For all the money spent, nobody can say if the offsets have done much to alleviate global warming.... Without government regulation and mandatory caps on emissions, all that is left to drive offset sales is guilt and marketing. Offset companies charge what the market will bear. "How much are you willing to spend to feel good or to impress your neighbors?" asked Marland, of Oak Ridge National Laboratory. For Katy Hansen, a 29-year-old law school student, the answer was $429.99. She wanted to offset the travel of the guests to her May wedding in Madison, Wis., and paid San Francisco-based...