Shipborne eddy covariance observations of methane fluxes constrain Arctic sea emissions

Super-Aerophilic Biomimetic Cactus for Underwater Dispersed Microbubble Capture, Self-Transport, Coalescence, and Energy Harvesting

Human ocean activities are inseparable from the supply of energy. The energy contained in the gas-phase components dispersed in seawater is a potential universal energy source for eupelagic or deep-sea equipment. However, the low energy density of bubbles dispersed in water introduces severe challenges to the potential energy harvesting of gas-phase components. Here, a super-aerophilic biomimetic cactus is developed for underwater dispersive microbubble capture and energy harvesting. The bubbles captured by the super-aerophilic biomimetic cactus spines, driven by the surface tension and liquid pressure, undergo automatic transport, coalescence, accumulation, and concentrated release. The formerly unavailable low-density dispersive surface free energy of the bubbles is converted into high-density concentrated gas buoyancy potential energy, thereby providing an energy source for underwater in situ electricity generation. Experiments show a continuous process of microbubble capture by the biomimetic cactus and demonstrate a 22.76-times increase in output power and a 3.56-times enhancement in electrical energy production compared with a conventional bubble energy harvesting device. The output energy density is 3.64 times that of the existing bubble energy generator. This work provides a novel approach for dispersive gas-phase potential energy harvesting in seawater, opening up promising prospects for wide-area in situ energy supply in underwater environments.

Passive Automatic Switch Relying on Laplace Pressure for Efficient Underwater Low-Gas-Flux Bubble Energy Harvesting

The buoyancy potential energy contained in bubbles released by subsea geological and biological activities represents a possible in situ energy source for underwater sensing and detection equipment. However, the low gas flux of the bubble seepages that exist widely on the seabed introduces severe challenges. Herein, a passive automatic switch relying on Laplace pressure is proposed for efficient energy harvesting from low-gas-flux bubbles. This switch has no moving mechanical parts; it uses the Laplace-pressure difference across a curved gas-liquid interface in a biconical channel as an invisible "microvalve". If there is mechanical equilibrium between the Laplace-pressure difference and the liquid-pressure difference, the microvalve will remain closed and prevent the release of bubbles as they continue to accumulate. After the accumulated gas reaches a threshold value, the microvalve will open automatically, and the gas will be released rapidly, relying on the positive feedback of interface mechanics. Using this device, the gas buoyancy potential energy entering the energy harvesting system per unit time can be increased by a factor of more than 30. Compared with a traditional bubble energy harvesting system without a switch, this system achieves a 19.55-fold increase in output power and a 5.16-fold enhancement in electrical energy production. The potential energy of ultralow flow rate bubbles (as low as 3.97 mL/min) is effectively collected. This work provides a new design philosophy for passive automatic-switching control of gas-liquid two-phase fluids, presenting an effective approach for harvesting of buoyancy potential energy from low-gas-flux bubble seepages. This opens a promising avenue for in situ energy supply for subsea scientific observation networks.

Doubly resonant sub-ppt photoacoustic gas detection with eight decades dynamic range

Photoacoustic spectroscopy (PAS) based gas sensors with high sensitivity, wide dynamic range, low cost, and small footprint are desirable in energy, environment, safety, and public health. However, most works have focused on either acoustic resonator to enhance acoustic wave or optical resonator to enhance optical wave. Herein, we develop a gas sensor based on doubly resonant PAS in which the acoustic and optical waves are simultaneously enhanced using combined optical and acoustic resonators in a centimeter-long configuration. Not only the lower detection limit is enhanced by the double standing waves, but also the upper detection limit is expanded due to the short resonators. As an example, we developed a sensor by detecting acetylene (C₂H₂), achieving a noise equivalent absorption of 5.7 × 10^-13 cm^-1 and a dynamic range of eight orders. Compared to the state-of-the-art PAS gas sensors, the developed sensor achieves a record sensitivity and dynamic range.

Ship-Borne Observations of Atmospheric CH4 and δ13C Isotope Signature in Methane over Arctic Seas in Summer and Autumn 2021

Determining the sources of methane emissions in the Arctic remains a complex problem, due to their heterogeneity and diversity. Information on the amount of emissions has significant uncertainties and may differ by an order of magnitude in various literature sources. Measurements made in the immediate vicinity of emission sources help to clarify emissions and reduce these uncertainties. This paper analyzes the data of three expeditions, carried out in the western Arctic seas during Arctic spring, summer, and early autumn in 2021, which obtained continuous data on the concentration of methane and its isotope signature δ13C. CH4 concentrations and δ13C displayed temporal and spatial variations ranging from 1.952 to 2.694 ppm and from −54.7‰ to −40.9‰, respectively. A clear correlation was revealed between the surface methane concentration and the direction of air flow during the measurement period. At the same time, even with advection from areas with a significant anthropogenic burden or from locations of natural gas mining and transportation, we cannot identify particular source of emissions; there is a dilution or mixing of gas from different sources. Our results indicate footprints of methane sources from wetlands, freshwater sources, shelf sediments, and even hydrates.

