I find "marine cloud brightening" [0] a lot more interesting. It aims to increase the albedo of Earth by creating more clouds over the oceans e.g. by spraying seawater.

Pro: "If implemented, the cooling effect is expected to be felt rapidly and to be reversible on fairly short time scales.", [0]

[0] https://en.wikipedia.org/wiki/Marine_cloud_brightening

> It aims to increase the albedo of Earth by creating more clouds over the oceans e.g. by spraying seawater.

Sort of; what we're usually doing, in general, is increase the aerosol burden which in turn increases the total number of nuclei available to form clouds. Given that the total liquid water content in a cloud system is fixed (i.e. controlled by factors that are harder for us to influence directly), this leads to a mixture of consequences:

1. The total number of cloud droplets increases 2. The average cloud droplet size decreases

Both (1) and (2) alone tend to increase the albedo of the cloud. They also produce secondary effects, e.g. they can suppress the formation of precipitation or modify microphysical properties that lead to clouds sticking around longer, thereby further increasing their radiative effect (more SW radiation reflected back to space and possibly more LW radiation at lower radiating heights).

We can guess by looking at the SO₂ deposited by volcanic eruptions

> [The 1991 eruption of Mount Pinatubo released] 17 megatons of sulfur dioxide into the atmosphere, causing global cooling by 0.5 °C (0.9 °F) between 1991–1993

> [It] oxidized in the atmosphere to produce a haze of sulfuric acid droplets, which gradually spread throughout the stratosphere over the year following the eruption

https://en.wikipedia.org/wiki/1991_eruption_of_Mount_Pinatub...

The vital difference is that vulcanic eruptions are basically a pulse-chase 'experiment', whereas geoengineering would require sustained injections with smaller doses.

The argument usually raised is that the volume is too small to matter (and a comparison to SO2 volcanic eruptions). I haven’t seen those studies so am at the moment dubious (but would like to be wrong).