Tuesday, April 22, 2014

Restoring the Arctic

There are numerous environmental concerns surrounding the progression of human-derived global warming. One of the most pressing is the persistent loss of Arctic ice. Due to a vast majority of global warming related heat being absorbed by the ocean all oceanic temperatures have increased, regardless of location, with the Arctic receiving the greatest temperature increase due to its lower base temperature. This increase has been significant enough that the ice extent at the summer minimum, which consistently occurs in September, has resulted in a net loss of 11% per decade since 1979 with a loss of 1.1 meters of mean ice thickness between 1980 and 2000.1,2 This loss of thickness has produced a general shift in the ice type from older multi-year ice to new single year ice resulting in an overall replacement of about 40% of the thick and old multi-year ice with single year ice.3 Coinciding with this empirical evidence various global and regional climate models have predicted that the situation will only get worse in the future.4

The chief purpose of ice in the Arctic, from a global warming standpoint, is to increase ocean albedo due to its reflective surface versus the darker surface of the water itself. When sunlight strikes the transparent/white surface of ice a vast majority of it is reflected back into the atmosphere. When sunlight strikes the dark blue, sometimes black, surface of Arctic water a vast majority of the light and its associated heat content is absorbed by the ocean rather than reflected back into the atmosphere. On a general level this heat absorption is a positive feedback effect where the more heat absorbed the more ice melts leading to even more heat absorbed, etc. Normally the ocean and its system of currents operate as a heat sink to control surface and atmospheric temperatures; however, this new massive heat absorption reduces sink efficiency allowing more heat to remain in the atmosphere increasing the detrimental effects associated with global warming. A secondary effect is that greater amounts of ice melt will increase global sea level rise in the future placing more coastal and even slightly inland cities at risk as well as negatively affecting Arctic wildlife by eliminating “land” surfaces for hunting and habitation.

With these near-future negative environmental events born from a lack of Arctic ice one would reason that it is important to find and execute a methodology that would increase Arctic ice volume and longevity. The most obvious means of increasing Arctic ice would be to eliminate the human derived excess heat, which would restore typical Arctic ocean temperatures seen in the 50s and 60s and even further past. One means of accomplishing this goal is to simply reverse the actions that lead to the heating. While reducing global carbon emissions is an important and critical step in addressing global warming, the realistic timetable for cooling the Arctic through carbon mitigation then reliance on natural processes is still decades if not even over a century away. Based on the rate of melting a more immediate solution will be required.

Recalling the albedo-heat feedback cycle from above, one method to break that cycle would be to increase the albedo of the ocean. Not surprisingly it is nearly impossible to change the natural color of the ocean due to its size and natural mixing, thus changing ocean albedo will require human intervention to change the surface albedo of the Arctic ocean. The easiest method is to mimic nature itself and increase surface ice by enhancing ice formation. Obviously enhancing ice formation will require large amounts of water; fortunately meeting this supply requirement is not a problem for water can be taken from the ocean itself and re-deposited on existing ice.

One of the principle reasons this strategy works is that ice is a quality thermal insulator, which can increase the speed of water freezing. In addition nucleation may also play a role in this ice formation enhancement where ice-forming nucleus tend to trigger freezing of under-cooled water droplets at higher temperatures when in solid contact versus liquid immersion.5-7 While the reason for this enhancement is unknown it is suspected that there are thermodynamically favorable interactions at the air-water interface8,9 leading to contact nucleation as a manifestation of an enhanced surface nucleation rate.5 Basically the liquid environment reduces the uniformity of the air-water interface reducing the efficiency of nucleation. Another important influencing factor may be that nucleation near the surface is greater because of a greater freedom of motion, thus the kinetic rate coefficient is larger at the surface than in the bulk (regardless of that bulk being solid or liquid); this change is important because the change in activation energy between phase changes is exponential.5 Overall the important point to take home is that water sprayed on to the surface of ice has a higher probability of freezing into new ice versus that water remaining adjacent or beneath the ice (all things being equal).

However, increasing ice formation will require managing the temperature increases that have lead to the reduced ice in the first place. There are two chief methods for addressing this temperature question. The first method is to take the water from the ocean and run it through a heat exchanger to remove a sufficient amount of heat to produce an appropriate freezing probability. The chief drawbacks to this method are the energy required to operate the heat exchanger and what to do with the heat absorbed from the water. The heat exchanger needs to be operated with an energy medium that has a very small carbon footprint otherwise the negative aspect of the added CO2 to the atmosphere through this method will more than likely exceed the benefits of adding more Arctic ice. In addition the heat removed from the water must be stored properly because if it is released to the environment it will either enter the atmosphere or the ocean, either result would largely mitigate any advantage to increasing Arctic ice.

The second method involves drawing ocean water not from the surface, but from deeper water near the bottom of the thermocline where the average temperature is much lower. The weakness of the first method is the reliance on the heat exchanger and its energy demands. Unfortunately while the second method eliminates the heat exchanger it cannot eliminate the need for additional energy usage because instead of using a heat exchanger a pump is required. The unknown question is which method will require more energy. Overall unless the first method is significantly more energy efficient, the second method should be favored because there is no excess heat to manage. While the power requirements for the pump and eventual energy consumption are easy to calculate experimentation will have to be conducted to identify the appropriate pumping rate, spray volume, and spray angle.

