<p>The United States Environmental Protection Agency Administrator finds that the current and projected concentrations of the six key well-mixed greenhouse gases&mdash;carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)&mdash;in the atmosphere threaten the public health and welfare of current and future generations.</p>