"...but maybe a screen of some sort could be placed in space to cast shadow on earth at various times and at least slow the process..."
Stationed between the Earth and sun at a point where the gravitational forces nearly cancel each other out, a 600,000-square-mile space "mirror" scatters sunlight with a mesh woven of fine metal wires.
From Popular Science, August 2005:
http://www.popsci.com/popsci/aviationspace/3afd8ca927d05010vgnvcm1000004eecbccdrcrd.html