Geoengineering (also known as climate engineering) proposals aim to reduce global temperatures in order to counteract global warming and stabilize the Earth's energy budget and climate. The proposed methods are diverse and vary greatly in terms of their technological characteristics and potential consequences. You can find some of them in the sketch below.
None of the geoengineering ideas can "save the planet." The ideas I am mainly interested in are known as "solar geoengineering." These ideas aim to either reduce the amount of solar radiation reaching the Earth's surface by injecting aerosol into the stratosphere or by brightening marine low clouds, or increase the amount of outgoing longwave radiation by modifying and/or decreasing cirrus cloud coverage (cirrus cloud thinning). However, none of these ideas can address all the problems of climate change, such as ocean acidification caused by increased carbon dioxide concentrations.
Solar geoengineering can be seen as a painkiller that temporarily prevents some of the climate damage, while we implement ambitious greenhouse gas reductions and thereby eliminate 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. Additionally, who can we trust with control over the global thermostat?
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 do not have a planet B where we can experiment with clouds, we have to rely on computer simulations of cirrus clouds to determine whether any of these ideas could potentially be effective, at least in climate models. These ideas are based on the mechanisms of cirrus formation. A cirrus cloud can be formed solely by the 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 climate. On the other hand, when solid aerosol particles such as dust are present, ice crystals form on them, resulting in a cirrus cloud that is composed of fewer, larger ice crystals.
These large ice crystals form a cloud that is more transparent to longwave radiation, resulting in a smaller warming effect compared to pure cirrus clouds. Additionally, the large ice crystals quickly precipitate out of the atmosphere, reducing the cloud's lifetime.
We could artificially introduce significant amounts of dust to locations where pure cirrus clouds are expected to form, promoting the formation of dusty cirrus clouds instead. This could potentially have a cooling effect on the climate.
In my research I address the following questions:
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