Cloud reflectivity modification

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Cloud reflectivity modification is a proposed process of altering clouds to make them more or less reflective, in order to change their effect on climate.

There are two hypothesised forms: cirrus stripping and marine cloud brightening. High, cold cirrus clouds tend to allow sunlight through, yet reflect infra-red from the Earth's surface. Therefore, thinning or removing these clouds reduces their reflectivity to infra-red and results in a net cooling effect on the Earth's climate.

Conversely, low cloud tends to be warm and highly reflective to sunlight, especially when the cloud is white due to the presence of smaller droplets (because of the Twomey effect). Therefore, modifying these clouds to make them more reflective cools the climate. This proposed technique is known as 'marine cloud brightening' or 'cloud whitening' on low cloud.

By modifying the reflectivity of clouds, the albedo of the Earth would be altered. The intention is that this technique, in combination with greenhouse gas emissions reduction (and possibly other climate engineering techniques) will be sufficient to control global warming. The effect is expected to be fully reversible, as the cloud condensation nuclei particles precipitate naturally. However, like any planetary-scale project dealing with the complex climate system, there is a non-trivial risk of unintended consequences.

Basic principles[edit]

Low level stratocumulus clouds cover approximately one-third of the oceans surface. These clouds possess Albedo levels of 0.3-0.7.[1] The levels of albedo present in the low-level marine stratocumulus clouds create a significant global cooling effect.[2] Theoretically, by increasing the albedo of the clouds, via spraying sea water(NaCl) droplets approximately 1 μm in size, the cooling effects of the clouds are increased and potentially their longevity is enhanced.[3] By creating cloud condensation nuclei, it might be possible to change the albedo of clouds to make them appear whiter.[4] This can be done using a variety of chemicals and techniques, with seawater sprays from ships being a simple example.[citation needed] Broadly speaking, the marine environment has a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea,[citation needed] so this technique is more effective over the ocean than over land.

Models[edit]

HadGAM1 general circulation model[edit]

A study by Jones, Latham, and Smith using the Hadley Center's General Circulation Model, suggests that warming due to a x2 increase in CO2 could potentially be compensated for by a doubling of the droplet concentration number in low-level marine stratocumulus clouds, accounting for anthropogenic aersol production, in three regions—off the coasts of the Americas and West Africa. The three regions combined cover approximately 3% of the Earths surface.[5]

Model of marine stratocumulus clouds developed by Bower, Jones and Choularton[edit]

Bower, Jones and Choularton created a model to analyze the effectiveness of albedo modification on clouds.[6] A 2006 study simplified the model.[7] Their model illustrates the practically of the technique. It demonstrates droplet size is not of that much importance, that location of clouds is of limited importance, and that significant cooling can be achieved with a level of .03.[7]

Proposed schemes[edit]

Seawater spray[edit]

Various schemes have been suggested,[8][9][10] such as that proposed by John Latham and Stephen Salter,[11][12] which works by spraying seawater in the atmosphere to increase the reflectiveness of clouds. The extra condensation nuclei created by the spray will change the size distribution of the drops in existing clouds to make them whiter.[13] The sprayers would use a fleet of around 1500 unmanned Rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth.[8][14] The whitening is achieved as a result of the Twomey effect. In order to significantly cool the earth, the vessels have to spray sea water droplets at a rate of 50 cubic meter per second over a large portion of the Earth's ocean surface.[15]

This technique can give >3.7 W/m2 of globally averaged negative forcing,[14][16] which is sufficient to reverse the warming effect of a doubling of CO2.

