National Public Radio: More than 20 years ago, an international treaty known as the Montreal Protocol phased out a group of chemicals that were destroying the Earth’s ozone layer. But since then, scientists have discovered that some of the chemicals developed to replace those destructive compounds might be contributing to another problem: global warming. These new chemicals are known as hydrofluorocarbons, or HFCs, and they’re used as coolants in refrigerators. HFCs have largely replaced older …