Theoretically, global warming can be reversed by reflecting more of the sun’s energy back into space. The cheapest and simplest solution to global warming I’ve seen is to increase cloud cover by spurting seawater up into the sky. This short video about it is well worth watching:
Another plan, the Global Albedo Enhancement Project, involves covering a vast area of desert in white plastic. Keeping the plastic white seems to be the hard part. The use of vacuum-cleaner robots has been considered.
There are also things that the average citizen can do to help raise the earth’s albedo.
What got me thinking about albedo was this article from Tree Hugger.