Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure. The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects. In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.

Fundamental principles and applications of natural gas hydrates

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain. © 2013 Macmillan Publishers Limited.

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Three decades of global methane sources and sinks

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.

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Abrupt climate change

Large amounts of methane are produced in marine sediments but are then consumed before contacting aerobic waters or the atmosphere1. Although no organism that can consume methane anaerobically has ever been isolated, biogeochemical evidence indicates that the overall process involves a transfer of electrons from methane to sulphate and is probably mediated by several organisms, including a methanogen (operating in reverse) and a sulphate-reducer (using an unknown intermediate substrate)2. Here we describe studies of sediments related to a decomposing methane hydrate. These provide strong evidence that methane is being consumed by archaebacteria that are phylogenetically distinct from known methanogens. Specifically, lipid biomarkers that are commonly characteristic of archaea are so strongly depleted in carbon-13 that methane must be the carbon source, rather than the metabolic product, for the organisms that have produced them. Parallel gene surveys of small-subunit ribosomal RNA (16S rRNA) indicate the predominance of a new archael group which is peripherally related to the methanogenic orders Methanomicrobiales and Methanosarcinales.

Methane-consuming archaebacteria in marine sediments

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).

For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters.

Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning.

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The global methane budget 2000–2017

Large-scale submarine slides occurred during the Holocene on the continental slope offshore mid-Norway, north and south of the Voring Plateau. The Traenadjupet slide event that affected an area of 14100 km2 is located north of the Voring Plateau. It occurred about 4,300 years B.P., 4000 years after the giant Storegga slide that affected an area of about 112,500 km2. A slope stability evaluation was performed in order to explain why the sliding took place on a very gentle slope (1 degree). This was done first with the deterministic approach using the Limit Equilibrium and the Finite Element methods, for static, pseudo-static and dynamic cases. Then the probabilistic approach was applied using the limit equilibrium method with the 1st and 2nd order reliability methods (FORM and SORM) and the Monte Carlo simulation to include the parameter uncertainties (soils parameters, seismic loading). The Finite Element modelling indicates that the slide triggering impacted preferably the upper 40 meters of the sediment column. The trigger could have been caused by one large earthquake of magnitude larger than M s 5.8 (retrogressive failures) but cyclic loading due to several earthquakes could also explain the slide, affecting the shearing resistance in the NYK contourite drift unit (weak layer) by excess pore pressure generation.

Slope Stability Assessment of the Trænadjupet Slide Area Offshore the Mid-Norwegian Margin

High-resolution 3D seismic exhibits new insights into the middle-late Pleistocene stratigraphic evolution and sedimentary processes of the Bear Island trough mouth fan

Iceberg ploughmarks are common features of glaciated continental margins worldwide. They form when iceberg keels plough through sediments on the ocean floor. They occur in various sizes and shapes, but generally exhibit a linear or curvilinear geometry. The SW Barents Sea is a heavily glaciated margin and its seafloor shows numerous ploughmarks among other glacial landforms (Andreassen et al. 2008). We present seafloor features formed during the late Weichselian glaciation (Solheim et al. 1990) imaged using high-resolution P-Cable 3D seismic data. P-Cable 3D seismic data have a spatial resolution of 6×6 m which is better than many hull-mounted multibeam systems, particularly in deep water. However, the cumulative seismic response of the seafloor is slightly different due to the lower-frequency bandwidth (30–350 Hz) of P-Cable compared to multibeam systems (up to 500 kHz). In addition, amplitude information is readily available and complements grid-based interpretations.

Numerous linear and curvilinear depressions occur on the seafloor near …

Iceberg ploughmarks in the SW Barents Sea imaged using high-resolution P-Cable 3D seismic data

Fluid escape features such as mud volcanoes have been discovered along many active continental margins but rarely along passive continental margins. Seismic and sidescan sonar data are presented from a fluid escape system situated on the passive Norwegian-Barents Sea continental margin (Fig. 1). This fluid system supports bacterial communities (Fig. 2), causes rock alteration via fluids and gas flow (Fig. 2), drives eruptive processes at surface (Fig. 3) and in subsurface sediments (Fig. 4) and contributes to the development of gas hydrates (Fig. 5) (Mienert et al. 2001). To understand the complex dyn amics of such a fluid system we need to understand its history. The Hakon Mosby mud volcano (HMMV) provides an opportunity for long-term observations of the dynamics of a fluid system developed on a passive margin. 
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Fig. 1.
Location map of the Hakon Mosby Mud Volcano (HMMV) on the Norwegian-Barents Sea continental margin


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Fig. 2.
Seafloor photograph of the inner part (caldera) of the Hakon Mosby Mud Volcano showing white bacteria mats and a heatflow probe measurement

Jürgen Mienert

Papers

Slope Stability Assessment of the Trænadjupet Slide Area Offshore the Mid-Norwegian Margin

High-resolution 3D seismic exhibits new insights into the middle-late Pleistocene stratigraphic evolution and sedimentary processes of the Bear Island trough mouth fan

Iceberg ploughmarks in the SW Barents Sea imaged using high-resolution P-Cable 3D seismic data

The Håkon Mosby Mud Volcano

Effect of gas hydrates melting on seafloor slope stability