Geoengineering proposals aim to reduce global temperatures to counteract global warming, stabilize the Earth's energy budget, and regulate the climate. The proposed methods vary greatly in terms of their technological characteristics and potential consequences. Some of these methods are illustrated in the sketch below.
I am interested in the idea of "solar geoengineering." These ideas aim to reduce the amount of solar radiation that reaches the Earth's surface. One method is to inject aerosols into the stratosphere. Another method is to brighten marine low clouds. A third method aims to increase outgoing longwave radiation by modifying and/or decreasing cirrus cloud coverage, a process known as cirrus cloud thinning.
However, none of these methods can "save the planet." They cannot solve all climate change problems, such as ocean acidification caused by increased carbon dioxide concentrations.
Solar geoengineering can instead be seen as a temporary solution that prevents some climate damage while we implement ambitious greenhouse gas reduction strategies, thereby eliminating the need for geoengineering.
Indeed, there are many reasons why geoengineering might not be such a great idea after all. You can find them summarized in table 1 of Robock, 2016.
Among these reasons, I personally find the prospect of commercializing geoengineering quite disturbing.
Cirrus cloud thinning
Since cirrus clouds warm the climate (see section on cirrus clouds), we could think of getting rid of them to counteract global warming.
Type 1: pure Ci
Type 2: dusty Ci
dust
However, since we don't have a Planet B to experiment with clouds, we must rely on computer simulations of cirrus clouds to determine if these ideas could be effective in climate models. These ideas are based on the mechanisms of cirrus formation. Cirrus clouds form only by freezing of small droplets or liquid aerosols. This type of cloud consists of numerous tiny ice crystals and has a significant greenhouse warming effect on the climate. Conversely, when solid aerosol particles, such as dust, are present, ice crystals form on them. This results in a cirrus cloud composed of fewer, larger ice crystals.
These large ice crystals form a cloud that is more transparent to longwave radiation. This results in a smaller warming effect than that of "pure cirrus" clouds. Additionally, the large ice crystals quickly fall out of the atmosphere, which reduces the cloud's lifetime.
We could introduce significant amounts of dust to locations where pure cirrus clouds are expected to form, which would promote the formation of "dusty cirrus" clouds. This could potentially cool the climate.
In my research I address the following questions using climate models:
Does cirrus cloud thinning really cool the climate? If so, by how much?
Are the uncertainties in simulating cirrus clouds currently too large to provide accurate answers on the efficacy of seeding? How can we decrease these uncertainties?
For more details please refer to:
To what extent can cirrus cloud seeding counteract global warming? (Gasparini et al., 2020, Env. Res. Lett.)
A cirrus cloud climate dial? (Lohmann and Gasparini, 2017, Science)
Is increasing ice crystal sedimentation velocity in geoengineering simulations a good proxy for cirrus cloud seeding? (Gasparini et al., 2017, ACP)
Why cirrus cloud seeding cannot substantially cool the planet (Gasparini and Lohmann, 2016, JGR)
By the way: chemtrails don't exist. Contrails do. Check http://contrailscience.com