There is an idea that is keeping me up at night. What if we could launch large sails to the L1 Lagrange Point to reduce the amount of radiation that reach the earth by only 2% ?
Aluminium foil is about 2g/cm2. That's 3e-3 g/cm2. 3e-2 kg/m2. 30,000kg/km2.
You'd need 66 million tonnes of aluminium.
Maybe you can make it 100 times lighter with better materials? So only 600,000 tonnes.
https://en.wikipedia.org/wiki/Lagrange_point
Maybe if we'd gotten into asteroid mining 100 years earlier...
EDIT: I've sort of come to the conclusion that this is the only solution that a single-party could take on that would meaningfully decrease the amount of heat in the atmosphere. Everything else is dependent on a ton of other people doing what they need to do, too.
I also think it is the only possible controllable way to control global warming. I am very happy to learn I am not the only one that thought of this.