One of the more dynamic issues regarding the progression of global warming and its ability to induce detrimental effects on society is the role of methane trapped in permafrost on land and methane hydrates in the ocean and their release into the atmosphere as surface and ocean temperatures increase. Methane garners such attention because some are concerned that once a significant and consistent amount of methane starts to discharge from natural sources a runaway effect will begin dramatically increasing the probability of detrimental environmental damage. Unfortunately the estimates surrounding this “tipping point” vary considerably with large uncertainty because no one actually understand how the environment will respond once these methane sources start emitting methane.
While surface permafrost trapped methane is important because the ocean is absorbing most of the initial additional heat created by the combustion of fossil fuels the near-term focus should be placed there. Due to millions of years of methane accumulation1-3 and sea level change during the last glacial maximum it is known that large amounts of methane are trapped in hydrates (a form of methane in a clathrate molecule containing water ice) on or beneath the sea floor; however, the actual amounts in both quantity and stability are unknown (methane estimates range from 700 to 10,000 Pg of C).1,4-6 The stability may be unknown, but more frequent plumbs of methane release, especially around the Eastern Siberian Arctic Shelf (ESAS) have begun to worry scientists.7,8 The excessive greenhouse potential of large scale methane release should be a concern, thus it is important to determine a counter-strategy to reducing the probability that significant “tipping point” amounts of methane are released from these hydrates.
Typical hydrate formation was driven by pressure as melting temperatures for given compounds increase with pressure. Most of the hydrates that formed over time did so at large ocean depths (a few hundred meters below the sea floor) due to this melting temperature change principle.1 The ocean water column also experiences a reduction in temperature with an increase in pressure. Therefore, most methane hydrates are “protected” by a double security blanket: the higher melting point and the cooler deep ocean temperatures that are further buffered by a sediment layer. There are two types of methane hydrates deposits: stratigraphic and structural with a majority of the formations being stratigraphic, which appear to contain less methane than structural.9
The ESAS is especially important in the issue of methane release because its hydrates are located in much shallower water (45-50 meters) than most others because instead of relying on pressure to induce temperature changes for formation, the ESAS, as well as some other parts of the Arctic, simply used existing lower temperatures as a driver for hydrate formation.1,8 Unfortunately now with the Earth influenced by global warming these shallower hydrates have a much higher probability of releasing methane over their deeper counterparts. Also while some climate scientists have shown concern about the land based permafrost in Siberia, the average temperature of the ESAS bottom seawater is 12 – 17 degree C warmer than the average surface temperature over land based permafrost8,10,11 making methane release from the ocean more probable than release from the land.
Of course methane release from the hydrates is only the first step for producing additional atmospheric methane and aggravating global warming. There are additional “safeguards” even after methane bubbles have formed from the hydrates. First, the production of bubbles associated with melting attempts to destabilize the sediment column, but fortunately the depth of sediment packing prevents such a catastrophic occurrence typically limiting rate of release.12 Second, because the sediment column does not collapse it acts as a physical barrier and typically remains cold enough that the methane bubbles migrating through it results in the dispersion of the bubble.1 Third, free flowing sulfate creates a chemical barrier that can oxidize the methane.1 Fourth, methanotrophic bacteria can react with the methane converting the methane to CO2 (clearly not an ideal situation).
Not surprisingly though these “safeguard” are not able to neutralize all of the released methane. The probability of successful migration is largely dependent on bubble volume as the larger bubbles can create a larger pressure differential between both the sediment and the water versus the bubble at the top and bottom of the bubble.1,8,9 Unfortunately this critical element of bubble volume is difficult to measure or model; this is one of the elements that make it difficult to accurately portray methane release.
Despite the lack of good information pertaining to creating accurate models of methane hydrate release, there is little uncertainty that continued warming of the ocean by releasing larger concentrations of CO2 and other greenhouse gases into the atmosphere will result in large amounts of methane release from methane hydrates. One of the trickier elements for this situation is that CO2 mitigation is a long-term solution, but not a short-term solution and methane release may be an all-term problem. The reason for this concern is that while reducing CO2 emissions will eventually result in a cooling atmosphere, oceanic release of heat through convection should proceed at a much slower pace maintaining the threat of methane release for a considerable period of time after CO2 mitigation is completed. Therefore, a strategy for mitigating this release probability beyond mitigation of CO2 emissions must be developed.
CO2 emission reduction is a longer-term strategy even if it rapidly occurs because of the physics of ocean heating and cooling. The surface layer of the ocean is warmed by sunlight penetration typically increasing the ocean surface temperature beyond the above atmosphere leading to heat loss. This rate of heat transfer is determined by the temperature gradient of “cool skin layer”, a thin viscous region of the ocean (0.1 to 1 mm thickness) that is in contact with the atmosphere.13,14 Due to the heat transfer between the atmosphere and the cool skin layer water molecules are forced together in a more organized formation limiting heat transfer to conduction only. When dealing with the thermodynamics of conduction temperature gradients are critical.
