In writing a recent article about nuclear winter, I ran across this outrageous statement quite a bit: "Couldn't we just cancel out global warming with nuclear winter?" The short answer? Yes. And you can also cure a hangnail with a meat cleaver, though it's probably not quite the fix you're looking for.
To be fair, no one is seriously advocating the use of thermonuclear weapons to save the environment. Most of the time, the suggestion is either a thoughtless joke or a shot at the theoretic (and therefore fallible) aspects of both global warming and nuclear winter. When it comes to understanding our atmosphere, there's a whole lot of room to wind up getting it wrong.
The bottom line is that, yes, scientists in a recent study theorize that the detonation of just 100 nuclear weapons in a regional conflict (such as between India and Pakistan) would cloak the globe in enough smoke to produce a three-year temperature drop of approximately 2.25 degrees F (1.25 degrees C). If you eject enough debris into the atmosphere, you could shield the surface from solar radiation and cut down on the heat.
Of course, there are ways to do this without killing millions or starting massive, city-destroying firestorms. In 2007, Wired blogger Brandon Keim spotlighted a proposal to unleash fleets of robot, cloud-seeding ships on the world's oceans. They'd simply blast seawater up into the atmosphere as a fine mist, creating the protective cloud cover.
Of course, we've all watched enough sci-fi to know what will happen if we create a fleet of robot super-ships: They'll either decide to kill all humans or pine for our understanding of love.
Treehugger offers a far simpler method of climate control: paint your roof white. This reflects solar radiation back up through the atmosphere, cutting down on cooling costs According to the Los Angeles Times, enough white roofs and roads in U.S. cities could offset 44 metric gigatons of greenhouse gases.