Aerosols were probably sulfuric acid. Unfortunately this has negative side-effects. Aerosol can also be produced by monoterpenes, e.g. pinene, but there is disagreement about whether this can be helpful.
The crazy part about this is that doing something like that is within reach of a number of wealthy individuals. All it takes is one delusional billionaire to throw caution, and a metric buttload of aerosols to the wind and we will find out whether it was a good idea or not.
Putting up a geosynchronous mirrored-foil tarp that shadows the entirety of one of earth's larger deserts would take about the same amount of effort — and be much easier to reverse if it turns out to be a bad idea.
I think the important part here is you could do the aerosol injection secretly. It's pretty much impossible to launch a rocket to geosync orbit without a major government getting involved in your business.
This has been studied in pretty extensive detail & what I remember is that it’d require a custom plane design to get to efficiencies where this is worth pursuing. And the number of flights required would be hard to hide from a motivated government adversary. Still worth pursuing. Any takers on here?
