JOHANNESBURG (AP) -- Greenhouse gases that are more powerful than carbon dioxide are the focus of a global gathering this week in Africa. Experts say cutting hydrofluorocarbons, or HFCs, is the fastest way to reduce global warming. The United States is among the countries that want the world to quickly phase out the use of HFCs, and Secretary of State John Kerry is there to encourage a deal. Hereamp;apos;s a look at a potential global climate agreement that has been described by the Nat...