The Response of Gas Hydrates to Tectonic Uplift

Pressure reduction following uplift may lead to dissociation of gas hydrates. The dynamics of hydrate dissociation in such settings, however, are poorly understood. We used TOUGH+HYDRATE to investigate the response of gas hydrates to an uplift of 0.009 myr$$^{-1}$$ - 1 over the last 8 kyrs, the approximate end of the postglacial sea-level rise. Geological parameters for the simulations are based on hydrate deposits from the Nankai Trough subduction zone. Our results suggest stabilisation from endothermic cooling, elevated pore pressure, and pore water freshening significantly slows hydrate dissociation such that the hydrate remains in place at its pre-uplift level. A shallower hydrate layer forms from upward-migrating gas when assuming moderate to high permeability (10$$^{-15}$$ - 15 and 10$$^{-13}$$ - 13 m$$^{2}$$ 2 ), while gas remains trapped for low permeability (10$$^{-17}$$ - 17 m$$^{2}$$ 2 ). In the latter case, we predict elevated pore pressure with potential implications for seafloor stability. Our findings suggest that following uplift, hydrates may exist outside the predicted regional gas hydrate stability field for thousands of years.

Using the BFAST Algorithm and Multitemporal AIRS Data to Investigate Variation of Atmospheric Methane Concentration over Zoige Wetland of China

The monitoring of wetland methane (CH4) emission is essential in the context of global CH4 emission and climate change. The remotely sensed multitemporal Atmospheric Infrared Sounder (AIRS) CH4 data and the Breaks for Additive Season and Trend (BFAST) algorithm were used to detect atmospheric CH4 dynamics in the Zoige wetland, China between 2002 and 2018. The overall atmospheric CH4 concentration increased steadily with a rate of 5.7 ± 0.3 ppb/year. After decomposing the time-series of CH4 data using the BFAST algorithm, we found no anomalies in the seasonal and error components. The trend component increased with time, and a total of seven breaks were detected within four cells. Six were well-explained by the air temperature anomalies primarily, but one break was not. The effect of parameter h on decomposition outcomes was studied because it could influence the number of breaks in the trend component. As h increased, the number of breaks decreased. The interplays of the observations of interest, break numbers, and statistical significance should determine the h value.

Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor

Widespread methane release from thawing Arctic gas hydrates is a major concern, yet the processes, sources, and fluxes involved remain unconstrained. We present geophysical data documenting a cluster of kilometer-wide craters and mounds from the Barents Sea floor associated with large-scale methane expulsion. Combined with ice sheet/gas hydrate modeling, our results indicate that during glaciation, natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates. Upon ice sheet retreat, methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters. We propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets.

Enhanced methane emissions from tropical wetlands during the 2011 La Niña

Year-to-year variations in the atmospheric methane (CH₄) growth rate show significant correlation with climatic drivers. The second half of 2010 and the first half of 2011 experienced the strongest La Niña since the early 1980s, when global surface networks started monitoring atmospheric CH₄ mole fractions. We use these surface measurements, retrievals of column-averaged CH₄ mole fractions from GOSAT, new wetland inundation estimates, and atmospheric δ¹³C-CH₄ measurements to estimate the impact of this strong La Niña on the global atmospheric CH₄ budget. By performing atmospheric inversions, we find evidence of an increase in tropical CH₄ emissions of ∼6–9 TgCH₄ yr⁻¹ during this event. Stable isotope data suggest that biogenic sources are the cause of this emission increase. We find a simultaneous expansion of wetland area, driven by the excess precipitation over the Tropical continents during the La Niña. Two process-based wetland models predict increases in wetland area consistent with observationally-constrained values, but substantially smaller per-area CH₄ emissions, highlighting the need for improvements in such models. Overall, tropical wetland emissions during the strong La Niña were at least by 5% larger than the long-term mean.

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape-integrated CH4 budget

Arctic ecosystems are characterized by a wide range of soil moisture conditions and thermal regimes and contribute differently to the net methane (CH₄ ) budget. Yet, it is unclear how climate change will affect the capacity of those systems to act as a net source or sink of CH₄ . Here, we present results of in situ CH₄ flux measurements made during the growing season 2014 on Disko Island (west Greenland) and quantify the contribution of contrasting soil and landscape types to the net CH₄ budget and responses to summer warming. We compared gas flux measurements from a bare soil and a dry heath, at ambient conditions and increased air temperature, using open-top chambers (OTCs). Throughout the growing season, bare soil consumed 0.22 ± 0.03 g CH₄ -C m^-2 (8.1 ± 1.2 g CO₂ -eq m^-2 ) at ambient conditions, while the dry heath consumed 0.10 ± 0.02 g CH₄ -C m^-2 (3.9 ± 0.6 g CO₂ -eq m^-2 ). These uptake rates were subsequently scaled to the entire study area of 0.15 km² , a landscape also consisting of wetlands with a seasonally integrated methane release of 0.10 ± 0.01 g CH₄ -C m^-2 (3.7 ± 1.2 g CO₂ -eq m^-2 ). The result was a net landscape sink of 12.71 kg CH₄ -C (0.48 tonne CO₂ -eq) during the growing season. A nonsignificant trend was noticed in seasonal CH₄ uptake rates with experimental warming, corresponding to a 2% reduction at the bare soil, and 33% increase at the dry heath. This was due to the indirect effect of OTCs on soil moisture, which exerted the main control on CH₄ fluxes. Overall, the net landscape sink of CH₄ tended to increase by 20% with OTCs. Bare and dry tundra ecosystems should be considered in the net CH₄ budget of the Arctic due to their potential role in counterbalancing CH₄ emissions from wetlands - not the least when taking the future climatic scenarios of the Arctic into account.