Model Study of the Effects of Climate Change on the Methane Emissions on the Arctic Shelves

Based on a regional ice-ocean model, we simulated the state of the water masses of the Arctic Ocean to analyze the transport of dissolved methane on the Arctic shelves. From 1970 to 2019, we obtained estimates of methane emissions at the Arctic seas due to the degradation of submarine permafrost and gas release at the ocean–bottom interface. The calculated annual methane flux from the Arctic shelf seas into the atmosphere did not exceed 2 Tg CH4 year−1. We have shown that the East Siberian shelf seas make the main contribution to the total methane emissions of the region. The spatial variability of the methane fluxes into the atmosphere is primarily due to the peculiarities of the water circulation and ice conditions. Only 7% of the dissolved methane originating from sediment enters the atmosphere within the study area. Most of it appears to be transported below the surface and oxidized by microbial activity. We found that increasing periods and areas of ice-free water and decreasing ice concentration have contributed to a steady increase in methane emissions since the middle of the first decade of the current century.

Nitrous oxide and methane in a changing Arctic Ocean

Human activities are changing the Arctic environment at an unprecedented rate resulting in rapid warming, freshening, sea ice retreat and ocean acidification of the Arctic Ocean. Trace gases such as nitrous oxide (N₂O) and methane (CH₄) play important roles in both the atmospheric reactivity and radiative budget of the Arctic and thus have a high potential to influence the region's climate. However, little is known about how these rapid physical and chemical changes will impact the emissions of major climate-relevant trace gases from the Arctic Ocean. The combined consequences of these stressors present a complex combination of environmental changes which might impact on trace gas production and their subsequent release to the Arctic atmosphere. Here we present our current understanding of nitrous oxide and methane cycling in the Arctic Ocean and its relevance for regional and global atmosphere and climate and offer our thoughts on how this might change over coming decades.

Atmospheric methane and nitrous oxide: challenges alongthe path to Net Zero

The causes of methane's renewed rise since 2007, accelerated growth from 2014 and record rise in 2020, concurrent with an isotopic shift to values more depleted in ¹³C, remain poorly understood. This rise is the dominant departure from greenhouse gas scenarios that limit global heating to less than 2°C. Thus a comprehensive understanding of methane sources and sinks, their trends and inter-annual variations are becoming more urgent. Efforts to quantify both sources and sinks and understand latitudinal and seasonal variations will improve our understanding of the methane cycle and its anthropogenic component. Nationally declared emissions inventories under the UN Framework Convention on Climate Change (UNFCCC) and promised contributions to emissions reductions under the UNFCCC Paris Agreement need to be verified independently by top-down observation. Furthermore, indirect effects on natural emissions, such as changes in aquatic ecosystems, also need to be quantified. Nitrous oxide is even more poorly understood. Despite this, options for mitigating methane and nitrous oxide emissions are improving rapidly, both in cutting emissions from gas, oil and coal extraction and use, and also from agricultural and waste sources. Reductions in methane and nitrous oxide emission are arguably among the most attractive immediate options for climate action. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Atmospheric methane removal: a research agenda

Atmospheric methane removal (e.g. in situ methane oxidation to carbon dioxide) may be needed to offset continued methane release and limit the global warming contribution of this potent greenhouse gas. Because mitigating most anthropogenic emissions of methane is uncertain this century, and sudden methane releases from the Arctic or elsewhere cannot be excluded, technologies for methane removal or oxidation may be required. Carbon dioxide removal has an increasingly well-established research agenda and technological foundation. No similar framework exists for methane removal. We believe that a research agenda for negative methane emissions-'removal' or atmospheric methane oxidation-is needed. We outline some considerations for such an agenda here, including a proposed Methane Removal Model Intercomparison Project (MR-MIP). This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Relevant methane emission to the atmosphere from a geological gas manifestation

Quantifying natural geological sources of methane (CH₄) allows to improve the assessment of anthropogenic emissions to the atmosphere from fossil fuel industries. The global CH₄ flux of geological gas is, however, an object of debate. Recent fossil (¹⁴C-free) CH₄ measurements in preindustrial-era ice cores suggest very low global geological emissions (~ 1.6 Tg year⁻¹), implying a larger fossil fuel industry source. This is however in contrast with previously published bottom-up and top-down geo-emission estimates (~ 45 Tg year⁻¹) and even regional-scale emissions of ~ 1–2 Tg year⁻¹. Here we report on significant geological CH₄ emissions from the Lusi hydrothermal system (Indonesia), measured by ground-based and satellite (TROPOMI) techniques. Both techniques indicate a total CH₄ output of ~ 0.1 Tg year⁻¹, equivalent to the minimum value of global geo-emission derived by ice core ¹⁴CH₄ estimates. Our results are consistent with the order of magnitude of the emission factors of large seeps used in global bottom-up estimates, and endorse a substantial contribution from natural Earth’s CH₄ degassing. The preindustrial ice core assessments of geological CH₄ release may be underestimated and require further study. Satellite measurements can help to test geological CH₄ emission factors and explain the gap between the contrasting estimates.