In 2012, CO2 accounted for about 82 percent of all U.S. greenhouse gas emissions, according to the EPA. "We know through high-accuracy instrumental measurements that there is an unprecedented increase in CO2 in the atmosphere. We know that CO2 absorbs infrared radiation [heat] and the global mean temperature is increasing," Keith Peterman, a professor of chemistry at York College of Pennsylvania, and his research partner, Gregory Foy, an associate professor of chemistry at York College of Pennsylvania, told Live Science in a joint email message.