Ocean foams[edit]

Creation of ocean foams has been suggested. When bubbles in the foams burst, they loft small droplets of seawater, which can modify the albedo of clouds.[17]

Ultrasonic excitation of a liquid using a piezo-electric transducer[edit]

This technique works by creating faraday waves at a free surface. If the waves are steep enough the droplets of sea water will be thrown from the crests and particles can enter into the clouds within a predictable area. However, a significant amount of energy is required.[18]

Electrostatic atomisation of seawater drops[edit]

This technique utilizes mobile spray platforms which move to adjust to changing weather conditions. A proposed idea is to use unmanned ships that shoot sea spray into the air.[19]

Cloud seeding yachts[edit]

Stephen Salter developed an unmanned yacht that uses Flettner rotors for propulsion. The rotors are vertical cylinders located on the deck of the yacht. They spray the tiny water droplets into the low level clouds to enhance their reflectivity. The power for the rotors and the ship is generated from underwater turbines. Approximately 1,500 of these ships would be required for the scheme to effectively cool the planet.[20] The Royal Society states that "approximately 1500 spray vessels would be required to produce a negative forcing of -3.7W m^-2. Each vessel would require approximately 150 kW of electrical energy to atomize and disseminate seawater at the necessary continuous rate (as well as to support navigation, controls, communications, etc.), so that the global power requirement is approximately 2.3x10^8 Watts." [21] This technique requires far less energy compared to many other geoengineering techniques.

Ocean sulfur cycle enhancement[edit]

Main article: sulfur cycle

Enhancing the natural sulfur cycle in the Southern Ocean by fertilizing a small portion with iron in order to enhance dimethyl sulfide production and cloud reflectivity.[22] The goal is to slow Antarctic ice from melting and raising sea level[23] Such techniques also tend to sequester carbon, but in this specific project the enhancement of cloud albedo was both the desired outcome and measured result.[24]

This technique can give only 0.016 W/m2 of globally averaged negative forcing,[16] which is practically insignificant as a contribution to reducing global warming. However, as it is a regionally acting technique its effects are concentrated in an influence on the climate of Antarctica.

Advantages and disadvantages[edit]

Advantages[edit]

  • The techniques use natural and reusable resources i.e. sea water and wind
  • Cloud albedo levels can be monitored via satellite and the sea spraying mechanisms could be adjusted to fit the data.[21]
  • Albedo enhancement is considerably cheaper than many other geoengineering techniques.[21]
  • It utilizes already existing technologies to send sea water droplets into low level oceanic clouds.
  • The location of the albedo enhancement of clouds can be controlled and localized. This could prevent ecological problems.
  • Cooling can happen in only the places in which it is required.

Disadvantages[edit]

  • Most of the information on albedo enhancement on clouds is from models and computer simulations. The actual results from spraying sea water into low level clouds may differ from the predicted effect.
  • Currently the two most commonly proposed plans for sea water dispersal are airplanes and turbine powered ships. The use of airplanes will be time consuming, labor intensive, and result in a large amount of carbon emissions. Turbine powered un-manned ships are the most ecologically friendly, but their capacity to fulfill the role is still not known.
  • The technique is strongly dependent on wind patterns.
  • If CO2 increases beyond the predicted rate then albedo enhancement of clouds may not be strong enough to cool the Earth.[25]
  • The effect of aerosols and its impact on albedo enhancement of clouds has not been significantly researched.[26]
  • Does not address the problem of ocean acidification caused by increased atmospheric concentrations of carbon dioxide.

Further research[edit]

  • The amount of water droplets that enter clouds and form additional droplets is not currently known and is affected largely by meteorological factors. Research must be done to assess what percentage of particles will be successful and what the effect of the weather is.[21]
  • Charging sea water particles to utilize the Earth's electric field may be beneficial.[21]
  • Climatological and meteorological ramifications must be studied as the technique will alter rainfall, temperature, static stability, and ocean currents.[21]
  • The effect of aerosols on low level clouds must be better understood. The aerosols may negatively impact efforts to enhance the albedo of clouds.[21]

Reducing cirrus trapping of outgoing infrared radiation[edit]

