Peter Eisenberger, a distinguished professor of earth and environmental sciences at Columbia University, has build two machines in Menlo Park, Calif., to pull carbon dioxide out of the air, like your car's catalytic converter, only giant-sized.

The challenging part was figuring out what to do with the CO2 once it was captured.

Growers pipe carbon dioxide into greenhouses. Oil companies pump it underground to help them squeeze out more oil. Soda companies use it to put bubbles in their drinks. These are mostly small-scale applications.

But using carbon dioxide to make fuel could someday be big. So Eisenberger's first project involves using CO2 to feed algae that churn out biofuel.

"Our first demonstration plant is being erected right now," he said, "in Alabama."

(READ or LISTEN to the story from NPR)