Is it possible for us to slow or even reverse the Earth's current global warming trend through the use of cloud seeding technology? The only reasonable answer to this question, based on our current knowledge and technical ability, is probably. Whether attempting to do so is actually worth trying however, is highly debatable. Not only in the scientific community, but in the political and public arenas, where the money would have to come from, as well. Because, while it may be technically feasible, it would almost certainly cost trillions of dollars to do.
Currently, cloud seeding is a controversial, weather modification process, primarily intended to increase precipitation (rainfall) in a targeted area. It is controversial because it is unclear whether it works or not; although the reality is that it almost certainly does work under some conditions in some places, while it does not in others. The controversy is more based on the usual; some people trying to con other people so they can profit off them. But this current use of cloud seeding technology relates to cloud formations relatively close to the Earth's surface, rather than those, like cirrus clouds, in the upper troposphere.
Cirrus clouds are those thin, wispy clouds that sometimes look like white feathers in an otherwise clear sky. The word cirrus comes from Latin and means curl, comparing them to the tendril-like curl of hair or a feather. To see them, the sky needs to be relatively clear except for them, because they usually form in the upper troposphere, above other cloud formations. When particularly thick, for them, they can present as a haze in the upper atmosphere.
Cirrus clouds make up a significant proportion of the clouds in our skies and can form during any season. They typically cover between 20 to 25 percent of the Earth's surface at any time and recent study using infrared satellite monitoring indicates that cirrus cloud formations at the tropics may cover as much as 70 percent of that region of the sky on average. Cirrus clouds usually form at altitudes of 8000 meters (26,000 feet) above sea level or higher. At this altitude the ambient air temperature varies between minus 20 to minus 30 degrees Celsius or minus 4 to 22 degrees Fahrenheit. Because of this, cirrus clouds are predominantly composed of ice crystals of varying size, formed by the freezing of super-cooled water droplets. Because they are composed mainly of ice crystals, they reflect rather than refract or focus light, as the liquid water droplets that may predominate in lower level clouds do. This means that they both reflect incoming sunlight back into space and return light reflected from the Earth's surface back down towards the Earth.
The seeding process that would be used on cirrus clouds would be a form of airborne static cloud seeding. High flying aircraft would be used to deliver microscopic particles, probably silver iodide, into atmospheric areas already containing cirrus cloud. Silver iodide has a crystalline structure similar to ice, water vapor and microscopic droplets of water within the cirrus cloud would have more particles to adhere to and freeze upon. This would increase both the number and size of ice particles within the cloud layer, the natural ice particles being supplemented by those artificially supplied.
While artificially supplemented cirrus cloud formations would reflect more of the light returning to space from the Earth's surface in the same manner as some greenhouse gases, by increasing their density and global coverage, the amount of sunlight reaching the Earth's surface in the first place should be reduced in proportion to the cloud seeding effort applied.
While this is technically viable, the effort required to supplement sufficient areas of cirrus cloud cover is not economically viable under today's technological constraints. The number of aircraft, fuel and seeding material needed to possibly reduce global warming, by reflecting sufficient sunlight in the upper troposphere, far exceeds the results that could be expected.
Trying to reduce global warming by this methodology might be equated to trying to put out an intense chemical fire with water. A bucket of water would not cost much, a garden hose would cost more, a fire hose wielded by firemen even more, but the multiple giant water buckets dumped by a helicopter, that just might possibly succeed, would cost the most.
What also needs to be considered is that this concept is about treating the symptoms rather than the cause. While this may be the traditional way we deal with problems, is it the way we should continue? Both the logical and common sense approach to dealing with global warming is to start by reducing the anthropogenic causes that are supplementing and speeding up this natural event. If minimizing our own detrimental impacts fails to procure a satisfactory result, then we can look at manipulating the natural world to improve the situation.