Addition of excess greenhouse gases to the atmosphere trap heat and redirect random percentages of the heat back to Earth including the ocean surface. This heat only penetrates the “cool skin layer” warming the top portion of the layer changing the temperature gradient.13,14 The change decreases the gradient between the atmosphere and the top portion of the “cool skin layer” and increases the gradient between the top portion and bottom portion of the “cool skin layer”. Due to these gradient changes heat will travel between the top portion and the bottom portion of the “cool skin layer” reducing the probability that heat is expelled back into the atmosphere. Thus the greenhouse gases have to be eliminated (through technological or natural processes) after mitigation to allow the ocean-atmosphere gradient to normalize for the ocean to start expelling heat into the atmosphere on a consistent basis.
There are two general short-term strategies for reducing the probability of methane release into the atmosphere: prevent the methane hydrate from melting in the first place or prevent the methane from reaching the surface and entering the atmosphere after melting. Despite potential protests from certain parties, the execution of these strategies will entail technological techniques that can be regarded as geoengineering. Two strategies come to mind when attempting to prevent the hydrates from melting: cloud thickening and increasing ocean surface albedo. One important aspect of strategy selection is to focus on locality to limit the costs and increase efficiency of the strategy. Such a consideration handicaps the injection of sulfuric aerosols into the atmosphere to promote cooling because over a significant period of time (multiple years) it is nearly impossible to maintain localization of these aerosols due to wind currents, thus the aforementioned two options become the most attractive.
Fortunately because the late fall, winter and early spring temperatures in the ESAS provide no threat to inducing methane hydrate melting any executed strategy would only need to be administered at most five months per year (early May to early October). The chief advantages of cloud thickening is its easy execution utilizing wind-propelled ships with reactants that are not environmentally detrimental and in very limited testing seems to reduce atmospheric temperatures. The chief disadvantage of cloud thickening is that it is a catalytic agent in that it only thickens existing clouds; it cannot create clouds in clear skies. Therefore, while this catalytic element is not a significant problem when considering cloud thickening for a global solar radiation management strategy, it could be significantly detrimental in its inconsistency for a local strategy.
Increasing ocean surface albedo is a little trickier because there are two chief possibilities: increase ice coverage and increase wake formation. Increasing ice coverage is almost a non-starter because it would involve fighting against decades of additional absorbed oceanic heat that has been reducing Arctic ice coverage including in the ESAS. Therefore, the increased albedo must come from something else. One possibility is increasing wake formation. While the process of creating a wake is theoretically simple, propeller generated vortices pressurize air creating submerged bubbles that rise to the surface,15,16 its overall reliability is questionable. For example to create the necessary speed to produce wake from ships would be counterproductive due to negative elements associated with the fueling components of those ships (relying on wind would not be appropriate).
Therefore, instead of directly applying a wake, one can indirectly create a wake through the production of a surface bubble layer. Bubbles require little energy to create, thus the operational costs for such a system are low.17,18 Bubbles increase ocean surface albedo by increasing the reflective solar flux by providing voids that backscatter light.17 In addition modeling the reflective behavior of bubbles is similar to aerosol water drops because light backscattering is cross-sectional versus mass or volume dependent and the spherical voids in the water column have the same refractive index characteristics. Note that ocean surface albedo varies with angle of solar incidence. Common values are less than 0.05 at 12:00, below 0.1 at 65 degrees solar zenith angle and a maximum albedo, which range from 0.2 to 0.5, at solar zenith angle 84 degrees.19-22
Experiments have already demonstrated the creation of hydrosols from the expansion of air saturated water moving through vortex nozzles, which applies the appropriate level of shearing forces creating a swirling jet of water.18 Also by using an artificial two-phase flow smaller microbubbles can be created to the point of even creating interfacial films through ambient fluid pressure reduction.19 Microbubbles can possibly form these films because they typically last longer than visible whitecap bubbles, which rise and bust in seconds. Note that whitecaps are froth created from breaking waves and can increase ocean albedo up to 0.22 from the common 0.05-0.1 values.23
While whitecaps from waves and wakes do provide increased surface albedo, the effect is ephemeral. Microbubble lifespan can be influenced by local surfactant concentration and fortunately the ESAS has limited surfactant concentration that would influence this lifespan, thus granting more control in the process of creating those bubbles (less outside factors that could unduly influence bubble lifespan). For example, if these bubbles are created through technological means additional elements can be added to the reactant water like a silane surfactant that could add hours to the natural lifespan.24 Bubble lifespan is probably the most important characteristic for this form of ocean albedo increase.
