From my reading I find that there are only two feasible, somewhat cost effective methods that may block solar radiation, and you can read in detail about them on Wikipedia: “Marine cloud brightening1” and “Stratospheric aerosol injection”2.
The most cost effective method of marine cloud brightening appears to be injecting salt water sprays into the atmosphere above the oceans several times a year by a fleet of specially designed ships. This may cost $500 billion dollars a year. That this may work is demonstrated by the contrails left by ships prior to 2020 when sulfur was emitted by ships using high sulfur oil3. A problem not discussed in the Wikipedia article is the possibility of not having as many oceanic cloud to brighten if higher ocean temperatures reduce their formation.
For stratospheric aerosol injection, the obvious and most cost effective solution is to inject sulphur dioxide (SO2) (or possibly some other gas or liquid) into the stratosphere each year or ever few years at the equator. Normal stratospheric winds will disperse the SO2 throughout the stratosphere where it will form small sulfuric acid drops. (The largest volcanic eruptions have done this, and caused drastic cooling, but also reductions in crop yields. The costs would perhaps be 100 to 500 billion of dollars each year.
Present jurisprudence and international agreements would not allow use of either of the above methods, nor is the finance available. However, given a future climate emergency, finance may well be obtained, and present legal limitations modified or ignored.
- https://en.wikipedia.org/wiki/Marine_cloud_brightening ↩︎
- https://en.wikipedia.org/wiki/Stratospheric_aerosol_injection ↩︎
- https://www.nature.com/articles/s43247-024-014 ↩︎
How large are short-term climate fluctuations, and what causes them?