Synoptic cirrus clouds may be capable of modification to reduce their lifetime and hence their net positive radiative forcing (opposite to the low-cloud scheme), using seeding mechanisms distributed by airliners.[27] It is believed that the synoptic cirrus in the upper troposphere are formed by homogeneous nucleation, resulting in large numbers of small ice crystals. If ice nuclei are introduced into this environment, the cirrus may instead form by heterogeneous nucleation. If the concentration of ice nuclei is tuned such that the resulting cloud particle density is less than for the natural case, the cloud particles should grow larger due to less water vapor competition and attain higher settling velocities. The net effect should be a reduced optical thickness for outgoing infrared radiation and a reduced cloud lifetime. The effects of this modification on the cloud radiative balance operate in different manner from that of the marine stratocumulus cloud brightening scheme. Instead of increasing the incoming shortwave reflectivity and lifetime of the cloud in order to increase the net radiative cooling effect, the outgoing infrared reflectivity and lifetime are reduced thereby decreasing the net radiative heating effect of cirrus. This authors claim that this scheme is sufficient to reverse the warming caused by a doubling of CO2.

See also[edit]

References[edit]

  1. ^ Schwartz and Slingo, 1996 S.E. Schwartz and A. Slingo In: P. Crutzen, Editor, Clouds, Chemistry and Climate: Proceedings of NATO Advanced Research Workshop (1996).
  2. ^ Latham, J. (2002). "Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds". Atmospheric Science Letters 3 (2–4): 52. doi:10.1006/asle.2002.0048.  edit
  3. ^ Computational Assessment Of A Proposed Technique For Global Warming Mitigation Via Albedo-enhancement Of Marine Stratocumulus Clouds K Bower - T Choularton - J Latham - J Sahraei - S Salter - Atmospheric Research - 2006.
  4. ^ Twomey, S. (1977). "The Influence of Pollution on the Shortwave Albedo of Clouds". J. Atmos. Sci. 34: 1149–1152. Bibcode:1977JAtS...34.1149T. doi:10.1175/1520-0469(1977)034<1149:TIOPOT>2.0.CO;2. 
  5. ^ Jones, A., Latham, J., Smith, M.H., submitted for publication. Radiative forcing due to the modification of marine stratocumulus clouds, Atmos. Sci. Letters.
  6. ^ Bower, K.N., Jones, A., and Choularton, T.W., 1999. A modeling study of aerosol processing by stratocumulus clouds and its impact on GCM parameterisations of cloud and aerosol. Atmospheric Research, Vol. 50, Nos. 3–4, The Great Dun Fell Experiment, 1995-special issue, 317–344.
  7. ^ a b Computational Assessment Of A Proposed Technique For Global Warming Mitigation Via Albedo-enhancement Of Marine Stratocumulus Clouds K Bower - T Choularton - J Latham - J Sahraei - S Salter - Atmospheric Research - 2006.
  8. ^ a b Latham, J. (1990). "Control of global warming" (PDF). Nature 347 (6291): 339–340. Bibcode:1990Natur.347..339L. doi:10.1038/347339b0. 
  9. ^ Latham, J.; Salter, S. Preventing global warming by increasing cloud albedo (PDF). Retrieved 2008-04-20.  (A brief handout, with artist's renderings.)
  10. ^ Keith Bower et al. (2006). "Assessment of a Proposed Technique for Global Warming Mitigation via Albedo-Enhancement of Marine Stratocumulus Clouds". Atmospheric Research 82 (1-2): 328–336. Bibcode:2006AtmRe..82..328B. doi:10.1016/j.atmosres.2005.11.013. 
  11. ^ Latham, J. (2002). "Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds" (PDF). Atmos. Sci. Lett. 3: 52–58. Bibcode:2002AtScL...3...52L. doi:10.1006/asle.2002.0099. 
