Carbon dioxide found popularity as a refrigerant for comfort cooling in the 1920s, before being replaced by more convenient CFCs (chlorofluorocarbons) in the 1930s and then HFCs (hydroflurocarbons) in the 1990s. But with the Kyoto Protocol, an international treaty requiring industrialized nations to lower their collective greenhouse gases 5 percent below 1990 levels by 2012, carbon dioxide has come back into the limelight as a hope for a safe and effective refrigerant replacement.