Instead of following your trick-or-treaters down the block in the car, consider walking the...

How much does it cost to pollute? Very often, it doesn't cost a dime. The principle of a carbon market is that it should: If companies or countries had to pay for the right to pollute, the thinking goes, they'd do less polluting. A carbon market functions much like any financial market, in which carbon shares (sometimes called pollution credits), representing the right to emit carbon dioxide, methane, and other greenhouse gases, are bought and sold. The market works in conjunction with a cap on allowable emissions; within the market, polluters that are below the cap can sell the "excess" emissions rights as credits, or shares, to others who are above the limit. One goal is to create a scarcity of shares, driving up the cost of emitting chemicals into the atmosphere through conventional, fossil-fuel-powered means, and encouraging further reduction and investment in alternative fuel sources. Experts disagree on how well this works in actual practice.

It is easy to throw caution to the wind when the office is paying for supplies, business travel...