I just finished watching the 'Mega Disasters: Mega Freeze' documentary in The History Channel which explains how global warming can disrupt the Thermohaline Circulation (THC)* causing an abrupt drop in temperatures in Europe and North America similar to what happened during the Younger Dryas** (12700 to 11500 years Before Present) setting off a chain of disasters that will reduce the estimated carrying capacity of the planet from the current 8 billion people to 2 billion.
If the THC is as important to maintaining the present climate as that documentary says it is, a simple (and perhaps simplistic) solution would be to throw more salt into the Ocean to maintain present levels of salinity. How much more salt and how to do this would be the challenge facing the world's scientists and engineers.
Update 08-11-2007: Via bookforum.com, a pointer to Mad scientists vs. global warming.
Update 01-01-2008: Via bookforum.com, a pointer to The Year's 10 Craziest Ways to Hack the Earth.
*For those who watched The Day After Tomorrow, the disruption of the Thermohaline Circulation is what triggered the Ice Age in that movie.
**Al Gore also talked about this in his movie, An Inconvenient Truth.