OK, they’re all dubious, but a new Bloomberg Businessweek article documents one unusual carbon offset transaction involving Chevrolet:

The automaker is paying almost $750,000 to bolster a state program in Maine that insulates homes for low-income families. The investment is enough to weatherize 170 houses and reduce carbon emissions by 1,224 tons through 2014, according to the Maine State Housing Authority. Yet Chevrolet is receiving credits for 45,738 tons worth of reduced carbon—the total savings Maine expects through 2014 from weatherizing all 5,500 homes in the program.

The difference illustrates a peril in using carbon offsets to achieve environmental goals, according to Anja Kollmuss, a Zurich-based scientist associated with the Stockholm Environment Institute and one of five co-authors of a handbook on carbon offset programs. Experts agree that carbon offsets are valuable only when they encourage emissions reductions that wouldn’t have happened otherwise. “The key is that decisions are actually altered by the creation of this carbon credit,” says Michael Wara, an assistant professor at Stanford Law School and a scholar of environmental law. “If they’re not, then we’re basically buying hot air.”