Recently, one of the most irksome members of the Senate, Joe Lieberman (I-Clowntown) expressed openness to one of the boldest and most effective climate-change policies possible. Some background:

A cap-and-trade system begins by placing a cap on carbon emissions and distributing permits (permission to emit a certain amount of CO2) equal to the capped amount. The notion is that permits will be bought and sold, allowing market forces to determine where emission reductions can be made fastest and easiest. The question...