Sunlight stimulates methane uptake and nitrous oxide emission from the High Arctic tundra

Many environmental factors affecting methane (CH₄) and nitrous oxide (N₂O) fluxes have been investigated during the processes of carbon and nitrogen transformation in the boreal tundra. However, effects of sunlight on CH₄ and N₂O fluxes and their budgets were neglected in the boreal tundra. Here, summertime CH₄ and N₂O fluxes in the presence and total absence of sunlight were investigated at the six tundra sites (DM1-DM6) on Ny-Ålesund in the High Arctic. The mean CH₄ fluxes at the tundra sites ranged from -4.7 to -158.6μg CH₄ m^-2h^-1 in the presence of light, indicating that a large CH₄ sink occurred in the tundra soils. However, enhanced CH₄ emission in total absence of light occurred at all the tundra sites. The mean N₂O fluxes ranged from 7.4 to 14.6μg N₂O m^-2h^-1 in the presence of light, whereas in the absence of light all the tundra sites generally released less N₂O, and even significant N₂O uptake occurred there. Soil temperature, chamber temperature and soil moisture showed no significant correlations with tundra CH₄ and N₂O flux. The presence of sunlight increased tundra CH₄ uptake by 114.2μg CH₄ m^-2h^-1 and N₂O emission by 10.9μg N₂O m^-2h^-1 compared with total absence of light. Overall our results showed that tundra ecosystem switched from CH₄ sink and N₂O emission source in the presence of light to CH₄ emission source and N₂O sink in the absence of light. Therefore sunlight had an important effect on CH₄ and N₂O budgets in the High Arctic tundra. The exclusion of sunlight might overestimate CH₄ budgets, but underestimate N₂O budgets in the Arctic tundra ecosystem.

Global atmospheric methane: budget, changes and dangers

A factor of 2.5 increase in the global abundance of atmospheric methane (CH(4)) since 1750 contributes 0.5 Wm(-2) to total direct radiative forcing by long-lived greenhouse gases (2.77 Wm(-2) in 2009), while its role in atmospheric chemistry adds another approximately 0.2 Wm(-2) of indirect forcing. Since CH(4) has a relatively short lifetime and it is very close to a steady state, reductions in its emissions would quickly benefit climate. Sensible emission mitigation strategies require quantitative understanding of CH(4)'s budget of emissions and sinks. Atmospheric observations of CH(4) abundance and its rate of increase, combined with an estimate of the CH(4) lifetime, constrain total global CH(4) emissions to between 500 and 600 Tg CH(4) yr(-1). While total global emissions are constrained reasonably well, estimates of emissions by source sector vary by up to a factor of 2. Current observation networks are suitable to constrain emissions at large scales (e.g. global) but not at the regional to national scales necessary to verify emission reductions under emissions trading schemes. Improved constraints on the global CH(4) budget and its break down of emissions by source sector and country will come from an enhanced observation network for CH(4) abundance and its isotopic composition (δ(13)C, δD(D=(2)H) and δ(14)C). Isotopic measurements are a valuable tool in distinguishing among various sources that contribute emissions to an air parcel, once fractionation by loss processes is accounted for. Isotopic measurements are especially useful at regional scales where signals are larger. Reducing emissions from many anthropogenic source sectors is cost-effective, but these gains may be cancelled, in part, by increasing emissions related to economic development in many parts of the world. An observation network that can quantitatively assess these changing emissions, both positive and negative, is required, especially in the context of emissions trading schemes.

Greenhouse gases in the Earth system: a palaeoclimate perspective

While the trends in greenhouse gas concentrations in recent decades are clear, their significance is only revealed when viewed in the context of a longer time period. Fortunately, the air bubbles in polar ice cores provide an unusually direct method of determining the concentrations of stable gases over a period of (so far) 800,000 years. Measurements on different cores with varying characteristics, as well as an overlap of ice-core and atmospheric measurements covering the same time period, show that the ice-core record provides a faithful record of changing atmospheric composition. The mixing ratio of CO(2) is now 30 per cent higher than any value observed in the ice-core record, while methane is more than double any observed value; the rate of change also appears extraordinary compared with natural changes. Before the period when anthropogenic changes have dominated, there are very interesting natural changes in concentration, particularly across glacial/interglacial cycles, and these can be used to understand feedbacks in the Earth system. The phasing of changes in temperature and CO(2) across glacial/interglacial transitions is consistent with the idea that CO(2) acts as an important amplifier of climate changes in the natural system. Even larger changes are inferred to have occurred in periods earlier than the ice cores cover, and these events might be used to constrain assessments of the way the Earth could respond to higher than present concentrations of CO(2), and to a large release of carbon: however, more certainty about CO(2) concentrations beyond the time period covered by ice cores is needed before such constraints can be fully realized.

Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic Shelf

Remobilization to the atmosphere of only a small fraction of the methane held in East Siberian Arctic Shelf (ESAS) sediments could trigger abrupt climate warming, yet it is believed that sub-sea permafrost acts as a lid to keep this shallow methane reservoir in place. Here, we show that more than 5000 at-sea observations of dissolved methane demonstrates that greater than 80% of ESAS bottom waters and greater than 50% of surface waters are supersaturated with methane regarding to the atmosphere. The current atmospheric venting flux, which is composed of a diffusive component and a gradual ebullition component, is on par with previous estimates of methane venting from the entire World Ocean. Leakage of methane through shallow ESAS waters needs to be considered in interactions between the biogeosphere and a warming Arctic climate.