  12. ^ Salter, S, G. Sortino & J. Latham (2008). "Sea-going hardware for the cloud albedo method of reversing global warming". Phil. Trans. R. Soc. A 366 (1882): 3989–4006. Bibcode:2008RSPTA.366.3989S. doi:10.1098/rsta.2008.0136. PMID 18757273. 
  13. ^ Panel on Policy Implications of Greenhouse Warming, National Academy of Sciences, National Academy of Engineering, Institute of Medicine (1992). Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base. The National Academies Press. ISBN 0-585-03095-2. 
  14. ^ a b Latham, J., P.J. Rasch, C.C.Chen, L. Kettles, A. Gadian, A. Gettelman, H. Morrison, K. Bower, T.W.Choularton (2008). "Global Temperature Stabilization via Controlled Albedo Enhancement of Low-level Maritime Clouds". Phil. Trans. Roy. Soc. A 366: 3969–87. Bibcode:2008RSPTA.366.3969L. doi:10.1098/rsta.2008.0137. PMID 18757272. 
  15. ^ BBC NEWS | Programmes | Futuristic Fleet of 'cloudseeders'" BBC News - Home. 15 Feb. 2007. 20 Nov. 2010. <http://news.bbc.co.uk/2/hi/programmes/6354759.stm>.
  16. ^ a b Lenton, T.M. and N.E. Vaughan (2009). "The radiative forcing potential of different climate geoengineering options". Atmos. Chem. Phys. Discuss 9: 2559–2608. doi:10.5194/acpd-9-2559-2009. 
  17. ^ Evans, J.; Stride, E.; Edirisinghe, M.; Andrews, D.; Simons, R. (2010). "Can oceanic foams limit global warming?". Climate Research 42 (2): 155–160. doi:10.3354/cr00885.  edit
  18. ^ Barreras et al., 2002 F. Barreras, H. Amaveda and A. Lozano, Transient high frequency ultrasonic water atomization, Exp. Fluids 33 (2002), pp. 405–413. View Record in Scopus | Cited By in Scopus (31)
  19. ^ BBC NEWS | Programmes | Futuristic Fleet of 'cloudseeders'" BBC News - Home. 15 Feb.2007. 20 Nov. 2010.
  20. ^ Latham, John (15 February 2007). "Futuristic fleet of 'cloudseeders'". BBC. Archived from the original on 2012-07-25. Retrieved 2012-07-25. 
  21. ^ a b c d e f g Global Temperature Stabilization Via Controlled Albedo Enhancement Of Low-level Maritime Clouds John Latham - Philip Rasch - Chih-Chieh Chen - Laura Kettles - Alan Gadian - Andrew Gettelman - Hugh Morrison - Keith Bower - Tom Choularton - Philosophical Transactions Of The Royal Society A: Mathematical, Physical And Engineering Sciences - 2008
  22. ^ Wingenter, O.W., Elliot, S.M., Blake, DR. (2007). "New Directions: enhancing the natural sulfur cycle to slow global warming". Atmospheric Environment 41: 7373–737. doi:10.1016/j.atmosenv.2007.07.021. 
  23. ^ Coale, K. H.; Johnson, K. S.; Buesseler, K.; Sofex Group (2002). "SOFeX: Southern Ocean Iron Experiments. Overview and Experimental Design". American Geophysical Union, Fall Meeting. 
  24. ^ T.S. Bates, B.K. Lamb, A. Guenther, J. Dignon and R.E. Stoiber (2004). "Sulfur emissions to the atmosphere from natural sourees". Journal of Atmospheric Chemistry 14 (1-4): 315–337. doi:10.1007/BF00115242. 
  25. ^ Global Temperature Stabilization Via Controlled Albedo Enhancement Of Low-level Maritime Clouds John Latham - Philip Rasch - Chih-Chieh Chen - Laura Kettles - Alan Gadian - Andrew Gettelman - Hugh Morrison - Keith Bower - Tom Choularton - Philosophical Transactions Of The Royal Society A: Mathematical, Physical And Engineering Sciences - 2008
  26. ^ Global Temperature Stabilization Via Controlled Albedo Enhancement Of Low-level Maritime Clouds John Latham - Philip Rasch - Chih-Chieh Chen - Laura Kettles - Alan Gadian - Andrew Gettelman - Hugh Morrison - Keith Bower - Tom Choularton - Philosophical Transactions Of The Royal Society A: Mathematical, Physical And Engineering Sciences - 2008
  27. ^ Mitchell, D. L.; Finnegan, W. (2009). "Modification of cirrus clouds to reduce global warming". Environmental Research Letters 4 (4): 045102. Bibcode:2009ERL.....4d5102M. doi:10.1088/1748-9326/4/4/045102.  edit