Ever seen the movie Snowpiercer? It was based on a comic called Le Transperceneige and they also more recently made a TV series about it, but I've only seen the movie and highly recommend it. It takes place on an Earth stuck in a planet-wide heavy winter due to an anti-global-warming scheme gone wrong. It's the first thing I think of whenever I hear of space umbrellas or these sorts of anti-warming ideas.

EDIT: Ah, someone sniped me.