Normally I find most geocliamate engineering proposals impractical and dangerous moonbattery, but Physicist and Sci-Fi writer Greg Benford has a practical idea...Dunno if it works or not.

"Benford has a proposal that possesses the advantages of being both one of the simplest planet-cooling technologies so far suggested and being initially testable in a local context. He suggests suspension of tiny, harmless particles (sized at one-third of a micron) at about 80,000 feet up in the stratosphere. These particles could be composed of diatomaceous earth. "That's silicon dioxide, which is chemically inert, cheap as earth, and readily crushable to the size we want," Benford says. This could initially be tested, he says, over the Arctic, where warming is already considerable and where few human beings live. Arctic atmospheric circulation patterns would mostly confine the deployed particles around the North Pole. An initial experiment could occur north of 70 degrees latitude, over the Arctic Sea and outside national boundaries. "The fact that such an experiment is reversible is just as important as the fact that it's regional," says Benford. You could do this for a hundred million bucks a year. You could do the whole planet for a couple of billion. That's amazingly cheap."

What I like is that his proposal is practical, affordable and scientific - that is, it's falsifiable.