An important secondary question is what should be done about the salt in the supply water? One possibility would involve removing the salt because salt “decreases” the freezing point of water making it more difficult to form ice and could even result in ice sheet perforation. An alternative strategy would involve retaining the salt, which would strengthen down-welling currents when the ice melts. The best means to determine the best strategy would simply be to test this ice formation methodology and closely observe how the rate of secondary ice formation changes depending on the current temperature and time of year without any salt removal. If the formation rate is not sufficient then the salt will need to be removed.

If water cannot be used due to energy requirements the other major option for creating a change in the ocean surface albedo in an environmental neutral method is cover the water surface with bubbles. One of the chief advantages of this second option is that bubbles require little energy to create, thus the operational costs for such a system are low.10,11 Bubbles increase ocean surface albedo by increasing the reflective solar flux by providing voids that backscatter light.10 In addition modeling the reflective behavior of bubbles is similar to aerosol water drops because light backscattering is cross-sectional versus being mass or volume dependent and the spherical voids in the water column have the same refractive index characteristics.10 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.12-15 Based on this comparison information the principle formula governing brightening is:

DeltaF = DeltaA * Io * So * (1-Cf) * Tu * Td

where DeltaF = change in brightening; DeltaA = change in albedo on water surface; Io = solar irradiance; So = cosine of solar zenith angle; Cf = fraction of cloud cover; Tu = upwelling transmissive; Td = down-welling transmissive;10

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.11 Also by using an artificial two-phase flow smaller microbubbles can be created, which can even result in interfacial films through ambient fluid pressure reduction.12 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.16

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 Arctic has limited natural 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.17 Bubble lifespan is probably the most important characteristic for this form of ocean albedo increase both from an economic and efficiency standpoint. However, while most surfactants and other agents like glycerin are typically not environmentally detrimental, the massive amounts required for increasing bubble longevity may make its use economically and environmentally unsustainable.

Another method for creating microbubbles comes from biomedical engineering where microfluidic procedures and sonication are used to enhance surfactant monolayers to stabilize microbubble formation.18 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, especially for the dramatic increase in production scale that will be required for Arctic use. Second, while sonication increases stabilizing time, it limits control of microbubble size distribution, which could limit the total reflectiveness of the bubbles.19,20

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, but this technique is currently only able to create single digit millimeter sized bubbles.18 However, the increased size may be offset by the increased stability of the bubble (less overall reflection, but longer residence time). Comparison testing will be required to make the appropriate judgment.

The final method for increasing ice formation involves devising a piece of technology that can absorb excess heat from the Arctic Ocean. At first thought such an idea seems unlikely due to the size of the Arctic Ocean and its environmental inputs. However, it may not be as far-fetched as it seems. The key to making such a strategy viable is efficiency and scale within the utilized technology.

Scale is achieved through a design that is small enough that it can be produced at reasonable cost with a reasonable level of speed. Efficiency is typically achieved through producing a device that is self-cycling and thereby producing an autonomous operation. If human involvement is required beyond “pushing the start button” then efficiency is significantly compromised. Tie that efficiency loss in a single unit and multiply it by the units required for scale and the result can be devastating in both the terms of cost and viability.

If the objective is to withdraw heat from the ocean the most important element in the device is what agent will be utilized to accomplish this task. Ironically water is one of the best insulators of heat, which is why it is used for cooling purposes in power plants, thus removing heat could prove difficult. Fortunately there is promising research that supports the idea of incorporating zeolite as the heat absorbent material. Zeolite is a mineral make up of SiO2-, various AlO2 groups and alkali-ions and is capable of absorbing gaseous molecules including water due to its crystalline structure. When zeolite absorbs a gas it retains heat due to the absorption enthalpy.21 In addition because zeolite is commonly produced synthetically for use as molecular sieves and washing detergents it is cheap (50 – 75 cents /kg) and environmentally neutral.21

A good example of how zeolite is used in heat absorption is seen through their use in absorption refrigerators. Absorption refrigerators consist of two connected but independent vessels, the evaporator and absorber. The evaporator vessel acts as a quasi-vacuum containing only the vapor pressure of a liquid, which is usually water. When the valve connecting the two vessels is opened the water vapor moves into the absorption vessel and is absorbed by the zeolite reducing the vapor pressure. The loss of pressure causes a phase change as the water become liquid. Eventually the zeolite becomes saturated ceasing the heat transfer between the zeolite and the water. In the refrigerator model at a later time the zeolite is superheated condensing the absorbed water vapor and returning it to the evaporator vessel.

