Methods

Methods which have been proposed include:
Releasing dust, sulfate particles, or reflecting micro-balloons into the stratosphere. This proposal, not unlike the others, carries with it considerable risks, including increased drought.
Enhancing low-level clouds.
Creating a Saturn-like ring of small particles.
Putting a very large diffraction grating (thin wire mesh) or lens in space, perhaps at the L1 point between the Earth and the Sun.
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.