Cirrus clouds are thin, wispy clouds that usually form above 18,000 feet at all latitudes. These clouds are the most common types of clouds, and constitute around 25 percent of the total cloud cover of this planet. They are especially dense in the tropics, where they constitute around 75 percent of the total cloud cover.
Cirrus clouds are composed of ice crystals and they play two important roles in determining the temperature of the earth. They first trap and reflect back some of the sun's energy, without allowing them to fall on to the earth's surface. This is the albedo effect. Secondly, they trap some of the heat in the form of infra radiation that touches the earth's surface and returns on its way to outer space. This process, known as the greenhouse effect and plays a significant role in warming the earth's atmosphere by around 15 degrees centigrade. Without these cirrus clouds the atmosphere of planet earth would have been 0 degrees centigrade, and thus inhospitable.
However, of late the phenomenon of global warming or simply put, the increased presence of greenhouse gasses in the atmosphere is warming the earth's surface and temperatures are on this rise from the normal. The average temperature of the earth has risen from 15 degree centigrade to 16.1 degree centigrade within the last decade. This has devastating effects on the earth in the form of melting ice caps, rising sea levels, destruction of crops and the likes. The concern now is to reduce the temperature, so that the earth would go back to its normal stage, or at least to limit the damage.
In such a scenario, some atmospheric physicists like David Mitchell propose that by dissipating the cirrus cloud cover, more of suns heat would go out to space, and this would reduce global warming and thus make up for the heating up of the earth by other sources.
The heat of the sun causes the air near the surface to rise, accompanied by water vapor from the oceans. As this moisture-laden hot air meets cooler air at higher altitudes, the moisture present in the air condenses to form tiny water droplets and ice crystals. These water droplets or ice crystals remain suspended in the air and form clouds. When the quantity of such water or ice becomes very heavy, they undergo precipitation and fall down as rain, hail, or snow. Cirrus clouds form in the same way.
The strategy proposed to dissipate cirrus clouds is to seed the existing cirrus clouds with special particles that would grow into large ice crystals. When this happens, the clouds would naturally precipitate and fall down from the sky as rain. This technique is similar to cloud seeding that farmers adopt to produce rain.
However, the fact of the matter is that this procedure is not a solution to global warming, for nature ensures that the process of cloud formation takes place repeatedly. This process at best buys some time to slow down the rate of global warming until people identify a more permanent solution.
The hypothesis of all leading climate models forecast is that as the atmosphere warms there should be an increase in high altitude cirrus clouds, which would amplify any warming caused by man made greenhouse gases.
However, research on this topic is not yet conclusive. The extent of global warming or the heat that the clouds trap depend on the size of the ice crystals, but it is not known for certainty whether the albedo effect, which actually cools the earth by preventing sunlight from touching the surface dominates, cancels out or overshadows the greenhouse effect that warms the earth. Most calculations severely underestimate the cirrus albedo effect.
Moreover, even assuming that the greenhouse effect overwhelms the albedo effect and cirrus cloud acts as a blanket that prevents the sun's heat from leaving the earth, the fact remains that around 80 percent of the natural greenhouse effect is due to water vapor and clouds, and those are largely under the control of precipitation systems. There are significant gaps in the scientific understanding of precipitation systems and their interactions with the climate, which needs looking into, before determining whether cirrus cloud seeding would succeed in combating global warming.