Barron’s: A KEY PIECE OF CLIMATE-CHANGE legislation is still up in the air, but that’s not hindering the rapid expansion of U.S. carbon markets. The extent of the expansion will depend on many factors, but key is the coming legislation for a mandatory U.S. "cap-and-trade" program, essentially a system for trading rights to emit the gases blamed for global warming. Although carbon trading is global, the majority of activity is so far in Europe, which has a four-year-old cap-and-trade …