Another method for creating microbubbles comes from biomedical engineering or biology arena where microfluidic procedures and sonication are used to enhance surfactant monolayers to stabilize microbubble formation.25 However, there are two common concerns about this method. First, it is used primarily in a laboratory largely for diagnostic and therapeutic applications, not in the field; therefore there may be questions about transition. Second, while sonication increases stabilizing time, it limits control of microbubble size distribution, which limits the total reflectiveness of the bubbles.26,27
An expanded and newer laboratory technique, electrohydrodynamic atomization, generates droplets of liquids and applies coaxial microbubbling to facilitate control over microbubble size. Unfortunately one concern with this technique is that as mentioned above ideal bubble size is in microns, this technique is currently only able to create single digit millimeter sized bubbles.25 However, the increased size may be offset by the increased stability of the bubble (less overall reflection, but longer residence times). Comparison testing will be required to make the appropriate judgment.
Initially the idea of cloud brightening was dismissed above due to its catalytic ability versus an inherent driving reactant ability, but this dismissal was based on cloud brightening as a standalone application in the ESAS. However, cloud brightening could be a useful secondary component to a microbubble system. Another point of note is that over time microbubble surface application will result in hastened cooling of the ocean, especially the surface, which should increase CO2 retention capacity. Basically increasing ocean albedo should result in a very small localized increase in CO2 absorption increasing ocean acidity.
Most of the above discussion has centered on preventing the hydrates from thawing versus preventing the released methane from reaching the atmosphere. The reason for this focus is quite obvious; preventing thawing is easier than preventing released methane from reaching the atmosphere, especially in the oceanic environment itself. There are two main methane “removal” reactions utilized by nature and neither one is appealing for eliminating ocean born methane.
The first and principle method of elimination involves the reaction of methane with a free hydroxyl radical (OH-) in the troposphere or stratosphere creating water vapor and CH3- radical. This CH3- radical usually later reacts with another hydroxyl radical to form formaldehyde. While this reaction almost exclusively occurs in the upper atmosphere transferring it to the ocean in some form will not improve upon the situation. Methane can also react with natural chlorine gas to produce chloromethane and hydrochloric acid (free radical halogenation), but this is another atmospheric reaction that probably cannot be effectively transferred to an ocean medium.
The chief ocean methane reaction involves metabolization by microorganisms known as methanotrophs (or methanophiles). There are two major types of methanotrophs (ribulose monophosphate users and serine carbon assimilators) divided into numerous additional groups, which use two principle reactions with selectivity governed by the availability of oxygen.28 Note that methanotrophs are also located in soils and landfills and aerobic/anaerobic methanotrophs are of different families.28 Both the aerobic and anaerobic basic reactions are shown below (the reactions have numerous intermediates and their efficiency is largely based on what type of monooxygenases (MMO) enzyme is utilized):28,29
CH4 + 2O2 → 2H2O + CO2 (1)
CH4 + SO4(2-) → HCO3- + HS- + H2O (2)
The aerobic reaction is the principle reaction between the two, but has two drawbacks. First, the consumption of oxygen limits its ability to scale and address large amounts of methane release from melting hydrates due to the creation of oxygen limiting factor regions (a.k.a. dead zones). However, this magnitude of this drawback is limited in the ESAS because of the limited amount of life by scale. Second, the reaction produces CO2 as a product, which may be a net detriment overall because of the lifespan of CO2 and increasing ocean acidity. Unfortunately this drawback is not as limited as the first because ocean mixing will de-localize the increase in acidity. The scaling efficiency of the anaerobic reaction is the chief problem with its use because most of the oceanic available SO4 is located near the ocean floor, which limits its usefulness once the methane release concentration begins to increase significantly. Therefore, it does not appear that either relying on existing methanotrophic or other methane-oxidizing bacteria or attempting to increase their numbers will be an effective strategy for addressing methane hydrate melting.
There is a bit of question whether or not methane in the ESAS is a genuine threat. Air sampling surveys have revealed large variability in methane concentrations versus the standard global background concentration of 1.85 ppm with average increases of 5-10%. Some have also calculated a total methane flux from the ESAS of 10.64 million tons of methane per year.30 However, modeling studies suggest that permafrost lags behind changes in surface temperature, thus current outgassing is tied to long-lasting warming initiated by permafrost submergence approximately 8000 years ago versus recent Arctic warming.31 Such a conclusion is possible, but the rationality seems far-fetched due to the magnitude of the time lag.
Overall the threat of significant methane release in the ESAS is a legitimate one, but not one that demands immediate strategy implementation. While immediate strategy implementation is not required, strategies to address this melting possibility must be studied to ensure a solid and effective plan when the time for implementation comes, which appears to be soon. Currently local production of microbubbles by some form of floating device (a buoy for example) initially appears to be the best strategy for preventing methane release, but as mentioned future study must be conducted to ensure the validity of this promise.
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