Space. This is a collection of ideas that are basically putting a translucent object between the Earth and the Sun, in order to filter the radiation. 

Imagine a giant glass object, shaped as disk. The disk has propulsors and can move around like a vessel, or alternatively, is towed by a vessel. The idea is that it would position itself between Earth and the Sun, at some distance where the disk's size is small enough to be constructed. There, it would filter the sunrays. This could aid in preventing melanoma or fight global warming. 

In another variant, the disk would be actually a thin hollow glass tank, filled with a solution. We would change the concentrations on the solution to our needs.

This is radical geoengineering: the risks are enormous. It would affect every living thing on the planet. It is not something you would do casually.