Cloud reflectivity modification

Cloud reflectivity enhancement is also known as 'marine cloud brightening' or 'cloud whitening' on low cloud. An opposite scheme exists to reduce the reflectivity of higher, colder cirrus clouds.^[1] It is a geoengineering technique that works by solar radiation management. By modifying the reflectivity of clouds, the albedo of the Earth is altered. The intention is that this technique, in combination with greenhouse gas emissions reduction (and possibly other geoengineering techniques) will be sufficient to control global warming. Compared to other climate modification strategies, this technique is relatively simple and benign, being based as it is on natural processes of 'ocean spray'. It can therefore be deployed quickly for further research, and can then be rolled out on an effective scale relatively cheaply after that. 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.

Contents 1 Basic principles 2 Models 2.1 HadGAM1 general circulation model 2.2 Model of marine stratocumulus clouds developed by Bower, Jones and Choularton 3 Proposed schemes 3.1 Seawater spray 3.1.1 Ultrasonic Excitation of a Liquid Using a Piezo-Electric Transducer 3.1.2 Electrostatic Atomisation of Seawater Drops 3.1.2.1 Cloud Seeding Yachts 3.1.3 Ocean Sulfur Cycle Enhancement 4 Advantages and Disadvantages 4.1 Advantages 4.2 Disadvantages 5 Further research 6 Reducing cirrus trapping of outgoing infrared radiation 7 See also 8 References

Basic principles

Low level stratocumulus clouds cover approximately one-third of the oceans surface. These clouds possess Albedo levels of 0.3-0.7.^[2] The levels of albedo present in the low-level marine stratocumulus clouds create a significant global cooling effect.^[3] 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.^[4] By creating cloud condensation nuclei, geoengineers can change the albedo of clouds to make them appear whiter.^[5] This can be done using a variety of chemicals and techniques, with seawater sprays from ships being a simple example. Broadly speaking, the marine environment has a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea, so this technique is more effective over the ocean than over land.

Models

HadGAM1 general circulation model

A study by Jones, Latham, and Smith using the Hadley Center's General Cirulation 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.^[6]

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

Bower, Jones and Choularton created a model to analyze the effectiveness of albedo modification on clouds.^[7] A 2006 study simplified the model.^[8] 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.^[8]

Proposed schemes

Seawater spray

Various schemes have been suggested,^[9][10][11] such as that proposed by John Latham and Stephen Salter,^[12][13] 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.^[14] 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.^[9][15] 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.^[16]

This technique can give >3.7 W/m² of globally averaged negative forcing,^[15][17] which is sufficient to reverse the warming effect of a doubling of CO2.

Ultrasonic Excitation of a Liquid Using a Piezo-Electric Transducer

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 with in a predictable area. However, a significant amount of energy is required.^[18]

Electrostatic Atomisation of Seawater Drops

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

Stephen Salter developed an unmanned yacht that uses Flettner rotors for propulsion. The rotors are vertical cylinders located on the deck of the yacht. The 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

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,^[17] 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

Advantages

• The techniques use natural and reusable resources i.e. sea water and wind
• You can monitor cloud albedo levels via satellite and adjust the sea spraying mechanisms 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

• Most of the information on abledo 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.
• The meteorological affects cannot be known until real world testing is conducted.
• 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]
• Consequent precipitation might salinize soils with seawater salt.

Further research

• 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

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.^[1] 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

• Geoengineering
• Solar radiation management
• Albedo
• Stratospheric sulfur aerosols (geoengineering)

References