|Fig. 1: Ship tracks satellite image retrieved by NASA's Terra MODIS instrument. (Courtesy of NASA)|
The Paris Climate accord aims to limit global warming increase to a maximum of 2°C over the baseline average. Country signatories aim to comply to this by implementing energy saving policies and technologies. However, there are some who state this is not enough - countries will move too slowly to comply, and possibly reach a tipping point where conditions will be irreversible.
The field of changing the climate using man-made means is known as geoengineering. One idea is to control the amount of sunlight reaching the earth, so called solar radiation management. One approach to do this is to change the reflectivity of clouds, making the brighter, so that more incoming sunlight is reflected out to space instead of hitting the earth. The basic idea is to send ocean spray into the atmosphere which will provide the optimum cloud particles to change the reflectivity of clouds so less sunlight reaches the earth by creating a small increase the brightness of clouds.
Clouds consist of droplets that are 10 μm in diameter, with 100 drops per cubic centimeter. These droplets are actually formed around very small hygroscopic crystals - nuclei of salt or sulphates.
As warm air rises above the ocean during the day, it then expands and cools and creates a small supersaturation. A large enough enveloped salt crystal will then convert into a cloud droplet. A collection of such droplets thus form clouds.
The greater the concentration of droplets, the greater the reflectance of clouds. We actually see this effect over oceans as ship tracks, where there is increased cloud concentration over ship sailing routes (Fig. 1). An ocean spray is caused as ships travel, and the salt water spray rises to the atmosphere to create clouds consisting of heavier droplets. The questions are (1) whether it is possible to replicate this process in scale and (2) whether it can be done economically.
|Fig. 2: Snow production at Camelback Ski Area, United States. (Source: Wikimedia Commons)|
If a spray could be made to create the right size droplets, how would we get them to the atmosphere? One idea is to have a tethered hot air balloon to pump water spray into the atmosphere.  Alternatively there have been several proposals to create a high pressure spray delivering an effervescent spray. The resulting spray consists of droplets where 98% are made up of the right size (40-400 nm). Cloud albedo is a measurement term for cloud reflectivity. Higher values of cloud albedo indicate that a cloud reflects a larger amount of solar radiation and transmits a smaller amount. Cloud albedo depends on the total mass of water, the size and shape of the droplets or particles and their distribution in space.
Another approach is the spray from ocean-going ships. A small change in cloud alebedo can redress the effects of global warming. Per Latham, the resulting estimated power increase of 3.7 W m-2 after a doubling of pre-industrial CO2 could be offset by a calculated albedo increase of only 1.1%, by dividing the power increase by the 24 hour solar input of 340 W m-2. [2,3]
A more recent idea is to insert the spray nozzles inside snow blowers currently designed for use on ski slopes (Fig.2), and then placing these snow blowers on ships to spray ocean water. [2,4] The estimated vertical throw would be 50 to 100 m, with 20-40 kW of fan power powering approximately 400 nozzles. The snow blowers are well established technology and could cost around $40k to retrofit a ship.
The total amount of sea salt emitted into the atmosphere is estimated to be in the range of 1.4 - 6.8 × 1012 kg/yr.  Let us assume a midway number of 4.1 × 1010 kg that needs to emitted, since we need to perturb the total by 1% to get a 1% change in the albedo. Seawater is 3.5% salts by weight, so the amount needed to be emitted will be 4.1 × 1010 kg / 0.035 = 1.171 × 1012kg (1.171 billion tonnes) of seawater per year.
If we have a ship that has 400 nozzles and they use all of the 40KW to throw water up to 100m, then the mass flow rate will be
A year's worth of that amounts to
or 1.29 million tonnes. The number of such ships required to get the desired 1% effect would thus be
However, this is a great oversimplification. For example, one would have to reckon in the how much area each ship could cover. It is thus likely that one would need many more ships. If we say we need 10 times more ships to obtain more coverage, then we would need 9140 ships.
If we assume ships already exist and we can pay them to put sprayers on the ships, then at $40k each, the capital cost for retrofitting 914 ships would be $40k x 914 = $36.56 million. The marginal energy cost per year (at $0.1 per kwH) would be
For our upper bound of 9140 ships, then there would be a capital cost of $357 million and an energy cost of $320 million. So for a range of 914 to 9140 ships, the costs look reasonable in the grand scheme of potential global warming damage. However, there would of course be costs for the ships themselves and associated fuel and personnel costs. But it maybe possible to add the sprayers to existing operating vessels that are in the sea for other purposes. For a reference point, there are in excess of 50,000 merchant ships trading internationally. The overall calculation is a great simplification but nevertheless this is a starting point.
Other potential applications include reducing the intensity of hurricanes by reducing the sea surface temperature. However, the level of intensity reduction has not been determined yet.  In 2017, the several hurricanes affecting the United States inflicted over $100 billion of damage. If the hurricanes could be reduced in intensity by even a small amount, there could easily be an economic deployment argument.
The geoengineering approach outlined here is controversial for several reasons, but particularly (1) the effort distracts efforts and funding from other energy saving policies and technologies, and (2) it is not yet determined whether there will be any adverse effects of man-made changes of climate in this way or whether the process is reversible. In contrast to these arguments, geoengineering proponents state that no large scale efforts should be undertaken until safety is assured but small scale experiments need to be done now in case geoengineering is the only option left. However, one has to ask whether any small scale experiment can be predictive of large scale implementations.
© Ronjon Nag. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Davidson et al., "Atmospheric Delivery System," U.S. Patent Application Publication US 2012/0241554 A1, 27 Sep 12.
 S. Salter, G. Sortino, and J. Latham, "Sea-Going Hardware For the Cloud Albedo Method of Reversing Global Warming," Phil Trans. R. Soc. A 366, 3989 (2008).
 J. Latham et al., "Marine Cloud Brightening," Phil Trans. R. Soc. A 370, 4217 (2012).
 R. Wood et al., "Could Geoengineering Research Help Answer One of the Biggest Questions in Climate Science?" Earth's Future 5, 659 (2017).
 O. Boucher et al.," Clouds and Aerosols," in Climate Change 2013: The Physical Science Basis, ed. by T. F. Stocker et al. (Cambridge University Press, 2013), p. 571.
 J. Latham et al., "Weakening of Hurricanes Via Marine Cloud Brightening (MCB)," Atmos. Sci. Lett. 13, 231 (2012).