hydrate-derived methane is reaching the atmosphere now, but more observational data and improved numerical models will better characterize the climate-hydrate synergy in the future
At high latitudes, the key factors contributing to overestimation of the contribution of gas hydrate dissociation to atmospheric CH4concentrations are the assumption that permafrost-associated gas hydrates are more abundant and widely distributed than is probably the case [Ruppel, 2015] and the extrapolation to the entireArctic Ocean of CH4emissions measured in one area. Appealing to gas hydrates as the source for CH4 emissions on high-latitude continental shelves lends a certain exoticism to the results but also feeds catastrophic scenarios. Since there is no proof that gas hydrate dissociation plays a role in shelfal CH4 emissions and several widespread and shallower sources of CH4could drive most releases, greater caution is necessary
the authors of this paper seem like a collection of smug ignoramuses.
1) Clathrates are associated with biological CH4 production. There are exceptions, such as the Yamal peninsula were it is clear that clathrates are formed by gas seeps from gas reservoirs, but they are not very important
Owing to the concentration of organic carbon on continental margins, these locations are where most gas hydrates occur (Figure 3), and gas hydrates are largely absent beneath abyssal plains. The organic carbon is delivered to the sediment both by the rain of phytoplankton to the seaﬂoor in highly productive continental margin waters and by export of terrestrial sediment from the continents. Remineralization of sedimentary organic carbon produces CO2, and most CH4 formed in sediments by microbial processes is the result of reducing this CO2. Microbial CH4, instead of thermogenic CH4 formed at higher temperatures via the same processes responsible for conventional natural gas, is the type most often found in recovered gas hydrates
2) Ocean biotic activity is concentrated in coastal zones due to the availability of essential nutrients and other ecosystem reasons. So clathrates (and carbon) accumulate mostly over coastal seabeds and not inner ocean seabeds
3) The Siberian shelf has an average depth of 50 meters which is not deep enough to preclude CH4 emissions to the atmosphere and there is plenty of evidence of CH4 evading to the surface. The fuss over whether this evasion is becoming catastrophic is not the issue. Other shelf regions will of necessity contain clathrate deposits that will be sufficiently shallow as to outgas into the atmosphere.
Some researchers do infer large amounts of PAGH beneath arctic continental shelves (e.g., 35 Gt C in hydrate beneath the Laptev Sea shelf) [ Shakhova et al., 2010a] ,but several assumptions used in making this estimate may not fully account for the complexity of PAGH systems. Shakhova et al. [2010a] also invoked anomalous shallow gas hydrates beneath the East Siberian Arctic shelf as a potential CH4 source and to explain elevated estimates of CH4 sequestered in gas hydrates. This area was not glaciated at the LGM, as is usually required for shallow gas hydrates to occur, and the origin and existence of possible anomalous gas hydrate deposits remain controversial and require further examination
Despite the expectation that upper continental slopes host the most climate-susceptible gas hydrate populations, widespread upper slope seepage has so far only been recognized on the West Spitsbergen margin [Westbrook et al., 2009], the U.S. Atlantic margin [Skarke et al., 2014], and the northwestern U.S. Paciﬁc margin[Johnson et al., 2015]
Upper continental slope seepage on the other margins has been interpreted in terms of warming of intermediate waters on time scales of years to centuries [Berndt et al., 2014; Biastoch et al., 2011; Brothers et al., 2014; Ruppel,2011a; Stranne et al., 2016b], but so far only the West Spitsbergen margin seepage has been ﬁrmly linked to dissociating gas hydrate [Berndt et al., 2014]
Users browsing this forum: No registered users and 8 guests