We could make starshades today if we made it a priority. It would take a shade roughly only about 0.09% the size of the cross-section of the Earth to change the climate as much as all of our CO2 emissions. That's only a roughly 200km in radius shade. If we were spending, say, a trillion dollars a year (1.3% of global GDP, a sum less than what is spent on many other things) we could almost certainly build and launch such a structure in at most a few decades.