Space Sunshades

Roger Angel at the University of Arizona has proposed launching trillions of space shades into what as known is the L-1 orbit between the sun and Earth.
These shades—each about two-feet in diameter and weighing only a gram—would collectively form a long, cylindrical cloud that would ultimately reduce sunlight hitting Earth by about two percent.
The project has been viewed as prohibitively expensive. Angel says the project could feasibly be deployed in 25 years for a few trillion dollars, which includes the cost of producing and blasting 20 million tons of shades into space.

Cloud Seeding

Another way to reflect more sunlight back into space is to increase reflectivity of the world's marine clouds, which cover a quarter of the ocean's surface. John Latham and Stephen Salter of the University of Edinburgh have proposed wind-powered yachts (pictured) that would spray seawater droplets into the air to produce more clouds.
Latham says that about a thousand of these vessels would be needed to make the plan effective, and that they should be deployed in the southern oceans, where most reflective marine stratocumulus clouds are. But more testing is necessary to better understand the ecological and meteorological consequences.