However, the secondary functionality of the above refrigerator design, zeolite recovery through heating, is not applicable in an oceanic environment. The water and resultant heat must be released from the zeolite so it can be reused, but this release will produce excess heat, which is similar to the problem of using a heat exchanger in the first strategy, there is no good place on the open ocean to store the heat without avoiding environmental release. One strategy to address this issue with a small movable device is when the zeolite becomes “full” the device can return, via a small battery powered motor, to a “mother” ship of sorts where the zeolite heat release process can be conducted. After restoring the zeolite to its rest state the device can return to the Arctic to withdraw more heat. After sufficient time the “mother” ship will be “full” of heat and would return to a land base, most likely Iceland due to its geothermal reserves as an energy source and well, to properly off-load the heat stores. Granted this method will place some limits on overall efficiency due to the trips between the Arctic and heat releasing stop over points, but necessary to manage the heat problem.

In the end the positive feedback associated with the warming-albedo reduction relationship is a legitimate threat to carbon mitigation and remediation strategies as a whole. Therefore, society needs to appreciate the time discrepancies associated with restoring colder temperatures to the Arctic Ocean in effort to preserve Arctic ice, especially during the summer. A technology-based solution will be required. Three possible strategies have been presented above in general detail to attempt to break this warming-albedo reduction relationship. One of the advantages of all of these strategies is that they can be experimentally explored with little overall detriment due to their ephemeral nature. Basically if the results are not similar to what is anticipated the experiments can be stopped with little environmental or economic damage. Overall something needs to be done about increased rate of warming in the Arctic and the dramatically increased rate of ice lost if global carbon mitigation strategies are going to be fully effective at reducing the detrimental effects of global warming.

Citations –

1. Perovich, D, and Richter-Menge, A. “Loss of sea ice in the Arctic.” Annu. Rev. Mar. Sci. 2009. 1:417–441.

2. Rothrock, D, Percival, D, and Wensnahan, M. “The decline in Arctic sea-ice thickness: Separating the spatial, annual, and interannual variability in a quarter century of submarine data.” J. Geophys. Res. 2008. 113:C05003.

3. Kwok, R. “Observational assessment of Arctic Ocean sea ice motion, export, and thickness in CMIP3 climate simulations.” J. Geophys. Res. 2011. 116:C00D05.

4. Bjork, G, Stranne, C, and Borenas, K. “The sensitivity of the Arctic Ocean sea ice thickness and its dependence on the surface albedo parameterization.” Journal of Climate. 2013. 26:1355-1370.

5. Shaw, R, Durant, A, and Mi, Y. “Heterogeneous surface crystallization observed in undercooled water.” Journal of Physical Chemistry B Letters. 2005. 109:9865-9868.

6. Vali, G. In Nucleation and Atmospheric Aerosols; Kulmala, M., Wagner, P., Eds.; Pergamon: New York, 1996.

7. Pruppacher, H, and Klett, J. Microphysics of Clouds and Precipitation, 2nd ed.; Kluwer Academic Pub.: Norwell, MA, 1997. Chapters 7 and 9.

8. Djikaev, Y, et Al. “Thermodynamic conditions for the surface-stimulated crystallization of atmospheric droplets.” J. Phys. Chem. A. 2002. 106:10247. doi:10.1021/jp021044s.

9. Tabazadeh, A, Djikaev, Y, and Reiss, H. “Surface crystallization of supercooled water in clouds.” PNAS. 2002. 99(25):15873-15878.

10. Seitz, F. “On the theory of the bubble chamber.” Physics of Fluids. 1958. 1: 2-10.

11. Seitz, F. “Bright Water: hydrosols, water conservation and climate change.” 2010.

12. Evans, J.R.G, et Al. “Can oceanic foams limit global warming?” Clim. Res. 2010. 42:155-160.

13. Davies, J. “Albedo measurements over sub-arctic surfaces.” McGill Sub-Arctic Res Pap. 1962. 13:61–68.

14. Jin, Z, et Al. “A parameterization of ocean surface albedo.” Geophys Res Letters. 2004. 31:L22301.

15. Payne, R. “Albedo of the sea surface.” J Atmos Sci. 1972. 29:959–970.

16. Moore, K, Voss, K, and Gordon, H. “Spectral reflectance of whitecaps: Their contribution to water-leaving radiance.” J. Geophys. Res. 2000. 105:6493-6499

17. Johnson, B, and Cooke, R. “Generation of Stabilized Microbubbles in Seawater.” Science. 1981. 213:209-211

18. Farook, U, Stride, E, and Edirisinghe, J. “Preparation of suspensions of phospholipid-coated microbubbles by coaxial electrohydrodynamic atomization.” J.R. Soc. Interface. 2009. 6:271-277.

19. Wang, W, Moser, C, and Weatley, M. “Langmuir trough study of surfactant mixtures used in the production of a new ultrasound contrast agent consisting of stabilized microbubbles.” J. Phys. Chem. 1996. 100:13815–13821.

20. Borden, M, et Al. “Surface phase behaviour and microstructure of lipid/PEG emulsifier monolayer-coated microbubbles.” Colloids Surf. B: Biointerfaces. 2004. 35:209–223.

21. Kreussler, S, and Bolz, D. “Experiments on solar adsorption refrigeration using zeolite and water.”

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