Use of solar shades is a proposed approach to the mitigation of global warming through planetary engineering. By intentionally changing the Earth's albedo, or reflectivity, scientists propose that we could reflect more heat back out into space, or intercept sunlight before it reaches the Earth through a literal shade built in space. A 0.5% albedo increase would roughly halve the effect of CO2 doubling.  As early as 1974, Russian expert Mikhail Budyko suggested that if global warming became a problem, we could cool down the planet by burning sulfur in the stratosphere, which would create a haze. Paul Crutzen suggests that this would cost 25 to 50 billion dollars/year. It would, however, increase the environmental problem of acid rain.
Creating a Saturn-like ring of small particles. 
Putting a very large diffraction grating (thin wire mesh) in space, perhaps at the L1 point between the Earth and the Sun. This plan was proposed in 1989 by J. T. Early,, and in 1997 by Edward Teller, Lowell Wood, and Roderick Hyde, In 2004, physicist and science fiction author Gregory Benford calculated that a rotating Fresnel lens 1000 kilometres across, floating in space at the L1 point, would reduce the solar energy reaching the Earth by approximately 0.5% to 1%. He estimated that this would cost around $10 billion.  In 2006, Benford outlined some possible side-effects of such a plan. 
The cooling effect that volcanic eruptions often have on the climate due to ash particles in the upper atmosphere can be seen as an analogy of how these methods might work.
A preliminary study by Edward Teller and others in 2002 presented the pros and cons of various relatively "low-tech" proposals to mitigate global warming through scattering/reflecting sunlight away from the Earth via insertion of various materials in the upper stratosphere, low earth orbit, and L1 locations. 
^ http://environment.newscientist.com/article/dn12397-sunshade-for-global-warming-could-cause-drought.html Sunshade' for global warming could cause drought 02 August 2007 New Scientist, Catherine Brahic
^ See footnote 23 in E. Teller, L. Wood, and R. Hyde, "Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change".
^ E. Teller, L. Wood, and R. Hyde, "Global Warming and Ice Ages: Prospects for Physics-Based Modulation of Global Change".
^ See Russell Dovey, "Supervillainy: Astroengineering Global Warming and Bill Christensen, "Reduce Global Warming by Blocking Sunlight".
^ In 2006, Benford pointed out one possible danger in this approach: if this lens were built and global warming were avoided, there would be less incentive to reduce greenhouse gases, and humans might continue to produce too much carbon dioxide until it caused some other environmental catastrophe, such as a chemical change in ocean water that could be disastrous to ocean life. (Comments at the 64th World Science Fiction Convention, August 2006.)