"According to the E.P.A. announcement, the proposed finding was based on rigorous scientific analysis of six gases — carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride — that have been widely studied by scientists around the world. Their studies showed that concentrations of these gases are at unprecedented levels as a result of human activity, the agency said, and these high levels are very likely responsible for the increase in average temperatures and other changes in the earth’s climate."
This decision was precipitated by a 5-4 (and by many accounts surprising) Supreme Court ruling, in the 2007 case of Massachusetts vs. Environmental Protection Agency which directed the EPA to determine if greenhouse gases were harmful to public health and the environment. While the Agency's scientists replied with the affirmative, the Bush Administration did what it needed to do to make sure no action was taken. But today is a new day.