...while cranking out carbon dioxide at business-as-usual rates we can geo-engineer our way back toward pre-industrial temperatures in short order, reaching 1900 levels in about five years. Not only that, it would be fairly cheap and easy to do.

Pumping 20 to 25 liters of aerosols per second to keep enough particles in the stratosphere would cool temperatures, causing the planet's carbon sinks to suck more carbon dioxide out of the atmosphere.

"That kind of flow rate can be handled by a single fire hose," said Caldeira. "For something like $100 million a year you could probably keep a hose in the stratosphere suspended by an array of balloons with pumps along the way."

The problem is what happens if we stop short or screw it up.

Bring the geo-engineering process to a halt, and those sun-warmed carbon sinks spit the carbon dioxide right back into the atmosphere. The rebound warming, to temperatures that would have been reached without the geo-engineering, would be 10 to 20 times the pace of today's global warming. The rapid warming, up to 7 degrees Fahrenheit per decade, would wreak havoc on the planet and threaten civilization.

To prevent disaster, the geo-engineering process would have to continue as long as carbon-dioxide levels were elevated. A quarter of the carbon dioxide that comes out of your car's tailpipe is still in the atmosphere a thousand years later, Caldeira said.

"We've never had systems work for a thousand years without failure," he added.