GeoEngineering using clouds?

The CLAW Hypothesis R. Charlson, J. Lovelock, M. Andreae and S The CLAW Hypothesis R. Charlson, J. Lovelock, M. Andreae and S. Warren (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326, 655-661. we now have evidence that some of the steps within the CLAW hypothesis are correct but we still don't know whether the system really operates as a negative feedback loop.  This makes it very difficult to represent the process in climate models and so we are still unsure quite how important DMS is to the cooling of our planet.

Dimethylsulfide "Upwelling": Upwelling usually occurs in the subtropics along the western continental coasts, where prevailing trade winds drive the surface water away from shore, drawing deeper water upward to take its place. Because of the abundance of krill and other nutrients in the colder waters, these regions are rich feeding grounds for a variety of marine and avian species. Upwelling can also occur in the middle of oceans where cyclonic circulation is relatively permanent or where southern trade winds cross the Equator. The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life in the planet.

Lenton & Vaughan, ACP, 2009

Lenton & Vaughan estimates of RF Ocean Sulfur Cycle Enhancement (Wikipedia summary) “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.”

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

Vessel concept

Vessel uses Flettner rotors – vertical spinning cylinders – instead of sails “The vessels will drag turbines resembling oversized propellers through the water to provide the means for generating electrical energy. Some will be used for rotor spin, but most will be used to create spray by pumping 30 kg s-1 of carefully filtered water through banks of filters and then to micro-nozzles with piezoelectric excitation to vary drop diameter. The rotors offer a convenient housing for spray nozzles with fan assistance to help initial dispersion.” “Electrical energy for spray and rotor drive will be generated by a pair of 2.4 m diameter axial-flow turbines on either side of the hull as shown in figure 10. These are very much larger than any propellers needed for a vessel of this size but can act as propellers for 10 hours in windless conditions using energy from a bank of Toshiba SCiB batteries.”

Salter et al’s. calculations

Salter et al’s. calculations

Where to deploy the sprayers? “Suitable sites for spraying need plenty of incoming sunshine to give something to reflect. They must have a high fraction of low-level marine stratocumulus cloud. They should have few high clouds because these will reduce incoming energy and send the reflected energy down again. There should be reliable but not extreme winds to give spray vessels sufficient thrust. There should be a low density of shipping and icebergs. It helps to have a low initial density of cloud condensation nuclei because it is the fractional change that counts. This suggests sea areas distant from dirty or dusty land upwind. Owing to a possible anxiety over the effect of extra cloud condensation nuclei on rainfall, areas upwind of land with a drought problem should be avoided.”