The federal Environmental Protection Agency said that in 2013, carbon dioxide accounted for more than 80 percent of all U.S. greenhouse gas emissions from human activities.

Researchers at Arizona State University are working on a way to take CO2 out of the air. Klaus Lackner is the director of the university’s Center for Negative Carbon Emissions.

EDITOR'S NOTE: This story originally aired December 1, 2015 and was re-aired July 4, 2016.