Clouds play a major part in the Earth’s heat balance. They act here in contradictory ways. On the one hand, they reflect part of the short-wave solar radiation and so have a cooling effect. On the other, they absorb long-wave heat radiation emitted from the Earth’s surface and radiate some of it back down. This greenhouse effect results in warming. RM methods that address both mechanisms have been discussed for over 20 years. It is unclear to what extent artificially altering the clouds on this scale would influence and change the water cycle and air circulation in the lower levels of the atmosphere. Scientists fear that it could trigger changes in the climate in various regions of the world.
Marine Cloud Brightening
As early as the 1980s, clouds forming around ships’ exhaust plumes above the sea under certain conditions were noticed on satellite images and those clouds reflect short-wave sunlight back into space. The reason why ship tracks cause clouds to form over remote marine areas especially is as follows: The air above these sea areas is usually particularly clean. It has hardly any particles for airborne moisture to condense around. Releasing particles artificially in such regions – as happens with ship exhausts – increases the number of condensation nuclei and more cloud droplets form. Some years ago, the idea was proposed of spraying saltwater over the oceans, as the salt crystals in the water make for good condensation nuclei. This would make it possible to create artificial clouds, of a cloud type known as stratocumulus, that intensify the reflection of short-wave sunlight and result overall in a cooling of the Earth. It is not yet known, however, if this would lead to regional or supraregional changes in the climate.
Potential and scale
This method is considered to have very large potential. It is estimated that about three percent of the Earth’s surface is particularly well suited for artificial modification of stratocumulus clouds. Southern Ocean regions are considered to be especially promising, and most of all the marine areas off Namibia and Peru. Seeding clouds there could offset up to 35 percent of the current greenhouse effect of CO2. The individual clouds only last for a few hours to a few days. Sea water would therefore have to be sprayed continuously above the surface of the tropical marine regions by thousands of vessels around the globe.
Application readiness and research needs
Further research is needed to determine whether this method is economically viable and environmentally compatible. Energy efficient spraying technologies would also be needed. It also has to be kept in mind that the method only makes sense if the equipment and vessels run on renewables-generated electricity. The basic technical challenge is how to propel the saltwater aerosols to altitudes at which stratocumulus form (from several hundred to 2,000 metres).
Thinning Of Arctic Winter Cirrus Clouds
Instead of increasing the cooling effect of the clouds, it is also possible to decrease their warming effect by influencing cloud dispersion. An RM method is thus being discussed that involves dispersing or at least altering cirrus clouds that occur at altitudes of 5,000 to 13,000 metres. Cirrus clouds consist of ice crystals that reflect both short-wave solar radiation and long wave heat radiation. Which way they act depends on their altitude, the geographical latitude, and also the shape of their ice crystals. In most cases, they reflect long-wave heat radiation back to the Earth’s surface. Cirrus clouds therefore tend to contribute more to global warming. This effect is particularly pronounced during the dark polar winter, when the cooling effect from reflection of solar radiation is not there. Dispersing or thinning the cirrus clouds then, or modifying the properties of their ice crystals, would allow long-wave heat radiation to escape into space: The Earth would be able to expel more heat. This could theoretically be achieved by seeding ice nuclei in the atmosphere. The artificial ice nuclei would cause the formation of larger ice crystals that fall more rapidly, thus depleting the clouds.
The method’s potential cannot currently be quantified. It is possible that polar cirrus clouds cannot be influenced in the desired manner and that the method is ineffective. In the optimum case, a number of models calculate a potential 1 °C global cooling effect. Dispersing cirrus clouds would have the basic advantage of affecting heat radiation rather than solar radiation. The method may therefore be better at offsetting the effect of greenhouse gases. Since the impact on the radiation balance as a result of cloud thinning or dispersal would vary considerably from region to region, scientists expect changes in both the climate and the water cycle in the affected regions. There could also be major meteorological side effects with an impact on regional weather.
Scientists estimate that the material expense for spreading the ice nuclei would be relatively small. It may be enough to spread the particles in suitable locations using commercial aircraft. The quantities of material needed would be in the range of a few kilograms per flight.
Application readiness and research status
Because not enough research has yet been done into the formation and properties of polar cirrus clouds, it is not known how well they can be dispersed in practice. It is consequently so far not possible to precisely quantify the method’s effectiveness.